ASNT Level III Visual Testing, VT

ASNT Level III Visual Testing, VT 

2014-August 

My Self Study Exam Preparatory Notes 

Charlie Chong/ Fion Zhang



Fion Zhang 

2014/August/15 

http://meilishouxihu.blog.163.com/ 


ASNT Certification Guide 

NDT Level III / PdM Level III- VT - Visual Testing 

Length: 2 hours Questions: 90 

1. Fundamentals 

• Vision and light 

• Ambient conditions 

• Test object characteristics 

2. Equipment Accessories 

• Magnifiers/microscopes 

•Mirrors 

• Dimensional 

• Borescopes 

• Video systems 

• Automated systems 

• Video technologies 

5 


• Machine vision 

• Replication 

• Temperature sensitive markers and surface comparators 

• Chemical aids 

• Photography 

•Eye 

3. Techniques/Calibration 

• Diagrams and drawings 

• Raw materials 

• Primary process materials 

• Joining processes 

• Fabricated components 

• In-service materials 

• Coatings 

• Other applications 

• Requirements 

6 


4. Interpretation/ Evaluation 

• Equipment including type and intensity of light 

• Material including the variations of surface finish 

• Discontinuity 

• Determination of dimensions (i.e.: depth, width, length, etc.) 

• Sampling/scanning 

• Process for reporting visual discontinuities 

• Personnel (human factors) 

• Detection 

5. Procedures and Documentation 

• Hard copy 

• Photography 

• Audio/video 

• Electronic and magnetic media 

7 


6. Safety 

• Electrical shock 

• Mechanical hazards 

• Lighting hazards 

• Chemical contamination 

• Radioactive materials 

• Explosive environments 

Reference Catalog Number 

NDT Handbook: Second Edition: Volume 8, 

Visual and Optical Testing 133 

ASNT Level III Study Guide: Visual and 

Optical Testing 2263 

ASM Handbook: Vol. 17, NDE and QC 105 

8 


SI Multiplier 

http://www.poynton.com/notes/units/ 


Other Reading: 

http://quizlet.com/29958394/visual-inspection-test-flash-cards/ 


Reading 1 


Key Points Only 


Reading 1 


Section 1: Introduction to Visual & Optical Testing 

Chapter 1: Fundamental of Light & Lighting 


About Vision 


Keywords: 

• Wavelength that excite retina 380~770 nm (380 x10 -9 ~ 770 x10 -9 m) 

• Electromagnetic theory also known as Maxwell Theory 

• Plank’s Quantum theory E= hv, h = plank’s constant (1.626 x 10 -34 Joule. 

Second), v = frequency (Hz) 


Keywords: 

• Light Spectrum 


Keywords: 

• Light Spectrum 


Keywords: 

• Scoptic & Photopic Visions 


Keywords: 

• Photopic--daylight--response of the human eye 

380nm 

770nm 


Keywords: 

• Scotopic- night response of the human eye. Note the loss of sensitivity to 

blue and red wavelengths. 


Keywords: 

• Scotopic & Photopic 

Charlie Chong/ Fion Zhang 

http://www.nature.nps.gov/night/science.cfm

Keywords: 

• Subtractive primaries 

• Additive primaries 


Magenta 


Cyan 


Keywords: 

Refraction Index n = V v / V m , 

Speed of light in medium, C = λv / n 

V v 

V m 

= Velocity of light in vacuum 

= Velocity of light in material 


Light Refraction: Refractive Index of Medium 

Refraction Index n = V v / V m , 

Speed of light in medium, C = λv / n 

Light Refraction: Snell Law 

Sin ϴ 1 / V 1 = Sin ϴ 2 / V 2 , Sin ϴ 1 n 1 = Sin ϴ 2 n 2 

As the speed of light is reduced in the slower medium, the wavelength is 

shortened proportionately. The frequency is unchanged; it is a characteristic 

of the source of the light and unaffected by medium changes. 



Sin ϴ1 n1 = Sin ϴ2 n2 



http://www.physicsclassroom.com/shwave/refraction.cfm 


Surface Luminance: Inverse Square Law 

Surface Luminance: 

E = I / d 2 

E = Source luminance, I= Source 

illuminances, d= distance 


Surface Luminance: Inverse Square & Lambert Law 


E = I / d 2 x Cosα 

E = Surface luminance, I= Source 

illuminances, d= distance, 

α = angle of incidence from normal 


Surface Luminance: Inverse Square & Lambert Law 


E = I / d 2 x Cosα 


Surface Luminance: Lambert’s Law 

In optics, Lambert's cosine law says that the radiant intensity or luminous 

intensity observed from an ideal diffusely reflecting surface or ideal diffuse 

radiator is directly proportional to the cosine of the angle θ between the 

observer's line of sight and the surface normal. The law is also known as the 

cosine emission law[3] or Lambert's emission law. It is named after Johann 

Heinrich Lambert, from his Photometria, published in 1760. 

A surface which obeys Lambert's law is said to be Lambertian, and exhibits 

Lambertian reflectance. Such a surface has the same radiance when viewed 

from any angle. This means, for example, that to the human eye it has the 

same apparent brightness (or luminance). It has the same radiance because, 

although the emitted power from a given area element is reduced by the 

cosine of the emission angle, the apparent size (solid angle) of the observed 

area, as seen by a viewer, is decreased by a corresponding amount. 

Therefore, its radiance (power per unit solid angle per unit projected source 

area) is the same. 


Surface Luminance: Lambert’s Cosine Law 

http://en.wikipedia.org/wiki/Lambert%27s_cosine_law 


Surface Luminance: Lambert’s Cosine Law 


E = I / d2 x Cos ϴ, 

Sin ϴ1 / V1 = Sin ϴ2 / V2 , 

Sin ϴ1 n1 = Sin ϴ2 n2 , 




Keywords: 

Absorptive Characteristic 

• Selective absorptive material – distinctive color 

• Non-selective absorptive material- black or gray appearance 

Transmitive Characteristics 

• Transparent materials- Transmitted light without apparent scatter. 

• Translucent materials- Transmitted large part of light with scattered some 

portion due to diffusion (diffusion?) 

• Opaque materials- Transmit no light, all of the spectrum is absorbed or 

reflected or combination of both. 



Flood the surface with as much 

light as possible and minimize 

shadow. 

Surface irregularities reflected the 

light toward from camera 



Surface irregularities reflected the light away from 

camera

Edges detection on opaque 

objects 

As with dark field front, this setup 

requires the camera to shown 

dark field. 


The lambert (symbol L, la or Lb) is a non-SI unit of luminance named for 

Johann Heinrich Lambert (1728–1777), a Swiss mathematician, physicist and 

astronomer. A related unit of luminance, the foot-lambert, is used in the 

lighting, cinema and flight simulation industries. The SI unit is the candela per 

square metre (cd/m²). 

http://en.wikipedia.org/wiki/Lambert_(unit) 


Chapter 2: Physiology of Vision 




http://starizona.com/acb/basics/observing_theory.aspx

The Eyes 


Keywords: 

Iris- Contracted, dilated 

Pupils- 

Chromatic aberration 

Spherical aberration 

Note: An aberration is something that 

deviates from the normal way. 


Rods and Cones 

The retina contains two types of photoreceptors, rods and cones. The rods 

are more numerous, some 120 million, and are more sensitive than the cones. 

However, they are not sensitive to color. The 6 to 7 million cones provide the 

eye's color sensitivity and they are much more concentrated in the central 

yellow spot known as the macula. In the center of that region is the " fovea 

centralis ", a 0.3 mm diameter rod-free area with very thin, densely packed 

cones. 

The experimental evidence suggests that among the cones there are three 

different types of color reception. Response curves for the three types of 

cones have been determined. Since the perception of color depends on the 

firing of these three types of nerve cells, it follows that visible color can be 

mapped in terms of three numbers called tristimulus values. Color perception 

has been successfully modeled in terms of tristimulus values and mapped on 

the CIE chromaticity diagram. 


Rod and Cone Density on Retina 

Cones are concentrated in the fovea centralis. Rods are absent there but 

dense elsewhere. 

Measured density curves for the 

rods and cones on the retina show 

an enormous density of cones in 

the fovea centralis. To them is 

attributed both color vision and the 

highest visual acuity. 

Visual examination of small detail involves focusing light from that detail onto 

the fovea centralis. On the other hand, the rods are absent from the fovea. At 

a few degrees away from it their density rises to a high value and spreads 

over a large area of the retina. These rods are responsible for night vision, 

our most sensitive motion detection, and our peripheral vision. 


Rod and Cone Density on Retina 


Cone Details 

Current understanding is that the 6 to 7 million cones can be divided into "red" 

cones (64%), "green" cones (32%), and "blue" cones (2%) based on 

measured response curves. They provide the eye's color sensitivity. The 

green and red cones are concentrated in the fovea centralis . The "blue" 

cones have the highest sensitivity and are mostly found outside the fovea, 

leading to some distinctions in the eye's blue perception. 

The cones are less sensitive to light than the rods, as shown a typical daynight 

comparison. The daylight vision (cone vision) adapts much more rapidly 

to changing light levels, adjusting to a change like coming indoors out of 

sunlight in a few seconds. Like all neurons, the cones fire to produce an 

electrical impulse on the nerve fiber and then must reset to fire again. The 

light adaption is thought to occur by adjusting this reset time. 

The cones are responsible for all high resolution vision. The eye moves 

continually to keep the light from the object of interest falling on the fovea 

centralis where the bulk of the cones reside. 


Rod Details 

The rods are the most numerous of the photoreceptors, some 120 million, 

and are the more sensitive than the cones. However, they are not sensitive to 

color. They are responsible for our dark-adapted, or scotopic, vision. The rods 

are incredibly efficient photoreceptors. More than one thousand times as 

sensitive as the cones, they can reportedly be triggered by individual photons 

under optimal conditions. The optimum dark-adapted vision is obtained only 

after a considerable period of darkness, say 30 minutes or longer, because 

the rod adaption process is much slower than that of the cones. 

The rod sensitivity is shifted toward shorter wavelengths compared to daylight 

vision, accounting for the growing apparent brightness of green leaves in 

twilight. 


While the visual acuity or visual resolution 

is much better with the cones, the rods are 

better motion sensors. Since the rods 

predominate in the peripheral vision, that 

peripheral vision is more light sensitive, 

enabling you to see dimmer objects in your 

peripheral vision. If you see a dim star in 

your peripheral vision, it may disappear 

when you look at it directly since you are 

then moving the image onto the cone-rich 

fovea region which is less light sensitive. 

You can detect motion better with your 

peripheral vision, since it is primarily rod 

vision. 

The rods employ a sensitive photopigment 

called rhodopsin. 


Keywords: 

• Cones are responsible for color perceptions and details 

• Rods are responsible for night vision, our most sensitive motion detection, 

and our peripheral vision. 

• Rods is one thousandth times more efficient photoreceptors than cones 

• Rods are efficient motion sensors 

• Rod adaptation is much slower than cones 

• Night vision is best at peripheral vision (not viewing the object right at the 

center of field of vision) 


Visual Acuity 

Resolution Acuity ( 识别视力 ) 

Recognition Acuity ( 识别视力 ) 

Temporal Resolution ( 瞬时清晰复 , 瞬时清晰度 ) 

Note: Temporal resolution refers to the accuracy of a particular measurement 

with respect to time. It is often in contest with spatial resolution which is a 

measure of accuracy with respect to the details of the space being measured. 


Visual Angle 


Snellen’s Acuity Fraction 


Visual Acuity: What is 20/20 Vision 

20/20 vision is a term used to express 

normal visual acuity (the clarity or 

sharpness of vision) measured at a 

distance of 20 feet. If you have 20/20 

vision, you can see clearly at 20 feet what 

should normally be seen at that distance. 

If you have 20/100 vision, it means that 

you must be as close as 20 feet to see 

what a person with normal vision can see 

at 100 feet. 

20/20 does not necessarily mean perfect 

vision. 20/20 vision only indicates the 

sharpness or clarity of vision at a distance. 

There are other important vision skills, 

including peripheral awareness or side 

vision, eye coordination, depth perception, 

focusing ability and color vision that 

contribute to your overall visual ability 


Approximate table of equivalent visual acuity notations for near vision 

http://courses.ttu.edu/edsp5383-ngriffin/letter.htm 


Color Vision: 

Photopic Vision depends on: 

■ 

■ 

■ 

Quantity of light 

Quality of light 

Adeptness of eye 

Inspection Color Temperature: 6700ºC with full spectrum is optimum 

Color Deficiencies affect approximately 

■ 10% of Male population. 

■ Women only constituted 0.5% of those affected. 


Ishihara Plates 


Ishihara Plates 


Color Deficiencies 


Optical Illusion-Due to Contrast 


Optical Illusion- The Logvinenko illusion. Although gray diamonds are identical, 

there appear to be light-gray ones and dark-gray ones (LOGVINENKO, 1999). 


Optical Illusion- Due to Contrast 


Optical Illusion- A & B 

http://en.wikipedia.org/wiki/Optical_illusion 


Optical Illusion- The square A is exactly the same shade of grey as 

square B. 


Optical Illusion- In this illusion, the coloured regions appear rather 

different, roughly orange and brown. In fact they are the same colour, and 

in identical immediate surrounds, but the brain changes its assumption 

about color due to the global interpretation of the surrounding image. Also, 

the white tiles that are shadowed are the same color as the grey tiles 

outside the shadow. 


Optical Illusion- Simultaneous Contrast Illusion. The background is a color 

gradient and progresses from dark grey to light grey. The horizontal bar 

appears to progress from light grey to dark grey, but is in fact just one colour. 


Optical Illusion- Simultaneous Contrast Illusion. The background is a color 

gradient and progresses from dark grey to light grey. The horizontal bar 

appears to progress from light grey to dark grey, but is in fact just one colour. 


Optical Illusion – Due to Brightness 

Bright objects look larger than the dark objects of the same size. 


Chapter 3: Fundamental of Imaging 


Keywords: 

• Plano 

• Concavo / Convexo 

• Convex 

• Concave 


Keywords: 

Thin lens: The thickness of the lens is small compare to its focal length. 

The thin lens equation: 

1/f = 1/d = 1/u 


Lenses and the focal lengths 


Chapter 4: Test Object Characteristics 


Keywords: 

Surface Features: Affected by (1) Form, (2) Waviness & (3) Roughness 


Surface Roughness with & without profile variation 

Profile = Form ? Waviness? 

Form- Variation i form or 

profile are typically 

controlled by the 

dimensional or geometric 

tolerance specifications. 


Surface Roughness 

Surface roughness, often shortened to roughness, is a component of surface 

texture. It is quantified by the vertical deviations of a real surface from its ideal 

form. If these deviations are large, the surface is rough; if they are small, the 

surface is smooth. Roughness is typically considered to be the highfrequency, 

short-wavelength component of a measured surface (see surface 

metrology). However, in practice it is often necessary to know both the 

amplitude and frequency to ensure that a surface is fit for a purpose. 

Roughness plays an important role in determining how a real object will 

interact with its environment. Rough surfaces usually wear more quickly and 

have higher friction coefficients than smooth surfaces (see tribology). 

Roughness is often a good predictor of the performance of a mechanical 

component, since irregularities in the surface may form nucleation sites for 

cracks or corrosion. On the other hand, roughness may promote adhesion. 


Although roughness is often undesirable, it is difficult and expensive to control 

in manufacturing. Decreasing the roughness of a surface will usually increase 

exponentially its manufacturing costs. This often results in a trade-off between 

the manufacturing cost of a component and its performance in application. 

Roughness can be measured by manual comparison against a "surface 

roughness comparator", a sample of known surface roughnesses, but more 

generally a Surface profile measurement is made with a profilometer that can 

be contact (typically a diamond styles) or optical (e.g. a white light 

interferometer). 

However, controlled roughness can often be desirable. For example, a gloss 

surface can be too shiny to the eye and too slippy to the finger (a touchpad is 

a good example) so a controlled roughness is required. This is a case where 

both amplitude and frequency are important. 


A roughness value can either be calculated on a profile (line) or on a surface 

(area). The profile roughness parameter (Ra, Rq,...) are more common. The 

area roughness parameters (Sa, Sq,...) give more significant values. 


Profile roughness parameters 

Each of the roughness parameters is calculated using a formula for 

describing the surface. Although these parameters are generally considered 

to be "well known" a standard reference describing each in detail is Surfaces 

and their Measurement. 

There are many different roughness parameters in use, but Ra is by far the 

most common though this is often for historical reasons not for particular merit 

as the early roughness meters could only measure Ra. Other common 

parameters include Rz, Rq,and Rsk. Some parameters are used only in 

certain industries or within certain countries. For example, the Rk family of 

parameters is used mainly for cylinder bore linings, and the Motif parameters 

are used primarily within France. 


Since these parameters reduce all of the information in a profile to a single 

number, great care must be taken in applying and interpreting them. Small 

changes in how the raw profile data is filtered, how the mean line is 

calculated, and the physics of the measurement can greatly affect the 

calculated parameter. With modern digital equipment it makes sense to look 

at the scan and make sure there aren't some obvious glitches that are 

skewing the values - and if there are, to re-measure 


Keyword: 

Ra: 

Roughness average is the universally recognised and most used international 

parameter of roughness. It is the arithmetic mean of the absolute departures 

of the roughness profile from the mean line. Ra is reported in microns. 

http://www.finetubes.co.uk/products/technical-reference-library/tube-surface-finishes/ 

Ra- Arithmetic average of absolute values 

Average distance of the profile to the mean line - Area under the curve 

between the surface profile and the surface mean after applying a 

mathematical filter to eliminate the effect of waviness. 

Ra- Arithmetic average of absolute values 

Area under the curve between the surface profile and the surface mean after 

applying a mathematical filter to eliminate the effect of waviness. 


Ra- Average distance of the profile to the mean line 


Comparison of approximately same Ra value with different profile with 

different roughness peak R p and roughness depth R v 

http://www.olympus-ims.com/en/knowledge/metrology/roughness/2d_parameter/ 


Keywords 

Anisotropic Surface: Periodic irregularity usually in one direction. 


Rz & Rmax 


Keywords: 

Anisotropic Surface- has a periodic irregularity usually in one direction 


Color & Gloss 


Color & Gloss 

Visual Comparison for Color Matching- The two most common systems 

are the (1) Natural color system and (2) Munsell color ordering system 

Color’s Variables: 

Colors order systems described colors as (1) hue, (2) value (3) saturation 

Hue- Chromaticness, describes color as its primary color constituents or mix 

of color constituents. expressed as redness, blueness and so forth 

Value- Described colors as lightness or darkness; Light color has high value 

and dark color has low value. 

Saturation- Measure of distance from natural corresponding color. It is often 

referred as color strength or intensity. 


Natural Color System 









Munsell Color System 


Geometry 

Datum Reference: X, Y, Z 


Geometric Tolerances 


Section 2: Material Science Basics and Visual 

Testing Applications 

Chapter 5: Types of Materials to be Tested 


Metal 

Cells → Crystals 


Metal 


Keywords: 

• Atoms 

• Cells 

• Crystals 

• Allotropic- Metal exhibits more than one cell structures 

• Macroscopic evaluation- 10X magnifications or less 

• Microscopic evaluation- >10X magnifications (50X ~ 200X) 


Mechanical Properties 


Keywords: 

Electrochemical Nature of Corrosion: 

General corrosion, Crevice corrosion and Galvanic corrosion are cause by 

the same mechanism. 


Chapter 6: 

Visual & Optimal Testing Applications 


Keywords: 

Metal Working Processes 

• Primary forming processes 

• Secondary forming processes 

• Finishing processes 

• Joining processes 

• Service 


• Metal Casting 

http://thelibraryofmanufacturing.com/metalcasting_basics.html 


Metal Casting 


• Forming processes 

Metal Manufacturing- Metal Rolling 

http://thelibraryofmanufacturing.com/metal_rolling.html 


• Primary forming processes - Metal Rolling 








• Secondary forming processes - Metal Rolling 






















• Finishing forming processes 








• Joining Process 






• Services 


• Services 


• Services 


Section 3: Inspection Planning & Equipment 

Chapter 7: 

Inspection Planning & Visual Inspection Tools 


Profilometer 


Profilometer 


Section 4: Documentation & Analysis 

Chapter 8: 

Documentation of Visual Testing 


Replication 

One of the purpose of performing in situ replication is to determine the extent 

of thermal degradation or thermal aging from the microstructure appearance. 

Prolonged exposure to high temperature will cause microstructures to 

decompose and eventually result in creep cracking. 

By replication technique, microstructure can be obtained at site nondestructively 

to safely judge the condition of the component. The results 

obtained can be used to identify previous heat treatment process and to verify 

the required temperature setting for post weld heat treatment (PWHT) for 

repair work purposes, and as a reference for future inspection and 

maintenance work arrangement. The processes of producing replica at site 

are based on international code and standard as follow: 


The processes of producing replica at site are based on international code 

and standard as follow: 

a. ASTM E3: Standard Guide for Preparation of Metallographic Specimens 

b. b. ASTM E407: Standard Practice for Microetching Metals and Alloys 

c. c. ASTM E1351: Standard Practice for Production and Evaluation of Field 

Metallographic Replicas 

d. ASTM E1558: Standard Guide for Electrolytic Polishing of Metallographic 

Specimens 


In-situ Metallographic Replication 






The techniques: 

A standardized technique (ASTM E 1351, ISO 3057) which can be 

implemented on most metallic materials using portable polishing and etching 

devices and a field optical microscope. The procedure to carry out 

metallographic replicas includes at least five stages 

1. Local grinding to eliminate surface layers (paint, decarburised layers, 

oxidation…) 

2. Mechanical polishing using abrasive papers and diamond paste 

3. Chemical or electrolytic etching of the polished area to reveal the 

microstructure 

4. Replication of the microstructure with a cellulose acetate film 

5. Observation of the structure with an optical microscope or a scanning 

electron microscope 



The End

ASNT Level III VT- Reading 2 

Pre-Exam Preparatory Notes 

My Self Study Notes 2014 August 


Reading 2 


Pump: 

A pump is a device that moves fluids (liquids or gases), or sometimes slurries, 

by mechanical action. Pumps can be classified into three major groups 

according to the method they use to move the fluid: direct lift, displacement, 

and gravity pumps. Pumps operate by some mechanism (typically 

reciprocating or rotary), and consume energy to perform mechanical work by 

moving the fluid. Pumps operate via many energy sources, including manual 

operation, electricity, engines, or wind power, come in many sizes, from 

microscopic for use in medical applications to large industrial pumps. 

Mechanical pumps serve in a wide range of applications such as pumping 

water from wells, aquarium filtering, pond filtering and aeration, in the car 

industry for water-cooling and fuel injection, in the energy industry for 

pumping oil and natural gas or for operating cooling towers. In the medical 

industry, pumps are used for biochemical processes in developing and 

manufacturing medicine, and as artificial replacements for body parts, in 

particular the artificial heart and penile prosthesis. 


Dynamic & Displacement Pumps 

http://wiki.answers.com/Q/Differences_between_dynamic_pump_and_positive_displacement_pump 

The main difference between them is the way that energy is added to the fluid 

to be converted to pressure increase. In dynamic pumps, energy is added to 

the fluid continuously through the rotary motion of the blades. These rotating 

blades raise the momentum of fluid and the momentum then is converted to 

pressure energy through diffuser in pump outlet. In positive displacement 

pumps, the energy is added periodically to the fluid. the pump has 

reciprocating motion by pistons for example. When the fluid enters the pump 

through valves, the reciprocating piston begins to press the fluid resulting in 

going out of the pump with pressure rise. 

Type of positive displacement pumps: gear pump, crescent gear pump, axialpiston 

pump, radial-piston pump, linear-piston pump, & vane pump 

Also, fuel injection pumps such as linear piston pumps and rotary piston 

pumps. 


Centrifugal Pumps 

The pump impeller rotates within the pump housing (sometimes called the 

volute), thus causing a reduced pressure at the inlet (suction) side of the 

pump. The rotary motion of the impeller drives the fluid to the outside of the 

pump volute, increasing its pressure, and sending it out of the pump 

discharge, as shown in the diagram. 

Both of these diagrams show a radial flow centrifugal pump, which has the 

flow pattern just described above. This is the most common centrifugal pump 

flow pattern. Another alternative is the axial flow centrifugal pump, which has 

an impeller shaped somewhat like a propeller, that draws fluid in along the 

pump axis and sends it out along the axis at the other side of the pump. 

http://upload.wikimedia.org/wikipedia/commons/6/69/Flexible_impeller_pump.gif 


Centrifugal Pump – Displacement pump 

http://en.wikipedia.org/wiki/Pump 


Seismic snubbers 


Seismic snubbers 


Forging: Rolling Defects Hear what the Expert say 

http://pmpaspeakingofprecision.com/tag/tears/ 


Miles Free

Forging: Rolled-in-Scales 

Pre-rolled 

scale 

Rolled-in-Scale 


Scabs 


Scabs 

Scabs are irregularly shaped, flattened protrusions caused by splash, boiling 

or other problems from teeming, casting, or conditioning.-AISI Technical 

Committee on Rod and Bar Mills, Detection, Classification, and Elimination of 

Rod and Bar Surface Defect 

(Teeming refers to the process of filling an ingot mold with molten steel from 

the ladle. We’ll point out some continuous casting analogs later in this post.) 

Scabs have scale and irregular surfaces beneath them; they tend to be round 

or oval shaped and concentrated to only certain blooms or billets. Scabs are 

always the same chemistry as the steel bloom or billet. 

(If the gross irregular surface protrusion characteristic is appearing on all 

product, it is not likely to be a scab. If the protrusion is a different analysis, it is 

likely to be mill shearing.) 

To differentiate between scabs and rolled in scale, scabs are ductile when 

bent while scale is brittle and crumbles. 

If the protrusion is brittle, it may be rolled in scale. 


Scabs 


Slivers 


Slivers On Rolled Steel Products “Slivers are elongated pieces of metal 

attached to the base metal at one end only. They normally have been hot 

worked into the surface and are common to low strength grades which are 

easily torn, especially grades with high sulfur, lead and copper.”- AISI 

Technical Committee on Rod and Bar Mills, Detection, Classification, and 

Elimination of Rod and Bar Surface Defects 

Slivers are loose or torn segments of steel that have been rolled into the 

surface of the bar. 




Slivers are often mistaken for shearing, scabs, and laps. 





Slivers often originate from short rolled out point defects or defects which 

were not removed by conditioning. 

Billet conditioning that results in fins or deep ridges have also been found to 

cause slivers and should be avoided. Feathering of deep conditioning edges 

can help to alleviate their occurrence. 

Slivers often appeared on mills operating at higher rolling speeds. 

When the frequency and severity of sliver occurrence varies between 

heats, grades, or orders, that is a clue that the slivers probably did not 

originate in the mill. Slivers are often mistaken for shearing, scabs and lap. 


Seams On Rolled Steel Products 



“Seams are longitudinal crevices that are tight or even closed at the surface, 

but are not welded shut. They are close to radial in orientation and can 

originate in steelmaking, primary rolling, or on the bar or rod mill.”- AISI 

Technical Committee on Rod and Bar Mills, Detection, Classification, and 

Elimination of Rod and Bar Surface Defects 

Seams may be present in the billet due to non-metallic inclusions, cracking, 

tears, subsurface cracking or porosity. During continuous casting loss of mold 

level control can promote a host of out of control conditions which can reseal 

while in the mold but leave a weakened surface. Seam frequency is higher in 

resulfurized steels compared to non-resulfurized grades. Seams are generally 

less frequent in fully deoxidized steels. 






Seams can be detected visually by eye, and magnaglo methods; electronic 

means involving eddy current (mag testing or rotobar) can find seams both 

visible and not visible to the naked eye. Magnaflux methods are generally 

reserved for billet and bloom inspection. 

Seams are straight and can vary in length- often the length of several barsdue 

to elongation of the product (and the initiating imperfection!) during rolling. 

Bending a bar can reveal the presence of surface defects like seams. 

An upset test (compressing a short piece of the steel to expand its diameter) 

will split longitudinally where a seam is present. 


“These long, straight, tight, linear defects are the result of gasses or bubbles 

formed when the steel solidified. Rolling causes these to lengthen as the steel 

is lengthened. Seams are dark, closed, but not welded”- my 1986 Junior 

Metallurgist definition taken from my lab notebook. We’ve a bit more 

sophisticated view of the causes now. 

The frequency of seams appearing can help to define the cause. Randomly 

within a rolling, seams are likely due to incoming billets. A definite pattern to 

the seams indicates that the seams were likely mill induced- as a result of 

wrinkling associated with the section geometry. However a pattern related to 

repetitious conditioning could also testify to billet and conditioning causationfailure 

to remove the original defect, or associated with a repetitive grinding 

injury or artifact during conditioning. 


My rule of thumb was that if it was straight, longitudinal, and when filed 

showed up dark against the brighter base metal it was a seam. 

Rejection criteria are subject to negotiation with your supplier, as are 

detection limits for various inspection methods, but remember that since 

seams can occur anywhere on a rolled product, stock removal allowance is 

applied on a per side basis. 

If you absolutely must be seam free, you should order turned and polished or 

cold drawn, turned and polished material. The stock removal assures that the 

seamy outer material has been removed. 

Metallurgical note: seams can be a result of propogation of cracks formed 

when the metal soidifies, changes phase or is hot worked. Billet caused 

seams generally exhibit more pronounced decarburization. 


Laps On Rolled Steel Products 

“Laps are longitudinal crevices at least 30 degrees off radial, created by 

folding over, but not welding material during hot working (rolling). A 

longitudinal discontinuity in the bar may exist prior to folding over but the 

defect generally is developed at the mill.”- AISI Technical Committee on Rod 

and Bar Mills, Detection, Classification, and Elimination of Rod and Bar 

Surface Defects 

http://pmpaspeakingofprecision.com/2012/05/15/laps-on-rolled-steel-products/ 


Laps On Rolled Steel Products 


Laps: In plain language, a lap is a ‘rolled over condition in a bar where a 

sharp over fill or fin has been formed and subsequently rolled back into the 

bar’s surface.’ 


Laps: An etch of the full section shows what is going on in the mill. Laps were 

often related to poor section quality on incoming billets, although overfill 

scratches, conditioning gouges from “chipping” have also been shown to 

cause laps. 


Laps: Laps are often confused with slivers, and mill shearing which we 

shall describe and post soon. The term ‘lap seam’ is sometimes used, but it 

is careless usage; it implies the lap is caused by a seam – it is not; a seam is 

a longitudinally oriented imperfection, and so is used in this mongrel term as 

a shorthand way of saying ‘longitudinal.’ 

Modern speakers sometimes try to use the word ‘lamination’ to describe laps 

but as we will see, not all lamination type imperfections are laps… 


Cross section of steel bar exhibiting laps (white angular linear indications). 

When two laps are present 180 degrees apart, the depth to which they are 

folded over can indicate where in the rolling the initial over fill ocurred. White 

indicates decarburization, which confirms my interpretation that this lapping 

occurred early in the rolling. 


Central Bursts, Chevroning in Cold Drawn and Extruded Steels 

In cold worked steels, failures can be broadly categorized in two categories. 

The first, are those nucleated by localized defects- such as seams, pipe, and 

exogenous inclusions. The second, are those which result from exceeding the 

strength of the material itself. 

The compressive stresses of cold working results in failures by shear along 

planes 45 degrees to the applied stress. These are known as shear failures. 

The presence of shear failures in an otherwise metallurgically normal material 

indicates excessive mechanical deformation. While often the result of tooling 

issues, conditions which lower material ductility including chemistry, 

macrostructure, nonmetallics, microstructure, aging, and hydrogen 

embrittlement have also been implicated in investigations of premature shear 

failure. 

http://pmpaspeakingofprecision.com/tag/steel-defects/ 


This post will focus on the central Bursts in the product of cold drawn steel, 

especially from the point of view of a shop making parts on automated 

equipment. 

Ignoring the steel factors that may play a role in triggering the central bursts 

or chevrons, the role of tooling is usually considered to be the root cause, as 

replacement of dies typically eliminates the central bursting. 

A bar which exhibited central bursting was saw cut lengthwise to show the 

internal ruptures. 


Burst- Chevron 


Chevron 


About Glaring: 


Polarization: (also polarisation) is a property of waves that can oscillate with 

more than one orientation. Electromagnetic waves such as light exhibit 

polarization, as do some other types of wave, such as gravitational waves. 

Sound waves in a gas or liquid do not exhibit polarization, since the oscillation 

is always in the direction the wave travels. 

In an electromagnetic wave, both the electric field and magnetic field are 

oscillating but in different directions; by convention the "polarization" of light 

refers to the polarization of the electric field. Light which can be approximated 

as a plane wave in free space or in an isotropic medium propagates as a 

transverse wave—both the electric and magnetic fields are perpendicular to 

the wave's direction of travel. 

http://upload.wikimedia.org/wikipedia/commons/thumb/4/41/Rising_circular.gif/200px-Rising_circular.gif 


About Bolt 


Bolt Naming 


Bolt Naming 


About Photogrammetry 


Photogrammetry 








Borescope 

• Objective lens 

• Relay lens 

• Eye pieces 


Borescope 

• Objective lens 

• Relay lens 

• Eye pieces 


Borescope 


Diffraction 

refers to various phenomena which occur when a wave encounters an 

obstacle or a slit. In classical physics, the diffraction phenomenon is 

described as the interference of waves according the Huygens Fresnel 

principle. These characteristic behaviors are exhibited when a wave 

encounters an obstacle or a slit that is comparable in size to its wavelength. 

Similar effects occur when a light wave travels through a medium with a 

varying refractive index, or when a sound wave travels through a medium with 

varying acoustic impedance. Diffraction occurs with all waves, including 

sound waves, water waves, and electromagnetic waves such as visible light, 

X-rays and radio waves. 

Since physical objects have wave-like properties (at the atomic level), 

diffraction also occurs with matter and can be studied according to the 

principles of quantum mechanics. Italian scientist Francesco Maria Grimaldi 

coined the word "diffraction" and was the first to record accurate observations 

of the phenomenon in 1660. 


Diffraction 


Diffraction 

http://physicshelp.co.uk/images/waves/single-slit.gif 


Diffraction 


Diffraction 


Diffraction 


Diffraction 


Diffraction 


Diffraction 


Diffraction 


Optical filters 

are devices that selectively transmit light of different wavelengths, usually 

implemented as plane glass or plastic devices in the optical path which are 

either dyed in the bulk or have interference coatings. 

Filters mostly belong to one of two categories. The simplest, physically, is the 

absorptive filter; interference or dichroic filters can be quite complex. 

Optical filters selectively transmit light in a particular range of wavelengths, 

that is, colours, while blocking the remainder. They can usually pass long 

wavelengths only (longpass), short wavelengths only (shortpass), or a band 

of wavelengths, blocking both longer and shorter wavelengths (bandpass). 

The passband may be narrower or wider; the transition or cutoff between 

maximal and minimal transmission can be sharp or gradual. There are filters 

with more complex transmission characteristic, for example with two peaks 

rather than a single band;[1] these are more usually older designs traditionally 

used for photography; filters with more regular characteristics are used for 

scientific and technical work 


Optical Filters 


Borescope 


Item#: E-TS083250-OZ2X 

Type: Ocular zoom with 1 to 2x adjustable magnification 

Diameter: 8mm (.236") 

Working Length: 32cm (12.59") 

Field of View: 50° 

Direction of View: Variable Viewing from 45°-115° 

32 mm diameter standard eyepiece (1.259") 

Full metal handle 

Focus adjustment 

Multilayer coated optical components 

Removable connectors for compatibility with other brands of light cables 

Optical systems optimized for each instrument diameter 

FEATURES INCLUDE 

Focusing Ring 

Scanning Ring: 45° fore-oblique to 115° retrograde direction of view. 

Viewing arc: 20° fore-oblique to 140° retrograde. 

Viewing orientation touch indictator 

Viewing orientation index in the image 

PVC Case 


Level II- Notes 

My self Study Notes 


About Steel Ingots 


Steel Ingots 


Steel Ingots 


Steel Ingots 


Stainless Steel Ingots 


Level II Question on Ingot (my mistake) 

Q49: An inherent discontinuity associated with the original solidification of 

metal in the ingot is called: 

a) A seam 

b) Thermal fatigues 

c) Hot tear (wrong! ) 

d) Porosity 


Ingot Discontinuities 

http://products.asminternational.org/fach/data/fullDisplay.do?database=faco&record=2081&trim=false 


Ingot Discontinuities 

http://www.substech.com/dokuwiki/doku.php?id=structure_of_killed_steel_ingot&DokuWiki=00c51b3aea35614ea05a35fd92dee0c3 


Metal – More Reading 

http://www.substech.com/dokuwiki/doku.php?id=metals 


Metal – More Reading 

Nondestructive Examination (NDE) Technology and Codes 

Student Manual 

Chapter 3.0 

Classification and Interpretation of Indications 

http://pbadupws.nrc.gov/docs/ML1214/ML12146A174.pdf 

http://pbadupws.nrc.gov/docs/ 


Bimetallic Thermometer: 

A temperature-measuring instrument in which the differential thermal 

expansion of thin, dissimilar metals, bonded together into a narrow strip and 

coiled into the shape of a helix or spiral, is used to actuate a pointer. Also 

known as differential thermometer. 

Read more: http://www.answers.com/topic/bimetallic-thermometer#ixzz3BJ3QtCnY 


Bimetallic Thermometer: 


Hyperthermia is elevated body temperature due to failed thermoregulation 

that occurs when a body produces or absorbs more heat than it dissipates. 

Extreme temperature elevation then becomes a medical emergency requiring 

immediate treatment to prevent disability or death. 

The most common causes include heat stroke and adverse reactions to drugs. 

The former is an acute temperature elevation caused by exposure to 

excessive heat, or combination of heat and humidity, that overwhelms the 

heat-regulating mechanisms. The latter is a relatively rare side effect of many 

drugs, particularly those that affect the central nervous system. Malignant 

hyperthermia is a rare complication of some types of general anesthesia. 

Hyperthermia can also be deliberately induced using drugs or medical 

devices and may be used in the treatment of some kinds of cancer and other 

conditions, most commonly in conjunction with radiotherapy.[1] 

Hyperthermia differs from fever in that the body's temperature set point 

remains unchanged. The opposite is hypothermia, which occurs when the 

temperature drops below that required to maintain normal metabolism. 


Hyperthermia Treatment 

Sitting in a bathtub of tepid or cool water (immersion method) can remove a 

significant amount of heat in a relatively short period of time. A recent study 

using normal volunteers has shown that cooling rates were fastest when the 

coldest water was used". 

In exertional heat stroke, studies have shown that although there are practical 

limitations, cool water immersion is the most effective cooling technique and 

the biggest predictor of outcome is degree and duration of hyperthermia. No 

superior cooling method has been found for non-exertional heat stroke. When 

the body temperature reaches about 40 °C, or if the affected person is 

unconscious or showing signs of confusion, hyperthermia is considered a 

medical emergency that requires treatment in a proper medical facility. In a 

hospital, more aggressive cooling measures are available, including 

intravenous hydration, gastric lavage with iced saline, and even hemodialysis 

to cool the blood. 


Photometry & Radioscopy 


Photometry is the science of the measurement of light, in terms of its 

perceived brightness to the human eye.[1] It is distinct from radiometry, which 

is the science of measurement of radiant energy (including light) in terms of 

absolute power. In modern photometry, the radiant power at each wavelength 

is weighted by a luminosity function that models human brightness sensitivity. 

Typically, this weighting function is the photopic sensitivity function, although 

the scotopic function or other functions may also be applied in the same way. 


In optics, radiometry is a set of techniques for measuring electromagnetic 

radiation, including visible light. Radiometric techniques characterize the 

distribution of the radiation's power in space, as opposed to photometric 

techniques, which characterize the light's interaction with the human eye. 

Radiometry is distinct from quantum techniques such as photon counting. 

Radiometry is important in astronomy, especially radio astronomy, and plays 

a significant role in Earth remote sensing. The measurement techniques 

categorized as radiometry in optics are called photometry in some 

astronomical applications, contrary to the optics usage of the term. 

Spectroradiometry is the measurement of absolute radiometric quantities in 

narrow bands of wavelength 


Stroboscope 


A stroboscope, also known as a strobe, is an instrument used to make a 

cyclically moving object appear to be slow-moving, or stationary. The 

principle is used for the study of rotating, reciprocating, oscillating or vibrating 

objects. Machine parts and vibrating strings are common examples. 

Intense flashing/pulsing light of various frequencies can trigger epileptic 

seizures in people who suffer from photosensitive epilepsy. 


A stroboscope 


Photometry & Radiometry: Photometry is the science of the measurement 

of light, in terms of its perceived brightness to the human eye.It is distinct from 

radiometry, which is the science of measurement of radiant energy (including 

light) in terms of absolute power. 

In modern photometry, the radiant power at each wavelength is weighted by a 

luminosity function that models human brightness sensitivity. Typically, this 

weighting function is the photopic sensitivity function, although the scotopic 

function or other functions may also be applied in the same way. 


Photopic (daytime-adapted, black curve) and scotopic (darkness-adapted, 

green curve) luminosity functions. The photopic includes the CIE 1931 

standard (solid), the Judd-Vos 1978 modified data (dashed), and the 

Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is 

wavelength in nm. 

scotopic 

Photopic 


Color 

Color or colour (see spelling differences) is the visual perceptual property 

corresponding in humans to the categories called red, blue, yellow, green and 

others. Color derives from the spectrum of light (distribution of light power 

versus wavelength) interacting in the eye with the spectral sensitivities of the 

light receptors. Because perception of color stems from the varying spectral 

sensitivity of different types of cone cells in the retina to different parts of the 

spectrum, colors may be defined and quantified by the degree to which they 

stimulate these cells. These physical or physiological quantifications of color, 

however, do not fully explain the psychophysical perception of color 

appearance. 

The science of color is sometimes called chromatics, colorimetry, or simply 

color science. It includes the perception of color by the human eye and brain, 

the origin of color in materials, color theory in art, and the physics of 

electromagnetic radiation in the visible range (that is, what we commonly refer 

to simply as light). 


Color 

Additive color is light created by mixing together light of two or more different 

colors. Red, green, and blue are the additive primary colors normally used in 

additive color systems such as projectors and computer terminals. 


Subtractive coloring uses dyes, inks, and pigments to absorb some 

wavelengths of light and not others. The color that a surface displays comes 

from the parts of the visible spectrum that are not absorbed and therefore 

remain visible. Without pigments or dye, fabric fibers, paint base and paper 

are usually made of particles that scatter white light (all colors) well in all 

directions. When a pigment or ink is added, wavelengths are absorbed or 

"subtracted" from white light, so light of another color reaches the eye. 

If the light is not a pure white source (the 

case of nearly all forms of artificial lighting), 

the resulting spectrum will appear a slightly 

different color. Red paint, viewed under blue 

light, may appear black. Red paint is red 

because it scatters only the red components 

of the spectrum. If red paint is illuminated by 

blue light, it will be absorbed by the red paint, 

creating the appearance of a black object. 


Metallurgical Microscope 

An optical microscope that is commonly use on the studies and observation of 

metals, ceramic and polymeric materials, plastics, minerals, precious stones, 

alloys and many other substances are known as Metallurgical Microscope. It 

differs from other microscope because it provides you a closer view on flat 

and highly polished surfaces like metals. 

This type of microscope is often use on Metallography, Archaeometallurgy, 

Crystallography and Gemology. Metallography is an ocular observation using 

metallurgical microscope on metal surfaces that can discovers relevant 

information about crystals, chemicals, minerals and the mechanical 

composition of the matter. 




Ocular 


Level III- Notes 

My self Study Notes 


Single Len Magnification 

Magnifying glass: The maximum angular magnification (compared to the 

naked eye) of a magnifying glass depends on how the glass and the object 

are held, relative to the eye. If the lens is held at a distance from the object 

such that its front focal point is on the object being viewed, the relaxed eye 

(focused to infinity) can view the image with angular magnification: 

Magnification = 25cm / f = 10 inches/ f 

Here, is the focal length of the lens in centimeters. The constant 25 cm is an 

estimate of the "near point" distance of the eye—the closest distance at which 

the healthy naked eye can focus. In this case the angular magnification is 

independent from the distance kept between the eye and the magnifying 

glass. If instead the lens is held very close to the eye and the object is placed 

closer to the lens than its focal point so that the observer focuses on the near 

point, a larger angular magnification can be obtained, approaching: 

Magnification = (25cm / f) + 1 

http://en.wikipedia.org/wiki/Magnification 


Single Lens magnification 


Astigmatism is an optical defect in which vision is blurred due to the inability 

of the optics of the eye to focus a point object into a sharp focused image on 

the retina. This may be due to an irregular or toric curvature of the cornea or 

lens. The two types of astigmatism are regular and irregular. Irregular 

astigmatism is often caused by a corneal scar or scattering in the crystalline 

lens, and cannot be corrected by standard spectacle lenses, but can be 

corrected by contact lenses. The more common regular astigmatism arising 

from either the cornea or crystalline lens can be corrected by eyeglasses or 

toric lenses. A 'toric' surface resembles a section of the surface of a Rugby 

ball or a doughnut where there are two regular radii, one smaller than the 

other one. This optical shape gives rise to astigmatism in the eye 


Astigmatism- Hyperobia & Myopia 


http://optical-casper-wyoming.com/vision/wp-admin/install.php

Farsightedness/ Hyperobia 


http://www.lasik.md/learnaboutlasik/refractiveerrors.php#.U_piqZCS3IU

Short Sightedness/ Myopia 


About Sampling Terms & Definitions 


Acceptance Sampling 

Sampling inspection in which decisions are made to accept or reject product.; 

also the science that deals with procedures by which decisions, dicisions to 

accept or reject are based on the results of the inspection of samples. 

Comment: Typical uses of acceptance sampling in manufacturing include 

making acceptance decisions about incoming raw materials lots, in-process 

sub-lots, and finished product lots 

Terms & Definitions from: 

Glossary and Tables for Statistical Quality Control - ASQC 

More on Terms & Reference Curves 

http://www.samplingplans.com/glossary.htm 


Acceptance Sampling Plan 

A specific plan that states the sample size or sizes to be used and the 

associated acceptance and rejection criteria. 

Comment: Most acceptance sampling plans in use are either attributes plans 

and variables plans. 


AOQ curve 

Acronym for Average Outgoing Quality. Useful to evaluate sampling plan 

applications where rejected lots are rectified by replacing or reworking 

defective items. The AOQ curve is the average quality of outgoing product as 

a function of the incoming quality. 

Comment: AOQ is the quality of an average outgoing lot. Therefore, you 

should expect half of the lots to be worse than AOQ. The AOQ calculation 

does not consider that the incoming quality usually varies. 


AOQL 

Acronym for Average Outgoing Quality Limit. The maximum AOQ over all 

possible values of incoming product quality, for a given acceptance sampling 

plan. 

Comment: Maximum of the AOQ curve. See AOQ 


AQL 

Acronym for Acceptable Quality Level. As used in the development of twopoint 

acceptance sampling plans, the values of AQL and alpha jointly define 

the producers point of the operating characteristic curve. 

If the value of a quality characteristic of a particular lot is exactly equal to the 

AQL of it's acceptance sampling plan, the probability that the plan will accept 

the lot is (Pa=1-alpha). 

Example of specifying AQL 

For a discussion of common confusions about AQL, see AQL Primer. 

"Glossary and Tables for Statistical Quality Control" - ASQC. 


AQL ..continues 

Comment: This definition of AQL is statistically exact and appropriate for use 

with two-point sampling plans, as supported by the software of H & H 

Servicco Corp. 

On the other hand, a more vague definition of AQL is typically used by 

documents that support one-point sampling plans. The most common of such 

documents are: 

Mil-Std-105, 

Mil-Std-414, 

ANSI/ASQC Z1.4, 

ANSI/ASQC Z1.9 

These one-point sampling plans do not make use of the consumer's point - 

they do not address the issue of accepting low-quality lots. They are 

particularly vulnerable to this for small sample sizes. 


ARL curve 

Acronym for Average Run Length. ARL is the average number of accepted 

lots between rejections. The ARL curve is a plot of ARL as a function of lot 

quality level. 

Comment: Use the ARL curve to assess the impact of an acceptance 

sampling plan on smoothness operations. 


ASN curve 

Acronym for Average Sample Number. ASN is the average number of sample 

units inspected per lot in reaching decisions to accept or reject. The ASN 

curve is a plot of ASN versus lot quality. 

Comment: Use ASN curves to evaluate sequential sampling plans to 

anticipate the amount of inspection that each plan will require. 


Audit sampling 

Sampling in which the goal is to estimate the value a quality characteristic but 

not provide a firm decision rule. The sample size n is chosen to provide a 

desired margin of error of the estimate. 

Comment: Many audit sampling situations involve more than one category, 

each having a different sample size. The categories having the smaller 

sample sizes will have estimates with larger margins of error. Conversely, the 

categories having the larger sample sizes will have estimates with smaller 

margins of error. 


Margin of Error 

The sampling error of the estimated statistic. The margin of error is usually 

expressed as half the the width of a confidence interval. 


OC curve 

Acronym for Operating Characteristic Curve. A curve showing, for a given 

sampling plan, the probability of accepting a lot, as a function of the lot quality 

level. It is knowledge (by the person who designs or selects the plan) of the 

oc curve that makes an acceptance sampling plan statistically valid. 


RQL 

Acronym for Rejectable Quality Level. As used in the development of twopoint 

acceptance sampling plans, the values of RQL and beta jointly define 

the consumers point of the operating characteristic curve. 

If the value of a quality characteristic of a particular lot is exactly equal to the 

RQL of it's acceptance sampling plan, the probability that the plan will accept 

the lot is (Pa=beta). 


Sequential Analysis, Sequential Sampling 

The technique by which we build up our sample one item at a time, and after 

inspecting each item, ask ourselves: "Can we be sure enough to accept or 

reject this batch on the information so far collected?" 

Its value is in enabling reliable conclusions to be wrung from a minimum of 

data. This was deemed sufficient to require that it be classified "Restricted " 

within the meaning of the Espionage Act during the war of 1939-45. 


Statistically Valid 

An acceptance sampling plan is statistically valid when the person who 

designs or selects it knows the probabilities that the plan will accept lots that 

were manufactured to various quality levels. These probabilities are shown by 

the operating characteristic curve 

http://www.samplingplans.com/glossary.htm 


Birefringence is the optical property of a material having a refractive index 

that depends on the polarization and propagation direction of light.[1] These 

optically anisotropic materials are said to be birefringent (or birefractive). The 

birefringence is often quantified as the maximum difference between 

refractive indices exhibited by the material. Crystals with asymmetric crystal 

structures are often birefringent, as well as plastics under mechanical stress. 

Birefringence is responsible for the phenomenon of double refraction whereby 

a ray of light, when incident upon a birefringent material, is split by 

polarization into two rays taking slightly different paths. This effect was first 

described by the Danish scientist Rasmus Bartholin in 1669, who observed 

it[2] in calcite, a crystal having one of the strongest birefringences. However it 

was not until the 19th century that Augustin-Jean Fresnel described the 

phenomenon in terms of polarization, understanding light as a wave with field 

components in transverse polarizations (perpendicular to the direction of the 

wave vector). 


About Light & Vision 


Birefringence 


Blue-light hazard is defined as the potential for a photochemical induced 

retinal injury resulting from electromagnetic radiation exposure at 

wavelengths primarily between 400 ~ 500 nm. This has not been shown to 

occur in humans, only inconclusively in some rodent and primate studies. The 

mechanisms for photochemical induced retinal injury are caused by the 

absorption of light by photoreceptors in the eye. Under normal conditions 

when light hits a photoreceptor, the cell bleaches and becomes useless until 

it has recovered through a metabolic process called the visual cycle. 


Absorption of blue light, however, has been shown in rats and a susceptible 

strain of mice to cause a reversal of the process where cells become 

unbleached and responsive again to light before they are ready. At 

wavelengths of blue light below 430 nm this greatly increases the potential for 

oxidative damage. For blue-light circadian therapy, harm is minimized by 

employing blue light at the near-green end of the blue spectrum. 1 ~ 2 min of 

408 nm and 25 minutes of 430 nm are sufficient to cause irreversible death of 

photoreceptors and lesions of the retinal pigment epithelium. The action 

spectrum of light-sensitive retinal ganglion cells was found to peak at 470 ~ 

480 nm, a range with lower damage potential, yet not completely outside the 

damaging range 


Classical interference microscopy (also referred to as quantitative 

interference microscopy) uses two separate light beams with much greater 

lateral separation than that used in phase contrast microscopy or in 

differential interference microscopy (DIC). 

In variants of the interference microscope where object and reference beam 

pass through the same objective, two images are produced of every object 

(one being the "ghost image"). The two images are separated either laterally 

within the visual field or at different focal planes, as determined by the optical 

principles employed. These two images can be a nuisance when they overlap, 

since they can severely affect the accuracy of mass thickness measurements. 

Rotation of the preparation may thus be necessary, as in the case of DIC. 


About Fillet Weld 


Fillet Weld 


Convex Fillet Weld= Weld Size – Weld Length 


Concave Fillet Weld= Weld Size < Weld Length 

Fillet weld measurements: L: Leg length, S: 

Fillet weld Size, T: Theoretical throat, V: 

Convexity, C: Concavity, W: Effective weld 

length 


Fillet Weld Legs Determine Size and Throat of Fillet Welds 

In heavy machinery, ships, and buildings, extensive frameworks and intricate 

angles may be composed of many kilometers of welded joints. Among them, 

fillet welds are used to join corners, Ts. and lap joints because they are more 

economical than groove welds. That is, fillet welded joints are simple to 

prepare from the standpoint of edge preparation and fit-up. 

The strength of a fillet weld is based, in the design, on the product (effective 

area of the weld: T x W) of the theoretical throat (design throat thickness) and 

effective weld length as shown in Fig. 1. Fillet weld legs determine fillet weld 

sizes. Fillet weld sizes are measured by the length of the legs of the largest 

right triangle that may be inscribed within the fillet weld cross section. 

http://www.kobelco-welding.jp/education-center/abc/ABC_2000-01.html 


Fig. 1 - Fillet weld measurements: 

L: Leg length, S: Fillet weld Size, 

T: Theoretical throat, V: 

Convexity, C: Concavity, W: 

Effective weld length 


Fillet weld sizes determine theoretical throat. The product of the size and 

cos45° in case where an isosceles right triangle may inscribe within the fillet 

weld cross section: S x cos45° = 0.7S, as shown in Fig. 2. 

Fig. 2 — How to calculate theoretical throat 


Fillet weld sizes must be large enough to carry the applied load, but the 

specified fillet weld size should not be excessive to minimize welding 

distortion and costs. AWS D1.1 (Structural Welding Code- Steel) specifies the 

minimum fillet weld size for each base metal thickness: e.g. 6-mm size for 

thickness over 12.7 up to 19.0 mm. AWS D1.1 also specifies the maximum 

convexity, because excessive convexity may cause stress concentration at 

the toes of the fillet weld, which may result in premature failure of the joint. In 

quality control of fillet welds on actual work, leg or size, throat, convexity, and 

concavity are inspected by using several types of welding gages. Fig. 3 

shows a multipurpose gage measuring a fillet weld leg. 


Fig. 3 - Measuring a fillet weld leg by means of a multipurpose welding gage 



Fastener Failures 

http://ipgparts.com/blog/common-failures-for-fasteners-head-studs-main-studs-rod-bolts-etc/#channel=f25869488a334be&origin=http%3A%2F%2Fipgparts.com 

http://ipgparts.com/blog/common-failures-for-fasteners-head-studs-main-studs-rod-bolts-etc/ 


Photoemissive, Photovoltaic, Photoconductive 


Photoemissive 

Phototube or photoelectric cell is a type of gas-filled or vacuum tube that is 

sensitive to light. Such a tube is more correctly called a 'photoemissive cell' to 

distinguish it from photovoltaic or photoconductive cells. Phototubes were 

previously more widely used but are now replaced in many applications by 

solid state photodetectors. The photomultiplier tube is one of the most 

sensitive light detectors, and is still widely used in physics research. 

Operating principles 

Phototubes operate according to the photoelectric effect: Incoming photons 

strike a photocathode, knocking electrons out of its surface, which are 

attracted to an anode. Thus current flow is dependent on the frequency and 

intensity of incoming photons. Unlike photomultiplier tubes, no amplification 

takes place, so the current that flows through the device is typically of the 

order of a few microamperes. 


The light wavelength range over which the device is sensitive depends on the 

material used for the photoemissive cathode. A caesium-antimony cathode 

gives a device that is very sensitive in the violet to ultra-violet region with 

sensitivity falling off to blindness to red light. Caesium on oxidised silver gives 

a cathode that is most sensitive to infra-red to red light, falling off towards 

blue, where the sensitivity is low but not zero. 

Vacuum devices have a near constant anode current for a given level of 

illumination relative to anode voltage. Gas filled devices are more sensitive 

but the frequency response to modulated illumination falls off at lower 

frequencies compared to the vacuum devices. The frequency response of 

vacuum devices is generally limited by the transit time of the electrons from 

cathode to anode. 


Photovoltaic 

Photovoltaics (PV) is a method of generating electrical power by converting 

solar radiation into direct current electricity using semiconductors that exhibit 

the photovoltaic effect. Photovoltaic power generation employs solar panels 

composed of a number of solar cells containing a photovoltaic material. Solar 

photovoltaics power generation has long been seen as a clean sustainable 

energy technology which draws upon the planet’s most plentiful and widely 

distributed renewable energy source – the sun. The direct conversion of 

sunlight to electricity occurs without any moving parts or environmental 

emissions during operation. It is well proven, as photovoltaic systems have 

now been used for fifty years in specialized applications, and grid-connected 

systems have been in use for over twenty years 


Photoconductivity is an optical and electrical phenomenon in which a 

material becomes more electrically conductive due to the absorption of 

electromagnetic radiation such as visible light, ultraviolet light, infrared light, 

or gamma radiation. 

When light is absorbed by a material such as a semiconductor, the number of 

free electrons and electron holes increases and raises its electrical 

conductivity. To cause excitation, the light that strikes the semiconductor must 

have enough energy to raise electrons across the band gap, or to excite the 

impurities within the band gap. When a bias voltage and a load resistor are 

used in series with the semiconductor, a voltage drop across the load 

resistors can be measured when the change in electrical conductivity of the 

material varies the current flowing through the circuit. 

Classic examples of photoconductive materials include the conductive 

polymer polyvinylcarbazole, used extensively in photocopying (xerography); 

lead sulfide, used in infrared detection applications, such as the U.S. 

Sidewinder and Russian Atoll heat-seeking missiles; and selenium, employed 

in early television and xerography. 


The End 


ASNT Level III- Visual & Optical Testing 

My Pre-exam Preparatory 

Self Study Notes Reading 3 

2014-August 


Reading 3 

On Nondestructive Examination (NDE) Technology and Codes 

Student Manual Volume 1 

Chapter 4.0 

Introduction to Visual Examination ML121461A175 

U.S. Nuclear Regulatory Commission (NRC) Regulatory agency for nuclear 

power production and other civilian uses of nuclear materials 

For my coming ASNT Level III VT Examination 


At Works 


VT as an NDT Method: 

• VT as an NDE Method prevailed, and the recommendation for training and 

experience were added to the 1988 edition of SNT-TC-1A. 

• In the late 1970's, the American Welding Society developed its Certified 

Welding Inspector (CWI) program to meet this need. 


Certifications- ASNT 

*Time in Method **Total time required in NDT 

SNT-TC-1A 

CP-189 

NOTES: 

1. Experience is based on the 

actual hours worked in the specific 

method. 

2. A person may be qualified 

directly to NDT Level II with no time 

as certified Level, providing the 

required training and experience 

consists of the sum of the hours 

required for NDT Level I and NDT 

Level II. 

3. The required minimum 

experience must be documented by 

method and by hour with supervisor 

or NDT Level III approval. 

4. While fulfilling total NDT 

experience requirement, 

experience may be gained in more 

than one (1) method. Minimum 

experience hours must be met for 

each method. 


Certifications – ASME XI (Nuclear) 

Section XI requires that personnel performing NDE be qualified and certified 

using a written practice prepared in accordance with ANSI/ANST CP-189 as 

amended by Section XI. IWA 2314 states that the possession of an ASNT 

Level III Certificate, which is required by CP-189, is not required by Section XI. 

Section XI also states that certifications to SNT-TC-1A or earlier editions of 

CP-189 will remain valid until recertification at which time CP-189 (1995 

Edition) must be met. 


Vision Factors 

The focus of the lens system in the eye can be changed like that of a 

camera. A diaphragm, the sight hole or iris, regulates the quantity of light 

admitted through the pupil. The retina is a light-sensitive plane upon which 

the image is formed. Adjustments of the focus are made by changing the 

thickness and curvature (i.e., the focusing power, of the lens). Increasing the 

lens thickness is called accommodation. This is done by the action of tiny 

muscles attached to the lens. 


The Eyes 

Working in Tandem: Human eyes normally work in tandem. Shine a light 

into one eye and both pupils become smaller. Look to the right and both eyes 

will look in that direction. 

Fovea: In the center of the retina is a small area called the fovea which is 

packed with about six million “cone” cells. These cone cells are only about 1.5 

microns in diameter and each connects directly to a neuron providing 

resolution sharpness and color perception provided sufficient illumination 

exists. 

Retina: Unlike the cone cells, rod cells work in groups to feed impulses to a 

neuron. A larger the group of rod cells working together for more sensitivity 

when the light is low. These peripheral parts of the retina are nearly one 

million times more sensitive to light than the central fovea. 


The Eyes 


Eye Adaptations 

When stepping from the bright sunlight into a dark theater, nothing can be 

seen at first, as the dark adaption process begins. Initially, there is 

a rapid rise in sensitivity for about 30 seconds followed by a slower increase 

until, after 5 to 9 minutes, sensitivity increases over 100 times. For the next 

20 to 30 minutes, sensitivity continues to increase by a factor of 1,000 to as 

much as 10,000 as the pigments in the rod cells regenerate. 

In addition to the 10,000 increase in sensitivity by the retinal rod cells, other 

changes in the eye, including the dilation of the pupil to allow more light to 

enter the eye, add to the effect so that the final result is to make the increase 

in light sensitivity equal to 100,000 times. 

It is interesting that the adaptation required when coming from the dark into 

the light is accomplished within only a few minutes. 


Starry Night 

Light sensitivity: 

10 minutes increase 100x 

30 minutes increase 1000x ~ 10000x 

30 minutes With pupil dilation increase 100000x 


Drill Rig at Night 


Drill Rig at Bright Sunny Morning 


Good Morning Shanghai 


Eye Refractivity 

At Relax Accommodation: In the normal eye the length of the eyeball and 

the refractive power of the cornea and lens are such that images of objects at 

a distance of 20 feet or more are sharply focused on the retina when the 

muscles of accommodation are relaxed. 


Myopia & Hyperopia: 

In a farsighted individual for instance, the situation can be corrected by 

glasses made of convex lenses. These bring light from distant objects to a 

focus without contracting the accommodation muscles which make the lens 

more convex. In the nearsighted person, light rays from distant objects focus 

in front of the retina. This causes a blurring of the image of all objects located 

beyond a critical distance from the eye. By use of concave lenses, thicker at 

the edge than in the center, distant object can be seen clearly. 


Myopia- Near Sightedness 


Myopia- Near Sightedness 


Hyperopia - Farsightedness 






Stereoscopic vision 

Stereoscopic vision depends, at least in part, upon the fact that each 

eye gets a slightly different view of close objects. 


The two kinds of light receptors in the retina, the rods and the cones, differ in 

shape as well as in function. At the point where the optic nerve enters the 

retina, there are no rods and cones. This portion of the retina, called the blind 

spot, is insensitive to light. On the other hand, the maximum visual acuity at 

high brightness levels exists only for that small portion of the image formed 

upon the center of the retina. This is the fovea centralis, or “spot of clear 

vision.” Here the layer of blood vessels, nerve fibers, and cells above the rods 

and cones is far thinner than in peripheral regions of the retina. Daylight 

vision, which gives color and detail, is performed by the cones, mainly in the 

fovea centralis. These have special nerve paths. At least three different kinds 

of cones are present, each of which is in some way activated by one of 

the three fundamental colors. 

Keywords: 

■ blind spot - optic nerve enters the retina 

■ fovea centralis- Cones, three different kinds, daylight vision 

■ peripheral regions of the retina- Rods, night vision 


Color: Human vision constructs the average colour from the responses of 

three types of colour receptors that detect different ranges of colour 

(corresponding roughly to the primary colours of light - red, green and blue). 


http://www.jpse.co.uk/sensory/colourtheory.shtml

q 



Pupil, Iris, Sclera 


The Eye 


The Eye 


Color Characteristics 

Every color has three physical characteristics: 

• tone or hue, 

• saturation or purity, 

• and brightness or luminosity. 

Hue is that characteristic of color associated with the color name, such as 

green or blue. It may be described by the wavelength of a hue in the 

spectrum which visually matches the dominant hue. Purples do not exist in 

the spectrum, but the spectrum furnishes a hue complementary to that of any 

purple. This is true whether the hue is lavender, magenta, or any other 

variation of the family of purple. Although an estimated seven million or 

more distinguishable colors exist, only a few main colors are distinguished for 

practical reasons. Their wavelengths are as follows, in nanometers (nm): 

violet, 380 to 450; indigo, 425 to 455; blue, 450 to 480; green, 510 to 550; 

yellow 570 to 590; orange, 590 to 630; red, 630 to 730. Light from a limited 

portion of the spectrum is called monochromatic. 


Keywords: 

Hue / Tone- Color 


Keywords: 



Keywords: 



Keywords: 



Color Characteristics 

Saturation: Another color characteristic is saturation. For example, if one 

adds more and more pure white paint to a pure blue paint, the dominant hue 

may remain fairly constant while a series of tints is produced. Beginning with 

100 percent saturation, the blue becomes less and less saturated. 


Color Saturation- Hue Saturation 


Saturation 

Brightness (value) 

Saturation 

(Chroma) 


Hue, Saturation & Brightness 


The Colors: Hue/ Saturation & Brightness 


Glare: 

Excessive brightness (or brightness within the field of view varying by more 

than 10 to 1) causes an unpleasant sensation called glare. Glare interferes 

with the ability of clear vision and critical observation and judgment. Glare 

can be avoided by using polarized light or other polarizing devices. 


Glare 


Glare can be avoided by using polarized light or other polarizing 

devices. 


Spectrum Limits of Visibility 

The eye perceives all the colors in the solar spectrum between violet (380 nm) 

and red (770 nm). Compared with the entire electromagnetic spectrum, only a 

rather minute portion is visible 



The Universe – The hidden colors 


18 percent neutral gray card: The examiner should have adequate 

illumination, either natural or artificial, while performing VT. This may be 

determined using a fine line, approximately 1/32 inch (0.8 mm) in width, 

drawn on a 18 percent neutral gray card. The card should be placed near the 

area under examination; if this fine line is distinctly visible, the illumination is 

adequate. 


18 percent neutral gray card 


Gray Scale Card 


Lighting: (applicable for NRC only) 

15 foot candles (fc) for general examination, and 

a minimum of 50 foot candles for the detection of small discontinuities. 


Magnifying Power = 10 / Focal Length 

http://www.mystd.de/album/calculator/magnification.html 


Magnification 


Magnification 


Visual Examination Code Requirements 


ASME-Section V 

A brief summary of the requirements for VT examination as contained in 

Article 9 follows: 

• A report of the demonstration that the procedure was adequate is required. 

• An annual vision test is required (J-1 letters). 

• Direct Visual Examination is defined as a VT where the eye can be placed 

within the 24" of the surface to be examined and at an angle not less than 

30° to the surface. 

• Minimum light intensity of 100 foot candles at the examination surface. 

• Remote Visual Examination is an acceptable substitute for Direct Visual 

Examination where accessibility is a problem. 

• Translucent Visual Examination is a supplement of Direct Visual 

• Examination and uses artificial lighting as an illumination to view a 

translucent object or material. 


The Jaeger eye chart is used for 

reading up close and for determining 

your near vision. When reviewing the 

chart, you will see the notation (1) next 

to the paragraph with the smallest text. 

Each progressive paragraph of larger 

text is noted with an increase in the 

number. As you progress to larger 

lettered paragraphs, the lettering size 

increases for lesser visual acuity. 


Jaeger J1 


ASME BPVC-XI-2010 

2010 ASME Boiler and Pressure Vessel Code (BPVC), Section XI: Rules for 

Inservice Inspection of Nuclear Power Plant Components 


ASME Section XI 

A summary of ASME IWA-2211 VT-1 requirements follows: 

• The VT-1 visual examination is conducted to detect discontinuities and 

imperfections on the surfaces of components, including such conditions as 

cracks, wear, corrosion, or erosion. 

• Direct VT-1 visual examination may be conducted when access is 

sufficient to place the eye within 24 inches of the surface to be examined 

and at an angle not less than 30° to the surface. Mirrors may be used to 

improve the angle of vision. Lighting, natural or artificial, shall be a 

minimum of 50 foot-candles (fc) and / or the ability to resolve a character 

of 0.044”. 

• Remote VT-1 visual examination may be substituted for direct examination. 

Remote examination may use aids, such as telescopes, borescopes, fiber 

optics, cameras, or other suitable instruments, provided such systems 

have a resolution capability at least equivalent to that attainable by direct 

visual examination. 



• The VT-2 visual examination is conducted to detect evidence of leakage 

from pressure retaining components, with or without leakage collection 

systems, as required during the conduct of system pressure test. 

• VT-2 visual examinations are conducted in accordance with IWA-5000, 

“System Pressure Tests”. 



• The VT-3 visual examination shall be conducted to determine the general 

mechanical and structural condition of components and their supports, by 

verifying parameters of clearances, settings, physical displacements, and 

to detect discontinuities and imperfections such as loss of integrity at 

bolted or welded connections, loose or missing parts, debris, corrosion, 

wear, or erosion. 

• VT-3 examinations also include examinations for conditions that could 

affect operability or functional adequacy of snubbers, and constant load 

and spring type supports. Lighting shall be a minimum of 50 fc and / or the 

ability to resolve a character of 0.105” 


AWS Certified Welding Inspector 

American Welding Society (AWS) created its Certified Welding Inspector 

(CWI) program in 1976. The program consists of: 

• Basic Body of Knowledge: 

- The Welding Inspector, 

- Documents Governing Welding Inspection and Control of Materials, 

- Weld Joint Geometry and Welding Terminology, 

- Welding and Nondestructive Testing Symbols, 

- Mechanical and Chemical Properties of Metals, 

- Destructive Testing, 

- Welding Metallurgy for the Welding Inspector, 

- Welding Procedure and Welder Qualification, 

- Welding, Brazing and Cutting Processes, 

- Weld and Base Metal Discontinuities, 

- Nondestructive Testing, and 

- Visual Inspection as an Effective Quality Control Tool. 


• Minimum of five (5) years relevant work experience, 

• Three part test covering fundamentals, practical on-the-job situations and 

a specific code (selected by the examinee). The test is administered at test 

sites around the country, and 

• Vision Test. 

Once the potential candidate meets the above requirements, he will be issued 

a certification from AWS. The certification is valid for three years. To renew 

the certification, the CWI must submit documentation showing continued work 

in the welding discipline or be re-examined. 


Direction of View (DOV) 


Halitation 





Self Study Notes Reading 4 Section 1 

2014-August 


Reading 4 

ASNT Nondestructive Handbook Volume 8 

Visual & Optical testing- Section 1 


2014-August 



Fion Zhang 

2014/August/15

SECTION 1 

FUNDAMENTALS OF VISUAL AND OPTICAL 

TESTING 


SECTION 1: FUNDAMENTALS OF VISUAL AND OPTICAL TESTING 

PART 1: Description of visual and optical tests 

1.1 Luminous Energy Tests 

1.2 Geometrical Optics 

PART 2: History of the borescope 

2.1 Development of the Borescope 

2.2 Certification of Visual Inspectors 

PART 3: Vision and light 

3.1 The Physiology of Sight 

3.2 Weber's Law 

3.3 Vision Acuity 

3.4 Vision Acuity Examinations 

3.5 Visual Angle 

3.6 Color Vision 

3.7 Fluorescent Materials 


PART 4: Safety for visual and optical tests 

4.1 Need for Safety 

4.2 Laser Hazards 

4.3 Infrared Hazards 

4.4 Ultraviolet Hazards 

4.5 Photosensitizers 

4.6 Damage to the Retina 

4.7 Thermal Factor 

4.8 Blue Hazard 

4.9 Visual Safety Recommendations 

4.10 Eye Protection Filters 


Part 1: DESCRIPTION OF VISUAL AND OPTICAL TESTS 

1.1.0 General: 

Nondestructive tests typically are done by applying a probing medium (such 

as acoustic or electromagnetic energy) to a material. After contact with the 

test material, certain properties of the probing medium are changed and can 

be used to determine changes in the characteristics of the test material. 

Density differences in a radiograph or location and peak of an oscilloscope 

trace are examples of means used to indicate probing media changes. In a 

practical sense, most nondestructive tests ultimately involve visual tests- a 

properly exposed radiograph is useful only when the radiographic interpreter 

has the vision acuity required to interpret the image. 

Likewise, the accumulation of magnetic particles over a crack indicates to the 

inspector an otherwise invisible discontinuity. The interface of visual testing 

with other nondestructive testing methods is discussed in more detail in a 

later section of this volume. 


For the purposes of this book, visual and optical tests are those that use 

probing energy from the visible portion of the electromagnetic spectrum. 

Changes in the light's properties after contact with the test object may be 

detected by human or machine vision. Detection may be enhanced or made 

possible by mirrors, magnifiers, borescopes or other vision enhancing 

accessories. 

Keywords; 

■ 

■ 

■ 

Visible Spectrum (380nm ~ 770nm), 

Human or machine vision, 

Vision enhancing tools- Borescope, mirror and other enhancing 

accessories. 


1.2.0 Luminous Energy Tests 

Visual testing was probably the first method of nondestructive testing. It has 

developed from its ancient origins into many complex and elaborate optical 

investigation techniques. Some visual tests are based on the simple laws of 

geometrical optics. Others depend on properties of light, such as its wave 

nature. A unique advantage of many visual tests is that they can yield 

quantitative data more readily than other nondestructive tests. 

Luminous energy tests are used primarily for two purposes: 

1. testing of exposed or accessible surfaces of opaque test objects (including 

a majority of partially assembled or finished products) and 

2. testing of the interior of transparent test objects (such as glass, quartz, 

some plastics, liquids and gases). For many types of objects, visual testing 

can be used to determine quantity, size, shape, surface finish, reflectivity, 

color characteristics, fit, functional characteristics and the presence of 

surface discontinuities. 


Keywords: 

Objects: 

• Testing of opaque objects 

• Testing of transparent objects 

VT is used to determined: 

• quantity, 

• size, 

• shape, 

• surface finish, 

• reflectivity, 

• color characteristics, 

• fit, 

• functional characteristics and the 

• presence of surface discontinuities. 

Question: 

Does VT covers Translucent object? 


1.3.0 Geometrical Optics 

1.3.1 Image Formation 

Most optical instruments are designed primarily to form images. In many 

cases, the manner of image formation and the proportion of the image can be 

determined by geometry and trigonometry without detailed consideration of 

the physics of light rays. 

This practical technique is called geometrical optics and it includes the 

formation of images by lenses and mirrors. The operation of microscopes, 

telescopes and borescopes also can be partially explained with geometrical 

optics. In addition, the most common limitations of optical instruments can 

be similarly evaluated with this technique. 

Keyword: 

Geometrical optics 


1.3.2 Light Sources 

The light source for visual tests typically emits radiation of a continuous or 

noncontinuous (line) spectrum. Monochromatic light is produced by use of a 

device known as a monochromator, which separates or disperses the 

wavelengths of the spectrum by means of prisms or gratings. 

Less costly and almost equally effective for routine tests are light sources 

emitting distinct spectral lines, These include mercury, sodium and other 

vapor discharge lamps. Such light sources may he used in combination with 

glass, liquid or gaseous filters or with highly efficient interference filters, for 

transmitting only radiation of a specific wavelength. 

Keywords: 

Continuous spectrum 

Non-continuous spectrum-Monochromatic light 

Monochromator used prisms or grating 


Keywords: 

Monochromatic light produces by vapor discharged lamp (Mercury/sodium 

etc.) with glass/ liquid & gaseous filter to produces only radition with specific 

wavelength 


Gas-discharge lamps 

are a family of artificial light sources that generate light by sending an 

electrical discharge through an ionized gas, a plasma. The character of the 

gas discharge depends on the pressure of the gas as well as the frequency of 

the current. Typically, such lamps use a noble gas (argon, neon, krypton and 

xenon) or a mixture of these gases. Most lamps are filled with additional 

materials, like mercury, sodium, and metal halides. In operation the gas is 

ionized, and free electrons, accelerated by the electrical field in the tube, 

collide with gas and metal atoms. Some electrons in the atomic orbitals of 

these atoms are excited by these collisions to a higher energy state. When 

the excited atom falls back to a lower energy state, it emits a photon of a 

characteristic energy, resulting in infrared, visible light, or ultraviolet radiation. 

Some lamps convert the ultraviolet radiation to visible light with a fluorescent 

coating on the inside of the lamp's glass surface. The fluorescent lamp is 

perhaps the best known gas-discharge lamp. 

http://en.wikipedia.org/wiki/Gas-discharge_lamp 


Compared to incandescent lamps, gas-discharge lamps offer higher 

efficiency, but are more complicated to manufacture, and require auxiliary 

electronic equipment such as ballasts to control current flow through the gas. 

Some gas-discharge lamps also have a perceivable start-up time to achieve 

their full light output. Still, due to their greater efficiency, gas-discharge lamps 

are replacing incandescent lights in many lighting applications. 


Vapor Discharged Lamp 






A monochromator is an optical device 

that transmits a mechanically selectable 

narrow band of wavelengths of light or 

other radiation chosen from a wider 

range of wavelengths available at the 

input. The name is from the Greek roots 

mono-, single, and chroma, colour, and 

the Latin suffix -ator, denoting an agent. 

http://en.wikipedia.org/wiki/Monochromator 








Monochromator 

used prisms or 

grating 








1.3.3 Stroboscopic Sources 

The stroboscope is a device that uses synchronized pulses of high intensity 

light to permit viewing of objects moving with a rapid, periodic motion. A 

stroboscope can be used for direct viewing of the apparently stilled test object 

or for exposure of photographs. The timing of the stroboscope also can be 

adjusted so that the moving test object is seen to move but at a much slower 

apparent motion. The stroboscopic effect requires an accurately controlled, 

intermittent source of light or may be achieved with periodically interrupted 

vision. 


1Stroboscopic Movement 


Stroboscopic Movement 


Stroboscopic Movement 


Stroboscopic Sources 


Stroboscopic Sources 


Stroboscopic Glasses 


千手观音 


千手观音 


千手观音 


1.3.4 Light Detection and Recording 

Once light has interacted with a test object (been absorbed, reflected or 

refracted), the resulting light waves are considered test signals that may be 

recorded visually or photoelectrically. Such signals may be detected by 

means of photoelectric cells, bolometers or thermopiles, photomultipliers 

or closed circuit television systems. Electronic image conversion devices 

often are used for the invisible ranges of the electromagnetic spectrum 

(infrared, ultraviolet or X-rays) but they also may he used to transmit 

visual data from hazardous locations or around obstructions. Occasionally, 

intermediary photographic recordings are made. 

The processed photographic plate can subsequently be evaluated either 

visually or photoelectrically. Some applications take advantage of the ability 

of photographic film to integrate low energy signals over long periods of time. 

Photographic film emulsions can be selected to meet specific test conditions, 

sensitivities and speeds. 


Keywords: 

Photoelectricity detection 

• photoelectric cells, 

• bolometers or 

• thermopiles, 

• photomultipliers 

• or closed circuit television systems. 


Bolometer consists of an absorptive element, such as a thin layer of metal, 

connected to a thermal reservoir (a body of constant temperature) through a 

thermal link. The result is that any radiation impinging on the absorptive 

element raises its temperature above that of the reservoir — the greater the 

absorbed power, the higher the temperature. 

The intrinsic thermal time constant, which sets the speed of the detector, is 

equal to the ratio of the heat capacity of the absorptive element to the thermal 

conductance between the absorptive element and the reservoir. The 

temperature change can be measured directly with an attached resistive 

thermometer, or the resistance of the absorptive element itself can be used 

as a thermometer. Metal bolometers usually work without cooling. They are 

produced from thin foils or metal films. Today, most bolometers use 

semiconductor or superconductor absorptive elements rather than metals. 

These devices can be operated at cryogenic temperatures, enabling 

significantly greater sensitivity. 


Bolometer 


Bolometer 


Bolometer 


Thermopiles 


Thermopiles 


Thermopiles 


Multi-Junction Thermopiles 

The thermopile is a heat sensitive device that measures radiated heat. The 

sensor is usually sealed in a vacuum to prevent heat transfer except by 

radiation. A thermopile consists of a number of thermocouple junctions in 

series which convert energy into a voltage using the Peltier effect. 

Thermopiles are convenient sensor for measuring the infrared, because they 

offer adequate sensitivity and a flat spectral response in a small package. 

More sophisticated bolometers and pyroelectric detectors need to be chopped 

and are generally used only in calibration labs. 


Photo Detector Comparisons 

http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/RYER/ch10.html 


Photomultiplier 

Photomultiplier tubes (photomultipliers or PMTs for short), members of the 

class of vacuum tubes, and more specifically vacuum phototubes, are 

extremely sensitive detectors of light in the ultraviolet, visible, and nearinfrared 

ranges of the electromagnetic spectrum. These detectors multiply the 

current produced by incident light by as much as 100 million times (i.e., 160 

dB), in multiple dynode stages, enabling (for example) individual photons to 

be detected when the incident flux of light is very low. Unlike most vacuum 

tubes, they are not obsolete. 

The combination of high gain, low noise, high frequency response or, 

equivalently, ultra-fast response, and large area of collection has earned 

photomultipliers an essential place in nuclear and particle physics, astronomy, 

medical diagnostics including blood tests, medical imaging, motion picture 

film scanning (telecine), radar jamming, and high-end image scanners known 

as drum scanners. Elements of photomultiplier technology, when integrated 

differently, are the basis of night vision devices. 


Semiconductor devices, particularly avalanche photodiodes, are alternatives 

to photomultipliers; however, photomultipliers are uniquely well-suited for 

applications requiring low-noise, high-sensitivity detection of light that is 

imperfectly collimated. 

http://en.wikipedia.org/wiki/Photomultiplier 



Secondary emission 

Photoelectric effect 

Photon 

Electrons multiplying 






Photoelectric Cell 

• Photovoltaic Cell 

• Photo emissivity 

Photovoltaic's (PV) is a method of generating electrical power by converting 

solar radiation into direct current electricity using semiconductors that exhibit 

the photovoltaic effect. Photovoltaic power generation employs solar panels 

composed of a number of solar cells containing a photovoltaic material. Solar 

photovoltaics power generation has long been seen as a clean sustainable[1] 

energy technology which draws upon the planet’s most plentiful and widely 

distributed renewable energy source – the sun. The direct conversion of 

sunlight to electricity occurs without any moving parts or environmental 

emissions during operation. It is well proven, as photovoltaic systems have 

now been used for fifty years in specialized applications, and grid-connected 

systems have been in use for over twenty years 



Photovoltaic Cell 


Photovoltaic Cell 


Photoemissive Cell- Photoemissive cell (electronics) 

A device which detects or measures radiant energy by measurement of the 

resulting emission of electrons from the surface of a photocathode. 


Photoemissive Cell 


Photoemissive Cell: Analysis of sodium levels in junk food by flame 

photometer 

http://www.pharmatutor.org/articles/analysis-sodium-levels-junk-food-flame-photometer?page=0,2 


1.3.5 Fluorescence Detection 

A material is said to fluoresce when exposure to radiation causes the material 

to produce a secondary emission of longer wavelength than the primary, 

exciting light. Visual tests based on fluorescence play a part in qualitative and 

quantitative inorganic and organic chemistry, as a means of quality control of 

chemical compounds, for identifying counterfeit currency, tracing hidden 

water flow and for detecting discontinuities in metals and pavement. 


Fluorescence Detection 


More Reading on Light 

Light Measurement Handbook 

http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/RYER/index.html 

http://homepages.inf.ed.ac.uk/rbf/CVonline/ 


Part 2: History of Borescope 

2.1.0 Introduction 

Development of the Borescope The development of self illuminated 

telescopic devices can be traced back to early interest in exploring the interior 

human anatomy without operative procedures. Devices for viewing the 

interior of objects are called endoscopes, from the Creek words for "inside 

view." Today the term endoscope in the United States is applied primarily to 

medical instruments. Nearly all of the medical endoscopes have an integral 

light source; some incorporate surgical tweezers or other devices. Industrial 

endoscopes are called horescopes because they were 'originally used in 

machined apertures and holes such as gun bores. There are both flexible 

and rigid, fiber optic and geometric light borescopes. 

Keywords: 

■ 

■ 

Endoscopes 

Horescopes 


2.1.1 Cystoscopes and Borescopes 

In 1806 Philipp Bozzini of Frankfurt announced the invention of his Lichtleiter 

(German for "light guide"). Having served as a surgeon in the Napoleonic 

wars, Bozzini envisioned using his device for medical research. It is 

considered the first endoscope. In 1876, Dr. Max Nitze, a urologist, developed 

the first practical cystoscope to view the human bladder.' A platinum loop in 

its tip furnished a bright light when heated with galvanic current. Two years 

later, Thomas Edison introduced an incandescent light in the United States. 

Within a short time, scientists in Austria made and used a minute electric bulb 

in Nitze's cystoscope, even before the electric light was in use in America. 

The early cystoscopes contained simple lenses but these were soon replaced 

by achromatic combinations. In 1900, Reinhold Wappler revolutionized the 

optical system of the cystoscope and produced the first American models. 

The forward oblique viewing system was later introduced and has proved 

very useful in both medical and industrial applications. Direct vision and 

retrospective systems were also first developed for cystoscopic use. 


Borescopes and related instruments for nondestructive testing have followed 

the same basic design used in cystoscopic devices. The range of borescope 

sizes has increased, sectionalized instruments have been introduced and 

other special devices have been developed for industrial applications. 


2.1.2 Gastroscopes and Flexible Borescopes 

A flexible gastroscope, originally intended for observing the interior of the 

stomach wall, was first developed by Rudolph Schindler' and produced by 

Georg Wolf in 1932. The instrument consisted of a rigid section and a flexible 

section. Many lenses of small focal distance were used to allow bending of 

the instrument to an angle of 34 degrees in several planes. The tip of the 

device contained the objective and the prism causing the necessary axial 

deviation of the bundle of rays coming from the illuminated gastric wall. The 

size of the image depended on the distance of the objective from the 

observed surface. It could be magnified, reduced or normal size but the 

image was sharp and erect with correct sides. Flexible gastroscopes are now 

available, with rubber tubes over the flexible portion, in diameters of 

approximately 14 mm (0.55 in.) and 8 mm (0.31 in.). 


Flexible borescopes for industrial use are more ruggedly constructed than 

gastroscopes, having flexible steel tubes instead of rubber for the outer tube 

of the flexible portion. A typical flexible borescope is 13 mm (0,5 in.) in 

diameter and has a 1 m (3 ft) working length, with flexibility in about 500 mm 

(20 in.) of the length. Extension sections are available in 1, 2 or 3 m (3, 6 or 9 

ft) lengths, permitting assembly of borescopes up to 10 m (30 ft) in length. In 

such flexible instruments the image remains round and sharp when the tube 

is bent to an angle of about 34 degrees. Beyond that limit, the image 

becomes elliptical but remains clear until obliterated at about 45 degrees of 

total bending. 


Keywords: Conventional Borescope Bend angles & Images 

• 34 Degree- Round and Clear 

• 34 ~ 45 Degree- Elliptical but Clear 

• > 45 Degree- Obliterated 


Digitized Borescope 


2.1.3 American Development of Borescopes 

After the early medical developments, certain segments of American industry 

needed visual testing equipment for special inspection applications. One of 

the first individuals to help fill this need was George Sumner Crampton. 

George Crampton (Fig. 1) was born in Rock Island, Illinois in 1874. He was 

said to have set up a small machine shop by the age of 10 and his first 

ambition was to become an electrical engineer. He chose instead to study 

medicine and received his M.D. from the University of Pennsylvania in 

1898. While he was interning at Pennsylvania Hospital, Crampton's 

mechanical and engineering abilities were recognized and he was advised to 

become an oculist.' He returned to the university, took a degree in 

ophthalmology and later practiced in Philadelphia, Pennsylvania and 

Princeton, New Jersey‘ In 1921, the Westinghouse Company asked 

Crampton to make a device that could be used to check for discontinuities 

inside the rotor of a steam turbine (Fig. 2). Crampton developed the 

instrument in his Philadelphia shop and delivered the prototype within a 

week- it was the first borescope produced by his company. 


Crampton continued to supply custom borescopes for testing inaccessible 

and often dark areas on power turbines, oil refinery piping, gas mains, soft 

drink tanks and other components (Fig. 3). Crampton soon was recognized 

for his ability to design and manufacture borescopes, periscopes and other 

optical equipment for specific testing applications. After retiring as emeritus 

professor of ophthalmology at the university Crampton continued private 

practice in downtown Philadelphia. At the same time, he worked on 

borescopes and other instruments in a small shop he had established in a 

remodeled nineteenth century coach house (Fig. 4). 


FIGURE 1. George Crampton, developer of the borescope 


FIGURE 2. Tests of forgings for a steam turbine generator shaft manufactured in the 1920s 

FIGURE 3. Inspectors use early borescopes to visually inspect piping at an Ohio oil refinery 


FIGURE 4. Periscope built in the 1940s is checked before shipment to a Texas chemical plant 


2.1.4 Wartime Borescope Developments 

After World War II began, Crampton devoted much of his energy to the war 

effort, filling defense orders for borescopes (Fig. 5). Crampton practiced 

medicine until noon, then went to the nearby workshop where he visually 

tested the bores of 37 mm antiaircraft guns and other weapons. During the 

war, borescopes were widely used for testing warship steam turbines 

(particularly their rotating shafts). The United States Army also used 

borescopes for inspecting the barrels of tank and antiaircraft weapons 

produced in Philadelphia. An even more challenging assignment lay 

ahead. 

The scientists working to develop a successful nuclear chain reaction in the 

top secret Manhattan Project asked Crampton to provide a borescope for 

inspecting tubes near the radioactive pile at its guarded location beneath the 

stadium seats at the University of Chicago's Stagg Field. Crampton devised 

an aluminum borescope tube 35 mm (1.4 in.) in diameter and 10 m (33 ft) 

long. The device consisted of 2 m (6 ft) sections of dual tubing joined by 

bronze couplings which also carried an 8 V lighting circuit. 


FIGURE 5. Using a borescope, an inspector at an automobile plant during World War H checks 

the interiors of gun tubes for 90 mm antiaircraft guns 


The inspector standing directly in front of the bore was subject to radioactive 

emissions from the pile, so Crampton mounted the borescope outside of a 

heavy concrete barrier. The operator stood at a right angle to the borescope, 

looking through an eyepiece and revolving the instrument manually. The 

borescope contained a prism viewing head and had to be rotated constantly. 

It was supported in a steel V trough resting on supports whose height could 

be varied. Crampton also mounted a special photographic camera on the 

eyepiece. 

The original Manhattan Project borescope was later improved with 

nondarkening optics and a swivel-joint eyepiece that permitted the operator to 

work from any angle (this newer instrument did not require the V trough). It 

also was capable of considerable bending to snake through the tubes in the 

reactor. A total of three borescopes were supplied fbr this epochal project and 

they are believed to be the first optical instruments to use glass resistant to 

radioactivity. 


Manhattan Project 






2.1.5 Borescopes and Aircraft Tests 

Aircraft inspection soon became one of the most important uses of borescope 

technology. In 1946, an ultraviolet light borescope was developed for 

fluorescent testing of the interior of hollow steel propeller blades. The 100 W 

viewing instniment revealed interior surface discontinuities as glowing green 

lines. Later, in 1958, the entire United States' B-47 bomber fleet was 

grounded because of metal fatigue cracks resulting from low level simulated 

bombing missions. Visual testing with borescopes proved to be the first step 

toward resolving the problem. The program became known as Project 

Milkbottle, a reference to the bottle shaped pin that was a primary connection 

between the fuselage and wing (Fig. 6). 

In the late 1950s, a system was developed for automatic testing of helicopter 

blades. The borescope, supported by a long bench, could test the blades 

while the operator viewed results on a television screen (Fig. 7). The system 

was used extensively during the Vietnam conflict and helicopter 

manufacturers continue to use borescopes for such critical tests. 


FIGURE 6. Inspector using a borescope to check for metal fatigue cracks in a B-47 bomber 

during grounding of the bomber fleet in 1958 

FIGURE 7. Visual testing of the frame of a 10 m (32 ft) long helicopter blade using a 10 m (32 ftj 

borescope; the inspector could view magnified results on the television screen at bottom left 


After a half century of pioneering work, George Crampton sold his borescope 

business to John Lang of Cheltenham, Pennsylvania, in 1962.6•7 Lang had 

developed the radiation resistant optics used in the Manhattan Project 

borescope, as well as a system for keeping it functional in high temperature 

environments. He also helped pioneer the use of closed circuit television with 

borescopes for testing the inner surfaces of jet engines and wings, hollow 

helicopter blades and nuclear reactors. In 1965, the company received a 

patent on a borescope whose mirror could he very precisely controlled. 

This borescope could zoom to high magnification and could intensely 

illuminate the walls of a chamber by means of a quartz incandescent lamp 

containing iodine vapor. The basic design of the borescope has been in use 

for many decades and it continues to develop, accommodating advances in 

video, illumination, robotic and computer technologies. 


2.2.0 Certification of Visual Inspectors 

2.2.1 Introduction 

The recognition of the visual testing technique and the development of formal 

procedures for educating and qualifying visual inspectors were important 

milestones in the history of visual inspection. Because visual testing can be 

performed without any intervening apparatus, it was certainly one of the first 

forms of nondestructive testing. In its early industrial applications, visual tests 

were used simply to verify compliance to a drawing or specification. This was 

basically a dimensional check. The soundness of the object was determined 

by liquid penetrant, magnetic particle, radiography or ultrasonic testing. 

Following World War II, few inspection standards included visual testing. By 

the early 1960s, visual tests were an accepted addition to the American 

Welding Society's code hooks. In NAV SHIPS 250-1500-1, the US Navy 

included visual tests with its specifications for other nondestructive testing 

techniques for welds. 


By 1965, there were standards for testing, and criteria for certifying the 

inspector had been established in five test methods: liquid penetrant, 

magnetic particle, eddy current, radiographic and ultrasonic testing. These 

five were cited in ASNT Recommended Practice No. SNT-TC-1A, introduced 

in the late 1960s. The broad use of visual testing hindered its addition to this 

group as a specific method- there were too many different applications on too 

many test objects to permit the use of specific acceptance criteria. It also was 

reasoned that visual testing would occur as a natural result of applying any 

other nondestructive test method. 


2.2.2 Expanded Need for Visual Certification 

In the early 1970s, the need for certified visual inspectors began to increase. 

Nuclear power construction was at a peak, visual certification was becoming 

mandatory and nondestructive testing was being required. In 1976, the 

American Society for Nondestructive Testing began considering the need for 

certified visual inspectors. ASNT had become a leading force in 

nondestructive testing and American industry had accepted its ASNT 

Recommended Practice No. SNT-TC-IA as a guide for certifying other NDT 

inspectors. In the spring of 1976, ASNT began surveying industry about their 

inspection needs and their position on visual testing. Because of the many 

and varied responses to the survey, a society task force was established to 

analyze the survey data. In 1977, the task force recommended that visual 

inspectors be certified and that visual testing be made a supplement to ASNT 

Recommended Practice No. SNT-TC-IA (1975). At this time, the American 

Welding Society implemented a program that, following the US Navy, was the 

first to certify inspectors whose sole function was visual weld testing. 


During 1978, ASNT subcommittees were formed for the eastern and western 

halves of the United States. These groups verified the need for both visual 

standards and trained, qualified and certified inspectors. In 1980, a Visual 

Methods Committee was formed in ASNT's Technical Council and the early 

meetings defined the scope and purpose of visual testing (dimensional testing 

was excluded). In 1984, the Visual Personnel Qualification Committee was 

formed in ASNT's Education and Qualification Council. In 1986, a training 

outline and a recommended reference list was finalized and the Board of 

Directors approved incorporation of visual testing into ASNT Recommended 

Practice No. SN T-TC -1 A. 


Part 3: VISION AND LIGHT 

3.1 The Physiology of Sight 

3.1.1 Visual Data Collection 

Human visual processing occurs in two steps. First the entire field of vision is 

processed. This is typically an automatic function of the brain, sometimes 

called pre-attentive processing. Secondly, focus is localized to a specific 

object in the processed field. Studies at the University of Pennsylvania 

indicate that segregating specific items from the general field is the foundation 

of the identification process. Based on this concept, it is now theorized that 

various light patterns reaching the eyes are simplified and encoded, as lines, 

spots, edges, shadows, colors, orientations and referenced locations within 

the entire field of view. The first step in the subsequent identification process 

is the comparison of visual data with the long-term memory of previously 

collected data. Some researchers have suggested that this comparison 

procedure is a physiological cause of deja vu, the uncanny feeling of having 

seen something before. 


The accumulated data are then processed through a series of specific 

systems. Certain of our light sensors receive and respond only to certain 

stimuli and transmit their data to particular areas of the brain for translation. 

One kind of sensor accepts data on lines and edges; other sensors process 

only directions of movement or color. Processing of these data discriminates 

between different complex views by analyzing their various components. 

By experiment it has been shown that these areas of sensitivity have a kind of 

persistence. This can be illustrated by staring at a lit candle, then diverting the 

eyes toward a blank wall. For a short time, the image of the candle is retained. 

The same persistence occurs with motion detection and can he illustrated by 

staring at a moving object, such as a waterfall, then at a stationary object like 

the river bank. The bank will seem to flow because the visual memory of 

motion is still present. 


3.1.2 Differentiation in the Field of View 

Boundary and edge detection can be illustrated by the pattern changes in Fig. 

8. When scanning the figure from left to right, the block of reversed Ls is 

difficult to separate from the upright Ts in the center but the boundary 

between the normal Ts and the tilted Ts is easily apparent. The difficulty in 

differentiation occurs because horizontal and vertical lines comprise the L and 

upright T groups, creating a similarity that the brain momentarily retains as 

the eye moves from one group to the other. 

On the other hand, the tilted Ts share no edge orientations with the upright Ts, 

making them stand out in the figure. Differentiation of colors is more difficult 

when the different colors are in similarly shaped objects in a pattern. The 

recognition of geometric similarities tends to overpower the difference in 

colors, even when colors are the object of interest. Additionally, in a grouping 

of different shapes of unlike colors, where no one form is dominant, a 

particular form may hide within the varied field of view. However, if the 

particular form contains a major color variance, it is very apparent. 

Experiments have shown that such an object may be detected with as much 

ease from a field of thirty as it is from a field of three. 


FIGURE 8. Pattern changes illustrating boundary and edge detection 


3.1.3 Searching the Field of View 

The obstacles to differentiation discussed above indicate that similar objects 

are difficult to identify individually. During pre-attentive processing, particular 

objects that share common properties such as length, width, thickness or 

orientation are not different enough to stand out. If the differences between a 

target object and the general field is dramatic, then a visual inspector requires 

little knowledge of what is to be identified. When the target object is similar to 

the general field, the inspector needs more specific detail about the target. In 

addition, the time required to detect a target increases linearly with the 

number of similar objects in its general field. When an unspecified target is 

being sought, the entire field must be scrutinized. If the target is known, it has 

been shown statistically that only about half of the field must be searched. 


The differences between a search for simple features and a search for 

conjunctions or combinations of features can also have implications in 

nondestructive testing environments. For example, visual inspectors may be 

required to take more time to check a manufactured component when 

the possible errors in manufacturing are characterized by combinations of 

undesired properties. Less time could be taken for a visual test if the 

manufacturing errors always produced a change in a single property. 

Another aspect of searching the field of view addresses the absence of 

features. The presence of a feature is easier to locate than its absence. For 

example, if a single letter 0 is introduced to a field of many Qs, it is more 

difficult to detect than a single Q in a field of Os. The same difficulty is 

apparent when searching for an open 0 in a field of closed Os. In this case 

statistics show that the apparent similarity in the target objects is greater and 

even more search time is necessary 


Experimentation in the area of visual search tasks encompasses several 

tests of many 'individuals. Such experiments start with studies of those 

features that should stand out readily, displaying the basic elements of early 

vision recognition. The experiments cover several categories, including 

quantitative properties such as length or number. Also included are search 

tasks concentrating on single lines, orientation, curves, simple forms and 

ratios of sizes. All these tests verify that visual systems respond more 

favorably to targets that have something added (Q versus 0) rather than 

something missing. 

In addition, it has been determined that the ability to distinguish differences in 

intensity becomes more acute with a decreasing field intensity. This is the 

basis of Weber's law. The features it addresses are those involved in the 

early visual processes: color, size, contrast, orientation, curvature, lines, 

borders, movement and stereoscopic depth. 


3.2 Weber's Law 

3.2.1 General 

Weber's law is widely used by psychophysicists and entails the following 

tenets: (1) individual elements such as points or lines are more important 

singly than their relation to each other and (2) closed forms appear to stand 

out more readily than open forms. To view a complete picture, the visual 

system begins by encoding the basic properties that are processed within the 

brain, including their spatial relationships. 

Each item in a field of view is stored in a specific zone and is withdrawn when 

required to form a complete picture. Occasionally, these items are withdrawn 

and positioned in error. This malfunction in the reassembly process allows the 

creation of optical illusions, allowing a picture to be misinterpreted. 


The diagram in Fig. 9 represents a model of the early stages of visual 

perception. The encoded properties are maintained in their respective spatial 

relationships and compared to the general area of vision. The focused 

attention selects and integrates these properties, forming a specific area of 

observation. In some cases, as the area changes, the various elements 

comprising the observance are modified or updated to represent present 

conditions. During this step, new data are compared to the stored information. 


FIGURE 9. Stages of visual perception 


http://art.nmu.edu/cognates/ad175/background.html 


3.3. Vision Acuity 

3.3.1 General 

Vision acuity encompasses the ability to see and identify, what is seen. Two 

forms of vision acuity are recognized and must be considered when 

attempting to qualify visual ability. These are known as near vision and far 

vision (acuity). 


3.3.2 Components of the Human Eye 

The components of the human eye (Fig. 10) are often compared to those of a 

camera. The lens is used to focus light rays reflected by an object in the field 

of view. This results in the convergence of the rays on the retina (film), 

located at the rear of the eyeball. The cornea covers the eye and protects the 

lens. The quantity of light admitted to the lens is controlled by the contraction 

of the iris (aperture). The lens has the ability to become thicker or thinner, 

which alters the magnification and the point of impingement of the light rays, 

changing the focus. 

Eye muscles aid in the altering of the lens shape as well as controlling the 

point of aim. This configuration achieves the best and sharpest image for the 

entire system. The retina consists of rod and cone nerve endings that lie 

beneath the surface. They are in groups that represent specific color 

sensitivities and pattern recognition sections. These areas may be further 

subdivided into areas that collect data from lines, edges, spots, positions or 

orientations. 


The light energy is received and converted to electrical signals that are 

moved by way of the optic nerve system to the brain where the data are 

processed. Because the light is being reflected from an object in a particular 

color or combination of colors, the individual wavelengths representing each 

hue also vary. Each wavelength is focused at different depths within the retina, 

stimulating specific groups of rods and cones (see Figs. 10 and 11). The color 

sensors are grouped in specific recognition patterns as discussed above. 


FIGURE 10. Components of the human eye in cross section 


FIGURE 11. Magnified cross section showing the blind spot of the human eye 


To ensure reliable observation, the eye must have all the rays of light in focus 

on the retina. When the point of focus is short or primarily near the inner 

surface of the retina closest to the lens, a condition known as 

nearsightedness exists. If the focal spot is deeper into the retina, 

farsightedness occurs. These conditions are primarily the result of the eyeball 

changing from nearly orb shaped to an elliptical or egg shape. In the case of 

the nearsighted person, the long elliptical diameter is horizontal, If the long 

diameter is in a vertical direction, farsightedness occurs. These clinical 

conditions result from a very small shift of the focal spot, on the order of 

micrometers (ten-thousandths of an inch). 


3.3.3 Determining Vision Acuity 

The method normally used to determine what the eye can see is based on the 

average of many measurements. The average eye views a sharp image when 

the object subtends an arc of five minutes, regardless of the distance the 

object is from the eye. The variables in this feature are the diameter of the 

eye lens at the time of observation and the distance from the lens to the retina. 

When vision cannot he normally varied to create sharp clear images, then 

corrective lenses are required to make the adjustment. While the eye lens is 

about 17 mm (0.7 in.) from the retina, the ideal eyeglass plane is about 21 

mm (0.8 in.) from the retina. Differences in facial features must therefore be 

considered when fitting for eyeglasses. Under various working conditions, the 

glass lenses may not stay at their ideal location. This can cause slight 

variations when evaluating minute details and such situations must be 

individually corrected. 

For the majority of visual testing applications, near vision acuity is required. 

Most visual inspections are performed within arm's length and the inspector's 

vision should be examined at 400 mm (15.5 in.) distance. Examinations for 

far vision are done at distances of 6 m (20 ft). 


Keywords: 

• The average eye views a sharp image when the object subtends an arc of 

five minutes, regardless of the distance the object is from the eye. 

• For the majority of visual testing applications, near vision acuity is required. 

• Near vision should be examined at 400 mm (15.5 in.) distance. 

• Far vision are done at distances of 6 m (20 ft). 


3.4 Vision Acuity Examinations 

3.4.1 General 

Visual testing may occur once or more during the fabrication or manufacturing 

cycle to ensure product reliability. For critical products, visual testing may 

require qualified and certified personnel. Certification of the visual test itself 

may also be required to document the condition of the material at the time of 

testing. In such cases, testing personnel are required to successfully 

complete vision acuity examinations covering specific areas necessary to 

ensure product acceptability. For certain critical inspections, it may be 

required for the eyes of the inspector to be examined as often as twice per 

year. 


3.4.2 Near Vision Examinations 

The examination distance should be 400 mm (16 in.) from the eyeglasses or 

from the eye plane, for tests without glasses. When reading charts are used, 

they should he in the vertical plane at a height where the eye is on the 

horizontal plane of the center of the chart. Each eye should be tested 

independently while the unexamined eye is shielded from reading the chart 

but not shut off from ambient light. The Jaeger" eye chart is widely used in the 

United States for near vision acuity examinations. The chart is a 125 X 200 

mm (5 x 8 in.) off-white or grayish card with an English language text 

arranged into groups of gradually increasing size. Each group is a few lines 

long and the lettering is black. In a vision examination using this chart, visual 

testing personnel may be required to read, for example, the smallest letters 

at a distance of 300 mm (12 in.). Near vision acuity examinations that are 

more clinically precise are described below. 


3.4.3 Far Vision Examinations 

Conditions are the same as those for near vision examinations, except that 

the chart is placed 6 m (20 ft) from the eye plane. Again, each eye is tested 

independently. 


3.4.4 Grading Vision Acuity 

The criterion for grading vision acuity is the ability to see and correctly identify 

7 of 10 optotypes of a specific size at a specific distance. The average 

individual should be able to read six words in four to five seconds, regardless 

of the letter size being viewed. 

The administration of a vision acuity examination does not necessarily require 

medical personnel, provided the administrator has been trained and qualified 

to standard and approved methods. In some instances specifications may 

require the use of medically approved personnel. In these cases, the 

administrator of the examination may be trained by medically approved 

personnel for this application. In no instance should any of these 

administrators try to evaluate the examinations. 


If an applicant does not pass the examination (fails to give the minimum 

number of correct answers required by specification), the administrator should 

advise the applicant to seek a professional examination. If the professional 

responds with corrective lenses or a written evaluation stating the applicant 

can and does meet the minimum standards, the applicant may be considered 

acceptable for performance of the job. 


An eye chart 

is a chart used to measure visual acuity. Types of eye charts include the 

logMAR chart, Snellen chart, Landolt C, Lea test and the Jaeger chart. 

Procedure 

Charts usually display several rows of optotypes (test symbols), each row in a 

different size. An optotype is a standardized symbol for testing vision. 

Optotypes can be specially shaped letters, numbers, or geometric symbols. 

The person is asked to identify the optotype on the chart, usually starting with 

large rows and continuing to smaller rows until the optotypes cannot be 

reliably identified anymore. Technically speaking, testing visual acuity with an 

eye chart is a psychophysical measurement that attempts to determine a 

sensory threshold (see also psychometric function). 


Ototype 


Snellen Chart- Far Vision Acuity 


Golovin-Sivtsev Table 


Jaeger chart 


3.4.5 Vision Acuity Examination Requirements 

There are some basic requirements to be followed when setting up a vision 

acuity examination system. The distances mentioned above are examples but 

there are also detailed requirements for the vision chart. The chart should 

consist of a white matte finish with black characters or letters. The 

background should extend at least the width of one character beyond any line 

of characters. Sloan letters as shown in Fig. 12 were designed to be used 

where letters must be easily recognizable. Each character occupies a five 

stroke by five stroke space. 


FIGURE 12. Letters used for acuity examination charts (measurements in stroke units) 


The background luminance of the chart should be 85 ± 5 cd•m -2 . The 

luminance is a reading of the light reflected from the white matte finish toward 

the reader. When projected images are used, the parameters for the size of 

the characters, the background luminance and the contrast ratio are the same 

as those specified for charts. In no case should the contrast or illumination of 

the projected image be changed. A projection lamp of appropriate wattage 

should be used. When projecting the image, room lighting is subdued. This 

should not cause any change in the luminance of the projected background 

contrast ratio to that of the characters. 

The room lighting for examinations using charts should be 800 lx (75 ftc). 

Incandescent lighting of the chart is recommended to bring the background 

luminance up to 85 ±5 cd•m -2 . Fluorescent lighting should not be used for 

vision acuity examinations. Incandescent lamps emit more light in the yellow 

portion of the visible spectrum. This makes reading more comfortable for the 

examinee. Fluorescent lamps, especially those listed as full spectrum, are 

good for color vision examinations. 


Many of the lighting conditions for vision acuity examinations can be met by 

using professional examination units. With one such piece of equipment, the 

examinee views slides under controlled, ideal light conditions. Another 

common design is used both in industrial and medical examinations. With this 

unit, the individual looks into an ocular system and attempts to identify 

numbers, letters or geometric differences noted in illuminated slides. The 

examinee is isolated from ambient light. The slides and their respective data 

were developed by the Occupational Research Center at Purdue University, 

based on many individuals tested in many different occupations. Categories 

were developed for different vocations and are provided as guides for 

examinations required by various industries. Such equipment is expensive 

and accordingly eye charts are still very popular. Table 1 compares the 

results of these three vision acuity examination systems. 


TABLE 1. Eye examination system conversion chart 


There are slight differences between the reading charts and the slides. The 

reading chart distance for one popular letter card is 400 mm (16 in.). The 

simple slide viewer is set for near vision testing at 330 min (13 in.). There also 

are some differences between individual examination charts. Most of the 

differences are the result of variances in typeface, ink and the paper's ink 

absorption rate. Regardless of the examination system that is used, the 

requirements for the lighting and contrast remain the same. 


3.5 Visual Angle 

3.5.1 Posture 

Posture affects the manner in which an object is observed—appropriate 

posture and viewing angle are needed to minimize fatigue, eyestrain and 

distraction. The viewer should maintain a posture that makes it easy to 

maintain the optimum view on the axis of the lens. 

3.5.2 Peripheral Vision 

Eye muscles may manipulate the eye to align the image on the lens axis. The 

image is not the same unless it impinges on the same set of sensors in the 

retina (see Fig. 13). As noted above, different banks of sensors basically 

require different stimuli to perform their functions with optimum results. Also, 

light rays entering the lens at angles not parallel to the lens axis are refracted 

to a greater degree. This changes the quality and quantity of the light energy 

reaching the retina. Even the color and contrast ratios vary and depth 

perception is altered 


FIGURE 13. Vision acuity of peripheral vision 


The commonly quoted optimum, included angle of five (5) minutes of arc is 

the average in which an individual encloses a sharp image. There are other 

angles to be considered when discussing visual testing. The angle of 

peripheral vision is not a primary consideration when performing detailed 

visual tests. It is of value under certain inspection conditions: 

(1) when surveying large areas for a discontinuity indication that 

(2) has a high contrast ratio with the background and 

(3) is observed to one side of the normal lens axis. 

The inspector's attention is drawn to this area and it can then he scrutinized 

by focusing the eyes on the normal plane of the lens axis. 


3.5.3 Visual Testing Viewing Angle 

The angle of view is very important during visual testing. The viewer should in 

all cases attempt to observe the target on the center axis of the eye. The 

angle of view should not vary more than 45 degrees from normal. Figure 14 

shows how the eye perceives an object from several angles and how the 

object appears to change or move with a change in viewing angle. 


FIGURE 14. Shifting eye positions change apparent object size and location 


The same principle applies to objects being viewed through accessories such 

as mirrors or borescopes. The field of view should be maintained much in the 

same way that it is when viewed directly. 

On reflective backgrounds, the viewing angle should be off normal but not 

beyond 45 degrees. This is done so that the light reflected off the surface is 

not directed toward the eyes, reducing the contrast image of the surface itself. 

It also allows the evaluation of discontinuities without distorting their size, 

color or location. This is very important when using optical devices to view 

areas not available to direct line of sight. 


3.6 Color Vision 

3.6.1 General 

There are specific industries where accuracy of color vision is important: paint, 

fabrics and photographic film are examples. Surface inspections such as 

those made during metal finishing and in rolling mills are to determine 

manufacturing discontinuities. Color changes are not indicative of 

such discontinuities and therefore, for practical purposes, color is not as 

significant in these applications. However, heat tints are sometimes important 

and colors may be crucial in metallography and failure analysis. When white 

light testing is performed, it must be remembered that white light is composed 

of all the colors (wavelengths) in the spectrum. If the inspector has color 

vision deficiencies, then the test object is being viewed differently than when 

viewed by an inspector with normal color vision. Color deficiency may be as 

critical as the test itself. During visual testing of a white or near white object, 

slight deficiencies in color vision may be unimportant. During visual testing of 

black or near black objects, color vision deficiencies make the test object 

appear darker 


3.6.2 Color Vision Examinations 

Ten percent of the male population have some form of color vision deficiency. 

The so-called color blind condition affects even fewer people truly color blind 

individuals are unable to distinguish red and green. But, there are many 

variations and levels of sensitivity between individuals with normal vision and 

those with color deficiencies. There are two causes of color deficiency: 

inherited and acquired. And each of these may be subdivided into specific 

medical problems. Most such subdivisions are typically discovered during the 

first vision examination. 

The most common color deficiencies are hereditary and occur in the redgreen 

range. About 0.5 percent of the affected individuals are female, in the 

red-green range. Women constitute about 50 percent of those affected in the 

blue-yellow range. Most such deficiencies occur in both eyes and in rare 

instances in only one eye. About 0.001 percent of the affected groups in the 

hereditary portion have their deficiency in the blue-green range. Individuals in 

the red green group may make misinterpretations of discontinuities in shades 

of red, browli, olive and gold. 


Color Vision Examinations-Ishihara Plates 



http://www.nature.com/nmeth/journal/v8/n6/full/nmeth.1618.html 




http://www.today.com/health/surprise-side-effect-new-specs-may-fix-color-blindness-1C8487550

Acquired color deficiency is a greater problem to good color vision testing. 

The acquired deficiencies may affect only one eye and a change from 

acceptable color vision to a recognizable problem may he very gradual. 

Various medical conditions can cause such a change to occur (Table 2 lists 

conditions that produce color vision deficiencies in particular color ranges). 

Most acquired color vision problems vary in severity and may be associated 

with ocular pathology. If the disease continues for an extended period of time 

without treatment, the deficiencies may become erratic in intensity and may 

vary from the red-green or blue-yellow ranges. Aging can also affect color 

vision. 


TABLE 2. Causes of acquired color vision deficiencies Color Vision Deficiency Cause of 

Deficiency 

Blue-yellow deficiency 

• Glaucoma 

• Myopic retinal degeneration 

• Retinal detachment 

• Pigmentary degeneration of the retina (including retinitis pigmentosa) 

• Senile macular degeneration 

• Chorioretinitis 

• Retinal vascular occlusion 

• Diabetic retinopathy 

• Hypertensive retinopathy 

• Papilledema 

• Methyl alcohol poisoning 

• Central serous retinopathy (accompanied by luminosity loss in red) 


TABLE 2. Causes of acquired color vision deficiencies Color Vision Deficiency Cause of 

Deficiency 

Red-green deficiency 

• Optic neuritis (including retrobulbar 

• neuritis) 

• Tobacco or toxic amblyopia 

• Leber's optic atrophy 

• Lesions of the optic nerve and pathway 

• Papillitis 

• Hereditary juvenile macular degeneration 

• {Stargardt's and Best's disease) 

• Blue-yellow deficiency 

• Dominant hereditary optic atrophy 

• Red-green or blue-yellow deficiency 

• Juvenile macular degeneration 


3.6.3 Color Vision Classifications 

Two functions that determine an individual's sensation range are their color 

perception and color discrimination. When a primary color is mistaken for 

another primary color, this is an error in perception. An error in discrimination 

is an error of lesser magnitude involving a mistake in hue selection. During a 

vision examination, these two functions are tested independently. 

A color vision examination performed with an anomaloscope allows the 

mixing of red and green lights to match a yellow light standard. Yellow and 

blue lights may be mixed to match a white light. An individual with normal 

vision requires red, blue and green light to mix and match colors of the entire 

color spectrum. A color deficient person may require fewer than the three 

lights to satisfy the color sensation. Table 3 indicates the type of deficiencies 

and the percent of the male population known to be affected 


Anomaloscope 


Anomaloscope Test 


TABLE 3. Classification of color vision deficiencies and percent of affected males 

Color Vision 

Hereditary deficiencies 

trichromatism 

three colors: red, green. blue) 

normal vision 

anomalous (defective) 

dichromatism (two colors)* 

protanopia (red lacking) 

deureranopia (green lacking) 

trianopia {blue lacking) 

tetratanopia (yellow lacking) 

Acquired deficiencies 

tritan (blue yellow) 

protan-deutan (red-yellow) 

Percent Males Affected 

92 

6 or 7 

1 

1 

rare 

very rare 

data not available 

data not available 

*Deficiency most often referenced when discussing color blindness 


TABLE 4. Naval Submarine Medical Research Laboratory color vision classification system 

Class 

Description 

0 Normal 

I 

Mild anomalous trichromat 

ll 

Unclassified anomalous trichromat 

(includes mild and moderate classes) 

III 

Moderate anomalous trichromat 

IV 

Severely color deficient {includes severe anomalous 

trichromats, dichromats andmonochromats) 


For the practical purpose of classifying personnel affected by hereditary color 

deficiencies, the Naval Submarine Medical Research Laboratory has 

developed the classifications shown in Table 4. about 50 percent of color 

deficient people can be categorized in accordance with this table. Class I 

covers 30 percent of the color deficient population and Class III accounts for 

20 percent. Individuals in Class I can judge colors used as standards for 

signaling, communication and identification as fast and as accurately as zero 

class persons can. The limitation of Class I people is when good color 

discrimination is necessary. Persons in Class III may be used in other areas 

such as radio repair, chemistry, medicine and surgery, electrical 

manufacturing or general painting. Class II encompasses staff members, 

managers or clerical help, whose need for color resolution is not critical. 

Individuals in Class IV must be restricted from occupations where color 

differentiation of any magnitude is required. 


As with vision acuity examinations, there are many different examinations for 

color vision. Color vision is often tested with pseudoisochromatic plates or 

cards on which the detection of certain figures depends on red-green 

discrimination. Unfortunately, most common vision acuity examinations were 

designed to identify hereditary red-green deficiencies and ignore blue-yellow 

deficiencies. 

A good, discriminating examination technique is illustrated in color Plates 1 to 

7. The diagrams show the sequence in which the colors are arranged in each 

photograph for each deficiency, differing from the sequence according to 

normal vision illustrated in Plate 1. 21 (Caution: These plates are provided for 

educational purposes only. Photography, print reproduction and chemical 

changes all cause colors to vary from the original and fade with time. Under 

no circumstances should illustrations in this book be used for vision 

examinations.) 


Pseudoisochromatic plates 

http://www.healthytimesblog.com/2011/04/facts-about-color-blindness/ 



http://www.healthytimesblog.com/2011/04/facts-about-color-blindness/

The exam consists of the examinee's arranging fifteen colored caps into a 

circle according to changes in hue progressing from a reference cap. To help 

evaluate the outcome, each cap is numbered on the back. A perfect score 

has the caps in numerical sequence. This test is used for those known to 

have a color vision deficiency. The test allows for the evaluation of the 

individual's ability and determines the specific area of the deficiency. The 

arrangement of colors allows confusion to exist across the quadrants of the 

circle. 

For instance, reds can be confused with blue-greens. One authority has 

stated that anyone who can pass this test should have no problem in any 

work requiring color vision acuity. Two types of red-green deficient patterns 

can be noted. 


Individuals in these categories confuse green (4) with redpurple (13) and 

blue-green (3) with red (12). The sequence then appears as 4, 13, 3 and 12. 

Persons with the blue-yellow deficiency confuse yellow-green (7) with purple 

(15), creating a sequence of 7, 15, 8, 14 and 9. 

As in the normal vision acuity examinations, lighting requirements and time 

must be controlled for color vision examinations. The illumination intensity of 

full spectrum fluorescent lighting should be no less than 200 lx (20 ftc). The 

rating of the light source is known as the color temperature. A low color 

temperature lamp such as an incandescent lamp makes it easier for persons 

with borderline color deficiencies to guess the colors correctly. A color 

temperature of 6,700 K is preferred. Too high a color temperature increases 

the number of reading errors. To eliminate glare, the light source should be 

45 degrees to the surface while the patient is perpendicular to it. The reading 

distance should be about 400 to 600 mm (15 to 24 in.) or arm's length. 


To perform such an examination, two minutes should be allotted to arrange all 

fifteen caps in their appropriate positions. In summary, color deficiency can be 

acquired or inherited. Some color deficiencies may be treated, alleviated or 

minimized. Pseudoisochromatic plates in conjunction with the progressive 

hue color caps provide an adequate test for most industrial visual inspectors. 

Full spectrum lighting (6,700 K) is necessary for accurate test results. 

It should be added that, because the visible spectrum is made up of colors of 

varying wavelengths and the black and white colors consist of various 

combinations of colors, deficiencies in any part of the color spectrum has an 

impact on certain black and white inspection methods, including X-ray film 

review It is recommended that all nondestructive testing personnel have their 

color vision tested annually, while taking their vision acuity examination. 


Caps for Color Vision Examinations 

The exam consists of the examinee's arranging fifteen colored caps into a 

circle by a change in hue progressing from a reference cap. To help evaluate 

the outcome, each cap is numbered on the back. A perfect score has the 

caps in numerical sequence. The diagrams show the sequence in which the 

colors are arranged in each photograph for each deficiency, differing from the 

sequence according to normal vision illustrated in Plate 1. 

(Caution: These plates are provided for instructional purposes only. 

Photography, print reproduction and chemical changes all cause colors to 

vary from the Original and fade with time. Under no circumstances should 

illustrations in this book be used for vision examinations.) 


PLATE 1. Colored caps for normal color vision examination 


PLATE 2. Colored caps for normal color vision with minor errors 


PLATE 3. Colored caps for normal color vision with one error 


PLATE 4. Colored caps for red blindness 


PLATE 5. Colored caps for green blindness 


PLATE 6. Colored caps for blue blindness 


PLATE 7. Colored caps for anomalous trichromatic vision 


3.7 Fluorescent Materials 

3.7-1 General 

Fluorescence is a complex phenomenon that occurs in gases, liquids and 

solids. It has also proved to be the greatest and most efficient source of the 

so-called cold light. For the purpose of visual nondestructive testing, 

fluorescence is used in conjunction with long wave ultraviolet radiation as an 

excitation source (see Fig. 15). Visible light rays are made up of billions of 

photons, packets of particle-like energy. Photons are so small they have no 

mass. They do however carry energy and this is what we see when a light 

bulb is energized—the photons have carried energy from the bulb to the eye. 

Photons have different energies or wavelengths which we distinguish as 

different colors. Red light photons are less energetic than blue light photons. 

Invisible ultraviolet photons are more energetic than the most energetic violet 

light that our eyes can see. 

Studies show that the intensity of fluorescence in most situations is directly 

proportional to the intensity of the ultraviolet radiation that excites it. 


Fluorescence is the absorption of light at one wavelength and reemission of 

this light at another wavelength. The whole absorption and emission process 

occurs in about a nanosecond and because it keeps happening as long as 

there are ultraviolet radiation photons to absorb, a glow is observed to begin 

and end with the turning on and off of the ultraviolet radiation. Care must be 

taken when using short wave or wide bandwidth ultraviolet sources. A safe, 

general operating principle is to always hold the lamp so the light is directed 

away from you. 

Long wave ultraviolet is generally considered safe. However, individuals 

should use adequate protection if they are photosensitive or subjected to long 

exposure times. Commercially available fluorescent dyes span the visible 

spectrum. Because the human eye is still the most commonly used sensing 

device, most nondestructive testing applications are designed to fluoresce as 

close as possible to the eye's peak response. Figure 16 shows the spectral 

response of the human eye, with the colors at the ends of the spectrum (red, 

blue and violet) appearing much dimmer than those in the center (orange, 

yellow and green). 


While the fundamental aspects of fluorescence are still incompletely 

understood, there is enough known to ensure that nondestructive testing 

methods using fluorescence will continue to improve with the development of 

new dyes or new solvents to increase brightness or eye response matching. 


Keywords: 

Studies show that the intensity of fluorescence in most situations is directly 

proportional to the intensity of the ultraviolet radiation that excites it. 


FIGURE 15. Electromagnetic spectrum and an enlargement of the ultraviolet region 










FIGURE 16. Human eye response at 1070 lx 1100 ftc) 


FIGURE 16. Human eye response to light 

Digital ambient light sensor 

http://www.michaelhleonard.com/sensorcape-reference/ 


FIGURE 16. Human eye response to light 


FIGURE 16. Human eye response at 1070 lx 1100 ftc) 


Eye Spectra Responds: http://www.telescope-optics.net/eye_spectral_response.htm 


Fluorescence Testing 














Part 4: SAFETY FOR VISUAL AND OPTICAL TESTS 

4.0 General: 

This information is presented solely for educational purposes and should not 

be consulted in place of current safety regulations. Note that units of measure 

have been converted to this book's format and are not those commonly used 

in all industries. Human vision can be disrupted or destroyed by improper use 

of any light source. Consult the most recent safety documents and the 

manufacturer's literature before working near any artificial light or radiation 

source. 


4.1 Need for Safety 

Developments in optical testing technology have created a need for better 

understanding of the potential health hazards caused by high intensity 'light 

sources or by artificial light sources of any intensity in the work area. The 

human eye operates optimally in an environment illuminated directly or 

indirectly by sunlight, with characteristic spectral distribution and range of 

intensities that are very different from those of most artificial sources. The eye 

can handle only a limited range of night vision tasks. 

Over time, there has accumulated evidence that photochemical changes 

occur in eyes under the influence of normal daylight illumination- short term 

and long term visual impairment and exacerbation of retinal disease have 

been observed and it is important to understand why this occurs. Periodic 

fluctuations of visible and ultraviolet radiation occur with the regular diurnal 

light-dark cycles and with the lengthening and shortening of the cycle as a 

result of seasonal changes. These fluctuations are known to affect all 

biological systems critically. 


The majority of such light-dark effects is based on circadian cycles and 

controlled by the pineal system, which can be affected directly by the 

transmission of light to the pineal gland or indirectly by effects on the optic 

nerve pathway. Also of concern are the results of work that has been done 

demonstrating that light affects immunological reactions in vitro and in vivo by 

influencing the antigenicity of molecules, antibody function and the reactivity 

of lymphocytes. 

Given the variety of visual tasks and illumination that confronts the visual 

inspector, it is important to consider whether failures in performance might be 

a result of excessive exposure to light or other radiation or even a result of 

insufficient light sources. A myth exists that 20/20 fovea vision, in the absence 

of color blindness, is all that is necessary for optimal vision. In fact, this is not 

so, there may be visual field loss in and beyond the fovea centralis for many 

reasons; the inspector may have poor stereoscopic vision; visual ability may 

be impaired by glare or reflection; or actual vision may be affected by medical 

or psychological conditions. 


4.2 Laser Hazards 

4.2.1 General 

Loss of vision resulting from retinal burns following observation of the sun has 

been described throughout history. Now there is a common technological 

equivalent to this problem with laser light sources. In addition to the 

development of lasers, further improvement in other high radiance light 

sources (a result of smaller, more efficient reflectors and more compact, 

brighter sources) has presented the potential for chorioretinal injury. It is 

thought that chorioretinal burns from artificial sources in industrial situations 

have been very much less frequent than similar burns from the sun. 


Because of the publicity of the health hazard caused by exposure to laser 

radiation, awareness of such hazards is probably much greater than the 

general awareness of the hazard from high intensity extended visible sources 

which may be as great or greater. Generally, lasers are used in specialized 

environments by technicians familiar with the hazards and trained to avoid 

exposure by the use of protective eyewear and clothing. 

Laser standards of manufacture and use have been well developed and 

probably have contributed more than anything else to a heightened 

awareness of safe laser operation. 


Laser standards of manufacture and use have been well developed and 

probably have contributed more than anything else to a heightened 

awareness of safe laser operation. 

Laser hazard controls are common sense procedures designed to (1) restrict 

personnel from entering the beam path and (2) limit the primary and reflected 

beams from occupied areas. Should an individual be exposed to excessive 

laser light, the probability of damage to the retina is high because of the high 

energy pulse capabilities of some lasers. 


However, the probability of visual impairment is relatively low because of the 

small area of damage on the retina. Once the initial flash blindness and pain 

have subsided, the resulting scotomas (damaged unresponsive areas) can 

sometimes be ignored by the accident victim. The tissue surrounding the 

absorption site can much more readily conduct away heat for small image 

sizes than it can for large image sizes. In fact, retinal injury thresholds (see 

Fig. 17) for less than 0.1 to 10 s exposure show a high dependence on the 

image size (0.01 to 0.1 W •mm - 2 for a 1,000 p.m image up to about 0.01 

KV•mm- 2 for a 20 μm image. To put the scale into perspective, the sun 

produces a 160 μm diameter image on the retina. 


4.2.2 High Luminance Visible Light Sources 

The normal reaction to a high luminance light source is to blink and to direct 

the eyes away from the source. The probability of overexposure to noncoherent 

light sources is higher than the probability of exposure to lasers, yet 

extended (high luminance) sources are used in a more casual and possibly 

more hazardous way. In the nondestructive testing industry, extended 

sources are used as general illumination and in many specialized applications. 

Unfortunately, there are comparatively few guidelines for the safe use of 

extended sources of visible light. 


FIGURE 1 7. Typical retinal burn thresholds 


4.3 Infrared Hazards 

Infrared radiation comprises that invisible radiation beyond the red end of the 

visible spectrum up to about 1 mm wavelength. Infrared is absorbed by many 

substances and its principal biological effect is known as hyperthermia, 

heating that can be lethal to cells. Usually, the response to intense infrared 

radiation is pain and the natural reaction is to move away from the source so 

that burns do not develop. 

Keywords: 

Hyperthermia 


4.4 Ultraviolet Hazards 

Before development of the laser, the principal hazard in the use of intense 

light sources was the potential eye and skin injury from ultraviolet radiation. 

Ultraviolet radiation is invisible radiation beyond the violet end of the visible 

spectrum with wavelengths down to about 185 nm. It is strongly absorbed by 

the cornea and the lens of the eye. Ultraviolet radiation at wavelengths 

shorter than 185 nm is absorbed by air, is often called vacuum ultraviolet and 

is rarely of concern to the visual inspector. Many useful high intensity arc 

sources and some lasers may emit associated, potentially hazardous, levels 

of ultraviolet radiation. With appropriate precautions, such sources can serve 

very useful visual testing functions. 

Keywords: 

380nm- 185nm Ultraviolet radiation damaging to eye 

< 185nm- Vacuum UV is absorbed by air and not a concern to the inspector 


Studies have clarified the spectral radiant exposure doses and relative 

spectral effectiveness of ultraviolet radiation required to elicit an adverse 

biological response. These responses include kerato-conjunctivitis (known as 

welder's flash), possible generation of cataracts and erythema or reddening 

of the skin. Longer wavelength ultraviolet radiation can lead to fluorescence 

of the eye's lens and ocular media, eyestrain and headache. These 

conditions lead, in turn, to low task performance resulting from the fatigue 

associated with increased effort. Chronic exposure to ultraviolet radiation 

accelerates skin aging and possibly increases the risk of developing certain 

forms of skin cancer. 


It should also be mentioned that some individuals are hypersensitive to 

ultraviolet radiation and may develop a reaction following, what would be for 

the average healthy human, suberythemal exposures. However, it is 

extremely unusual for these symptoms of exceptional photosensitivity to be 

elicited solely by the limited emission spectrum of an industrial light source. 

An inspector is typically aware of such sensitivity because of earlier 

exposures to sunlight. In industry, the visual inspector may encounter many 

sources of visible and invisible radiation: incandescent lamps, compact arc 

sources (solar simulators), quartz halogen lamps, metal vapor (sodium and 

mercury) and metal halide discharge lamps, fluorescent lamps and flash 

lamps among others. Because of the high ultraviolet attenuation afforded by 

many visually transparent materials, an empirical approach is sometimes 

taken for the problem of light sources associated with ultraviolet: the source is 

enclosed and provided with ultraviolet absorbing glass or plastic lenses. 


If injurious effects continue to develop, the thickness of the protective lens is 

increased. The photochemical effects of ultraviolet radiation on the skin and 

eye are still not completely understood. Records of ultraviolet radiation's 

relative spectral effectiveness for eliciting a particular biological effect 

(referred to by photohiologists as action spectra) are generally available. 

Ultraviolet irradiance may be measured at a point of interest with a portable 

radiometer and compared with the ultraviolet radiation hazard criteria (Table 

5). For the near ultraviolet region (from 320 nm to the edge of the visible 

spectrum), the total irradiance incident on the unprotected skin or eye should 

not exceed 1 mW.cm -2 for periods greater than 1,000 s. For exposure times 

less than 1,000 s, incident irradiance on unprotected skin or eye should not 

exceed 1 J.cm -2 within an eight hour period. These values do not apply to 

exposures of photosensitive people or those simultaneously exposed to 

photosensitizing Agents. 


For purposes of determining exposure levels, it is important to note that most 

inexpensive, portable radiometers are not equally responsive at all 

wavelengths throughout the ultraviolet spectrum and are usually only 

calibrated at one wavelength with no guarantees at any other wavelength. 

Such radiometers have been designed for a particular application using a 

particular lamp. 

A common example in the nondestructive testing industry is the so-called 

blacklight radiometer used in fluorescent liquid penetrant and magnetic 

particle applications. These meters are usually calibrated at 365 nm, the 

predominant ultraviolet output of the filtered 100 W medium pressure mercury 

vapor lamp commonly used in the industry. Use of the meter at any other 

wavelength in the ultraviolet spectrum may lead to significant errors. To 

minimize problems in assessing the hazard presented by industrial lighting, it 

is important to use a radiometer that has been calibrated with an ultraviolet 

spectral distribution as close as possible to the lamp of interest. 


If the inspector is concerned about the safety of a given situation, ultraviolet 

absorbing eye protection and face wear is readily available from several 

sources. An additional benefit of such protection is that it prevents the 

annoyance of lens fluorescence and provides the wearer considerable 

protection from all ultraviolet radiation. In certain applications, tinted lenses 

can also provide enhanced visibility of the test object. 


TABLE 5. Threshold limit values for ultraviolet radiation' within an eight hour 

period* 

Wavelength 

(nanometers) 

200 

210 

220 

230 

240 

250 

254 

260 

270 

280 

290 

300 

305 

310 

315 

Threshold Limit Values 

100 

40 

25 

16 

10 

7 

6 

4.6 

3 

3.4 

4.7 

10 

50 

200 

1,000 

*These values are presented for 

instructional purposes and not as 

guidelines. They do not apply to 

exposure of photosensitive people or 

those simultaneously exposed to 

photosensitizing agents. Consult 

current safety regulations, 

manufacturers data and inspection 

codes before any exposure to 

ultraviolet radiation. 


4.5 Photosensitizers 

While ultraviolet radiation from most of the high intensity visible light sources 

may be the principal concern, the potential for chorioretinal injury from visible 

radiation should not be overlooked. 

Over the past few decades, a large number of commonly used drugs, food 

additives, soaps and cosmetics have been identified as phototoxic or 

photoallergenic agents even at the longer wavelengths of the visible spectrum. 

Colored drugs and food additives are possible photosensitizers for organs 

below the skin because longer wavelength visible radiations penetrate deeply 

into the body. 


4.6 Damage to the Retina 

It is possible to multiply the spectral absorption data of the human retina by 

the spectral transmission data of the eye's optical media at all wavelengths to 

arrive at an estimate of the relative absorbed spectral dose in the retina and 

the underlying choroid for a given spectral radiant exposure of the cornea. 

The computation should provide a relative spectral effectiveness curve for 

chorioretinal burns. In practice, the evaluation of potential chorioretinal burn 

hazards may be complicated or straightforward, depending on the maximum 

luminance and spectral distribution of the source; possible retinal image sizes; 

the image quality; pupil size; spectral scattering and absorption by the cornea, 

aqueous humor, the lens and the vitreous humor; and absorption and 

scattering in the various retinal layers. For convenience, the product of total 

transmittance of the ocular media Tx and the total absorptance of the retinal 

pigment epithelium and choroid α λ , over all wavelengths may be defined as 

the relative retinal hazard factor R: 


where L λ , could be any other spectral quantity. Qualitatively, the ocular media 

transmission rises steeply from somewhat less than 400 nm and does not fall 

off again until about 900 nm in the near infrared after which a peak at about 

1,100 nm is exhibited. These values finally fall off to virtually zero at about 

1,400 nm thus defining the potential hazardous wavelength range. For most 

extended visible sources, the retinal image size can be calculated by 

geometrical optics. As shown in Fig. 18, the angle subtended by an extended 

source defines the image size. Knowing the effective focal length f of the 

relaxed normal eye (17 mm), the approximate retinal image size d λ can be 

calculated if the viewing distance r and the dimensions of the light source D L , 

are known. 


This analysis strictly holds only for small angles—corrections must be made 

at angles exceeding about 20 degrees. Because the solid angles Ω 

subtended by the source and retinal image are clearly identical, the retinal 

illuminance area A L and source luminance area A r are likewise proportional. 

The source luminance L is related to the illuminance at the cornea E r as 

follows: 

FIGURE 18. The extended source of length D L imaged on the retina 

with length d r 


Calculation of the permissible luminance from a permissible retinal 

illuminance for a source breaks down for very small retinal image sizes or for 

very small hot spots in an extended image caused by diffraction of light at the 

pupil, aberrations introduced by the cornea and lens and scattering from the 

cornea and the rest of the ocular media. Because the effects of aberration 

increase with increasing pupil size, greater blur and reduced peak retinal 

illuminance are noticed for larger pupil sizes and for a given corneal 

illumination. 


4.7 Thermal Factor 

Visible and near infrared radiation up to about 1,400 nm (associated with 

most optical sources) is transmitted through the eye's ocular media and 

absorbed in significant doses principally in the retina. These radiations pass 

through the neural layers of the retina. A small amount is absorbed by the 

visual pigments in the rods and cones, to initiate the visual response, and the 

remaining energy is absorbed in the retinal pigment epithelium and choroid. 

The retinal pigment epithelium is optically the most dense absorbent layer 

(because of high concentrations of melanin granules) and the greatest 

temperature changes arise in this layer. For short (0.1 to 100 s) accidental 

exposures to the sun or artificial radiation sources, the mechanism of injury is 

generally thought to be hyperthermia resulting in protein denaturation and 

enzyme inactivation. Because the large, complex organic molecules 

absorbing the radiant energy have broad spectral absorption bands, the 

hazard potential for chorioretinal injury is not erected to depend on the 

coherence or monochromaticity of the source. Injury from a laser or a 

nonlaser radiation source should not differ if image size, exposure time and 

wavelength are the same. 


Because different regions of the retina play different roles in vision, the 

functional loss of all or part of one of these regions varies in significance. The 

greatest vision acuity exists only for central (foveal) vision, so that the loss of 

this retinal area dramatically reduces visual capabilities. In comparison, the 

loss of an area of similar size located in the peripheral retina could be 

subjectively unnoticed. The human retina is normally subjected to irradiances 

below 1 1.1.W•mm -2 , except for occasional momentary exposures to the sun, 

arc lamps, quartz halogen lamps, normal incandescent lamps, flash lamps 

and similar radiant sources. The natural aversion or pain response to bright 

lights normally limits exposure to no more than 0.15 to 0.2 s. In some 

instances, individuals can suppress this response with little difficulty and stare 

at bright sources, as commonly occurs during solar eclipses. 


Fortunately, few arc sources are sufficiently large and sufficiently bright 

enough to be a retinal burn hazard under normal viewing conditions. Only 

when an arc or hot filament is greatly magnified (in an optical projection 

system, for examlarge can hazardous irradiance be imaged on a sufficiently 

large area of the retina to cause a burn. Visual inspectors do not normally 

step into a projected beam at close range or view a welding arc with 

binoculars or a telescope. Nearly all conceivable accident situations require a 

hazardous exposure to be delivered within the period of a blink reflex. If an 

arc is stnick while an inspector is located at a very close viewing range, it is 

possible that a retinal burn could occur. At lower exposures, an inspector 

experiences a short term depression in photopic (daylight) sensitivity and a 

marked, longer term loss of scotopic (dark adapted) vision. That is why it is so 

important for visual inspectors in critical fluorescent penetrant and magnetic 

particle test environments to undergo dark adaptation before actually 

attempting to find discontinuities. Not only does the pupil have to adapt to the 

reduced visible level in a booth but the actual retinal receptors must attain 

maximum sensitivity. This effect may take half an hour or more, depending on 

the preceding state of the eye's adaptation. 


4.8 Blue Hazard 

The so-called blue hazard function has been used in conjunction with the 

thermal factor to calculate exposure durations that do not damage the retina. 

The blue hazard is based on the demonstration that the retina can be 

damaged by blue light at intensities that do not elevate retinal temperatures 

sufficiently to cause a thermal hazard. It has been found that blue light can 

produce 10 to 100 times more retinal damage (permanent decrease in 

spectral sensitivity in this spectral range) than longer visible wavelengths. 

Note that there are some common situations in which both thermal and blue 

hazards may be present. 


4.9 Visual Safety Recommendations 

The American Conference of Governmental Industrial Hygienists (ACGIH) 

has proposed two threshold limit values (TLVs) for noncoherent visible light, 

one covering damage to the retina by a thermal mechanism and one covering 

retinal damage by a photochemical mechanism. Threshold limit values for 

visible light, established by the American Conference of Governmental 

Industrial Hygienists, are intended only to prevent excessive occupational 

exposure and are limited to exposure durations of 8 h or less. They are not 

intended to cover photosensitive individual. 


4.10 Eye Protection Filters 

Because continuous visible light sources elicit a normal aversion or pain 

response that can protect the eye and skin from injury, visual comfort has 

often been used as an approximate hazard index and eye protection and 

other hazard controls have been provided on this basis. Eye protection filters 

for various workers were developed empirically but now are standardized as 

shades and specified for particular applications. 

Other protective techniques include use of high ambient light levels and 

specialized filters to further attenuate intense spectral lines. Laser eye 

protection is designed to have an adequate optical density at the laser 

wavelengths along with the greatest visual transmission at all other 

wavelengths. Always bear in mind that hazard criteria must not be considered 

to represent fine lines between safe and hazardous exposure conditions. To 

be properly applied, interpretation of hazard criteria must he based on 

practical knowledge of potential exposure conditions and the user, whether a 

professional inspector or a general consumer. Accuracy of hazard criteria is 

limited by biological uncertainties including diet, genetic photosensitivity and 

the large safety factors required to be built into the recommendations. 






2014-August 



2014-August 


Reading 4 




2014-August 



Fion Zhang 

2014/August/15

SECTION 2 

THE PHYSICS OF LIGHT 


SECTION 2: The Physics of Light 

PART 1: THE PHYSICS OF LIGHT 

1.1 Radiant Energy Theories 

1.2 Light and the Energy Spectrum 

1.3 Blackbody Radiation 

1.4 Atomic Structure and Radiation 

1.5 Luminous Efficiency of Radiant Energy 

1.6 Luminous Efficiency of Light Sources 


SECTION 2: The Physics of Light 

PART 2: Measurement of the properties of light 

2.1 Photovoltaic Cells 

2.2 Photoconductor Cells 

2.3 Photoelectric Tubes 

2.4 Photodiodes and Phototransistors 

2.5 Photometry 

2.6 Principles of Photometry 

2.7 Photometers 

2.8 Photovoltaic Cell Meters 

2.9 Meters Using Photomultiplier Tubes 

2.10 Equivalent Sphere Illumination Photometers 

2.11 Reflectometers 

2.12 Radiometers 

2.13 Spectrophotometers 

2.14 Types of Photometers 


PART 1: THE PHYSICS OF LIGHT 

1.0 General: 

Light can be defined as radiant energy capable of exciting the human retina 

and creating a visual sensation. From the viewpoint of physics, light is defined 

as that portion of the electromagnetic spectrum with wavelengths between 

380 and 770 nm. Visually, there is some variation in these limits among 

individuals. Radiant energy at the proper wavelength makes visible anything 

from which it is emitted or reflected in sufficient quantity to activate the 

receptors in the eye. The quantity of such radiant energy may be evaluated in 

many ways, including: radiant flux (measured in joules per second or in watts) 

and luminous flux (measured in lumens). 


1.1 Radiant Energy Theories 

Several theories describing radiant energy have been proposed and the text 

below briefly discusses the primary theories. 

Corpuscular Theory 

The corpuscular theory was advocated by Sir Isaac Newton and is based on 

the following premises. 

1. Luminous bodies emit radiant energy in particles. 

2. These particles are intermittently ejected in straight lines. 

3. The particles act on the retina of the eye, stimulating the optic nerves to 

produce the sensation of light. 


Wave Theory 

The wave theory of radiant energy was advocated by Christian Huygens and 

is based on these premises. 

1. Light results from the molecular vibration in luminous material. 

2. The vibrations are transmitted through the ether in wavelike movements 

(comparable to ripples in water). 

3. The vibrations act on the retina of the eye, stimulating the optic nerves to 

produce visual sensation. 

The velocity of a wave is the product of its wavelength and its frequency. 


Electromagnetic Theory 

The electromagnetic theory was advanced by James Clerk Maxwell and is 

based on these premises. 

1. Luminous bodies emit light in the form of radiant energy. 

2. This radiant energy is propagated in the form of electromagnetic waves. 

3. The electromagnetic waves act on the retina of the eye, stimulating the 

optic nerves to produce the sensation of light. 


Quantum Theory 

The quantum theory is an updated version of the corpuscular theory. It was 

advanced by Planck and is based on these premises. 

1. Energy is emitted and absorbed in discrete quanta (photons). 

2. The energy in each quantum is hv. 

The term h is known as Planck's constant and is equal to 6.626 x 10 -34 

joule•second. The term v is the frequency in hertz. 


Unified Theory 

The unified theory of radiant energy was proposed by De Broglie and 

Heisenberg and is based on the premise that every moving element of mass 

is associated with a wave whose length is given by: 

λ = h / mv (Eq. 1) 

Where: 

λ 

h 

m 

v 

= wavelength of the wave motion (meters); 

= Planck's constant or 6.626 x 10 -34 Joule.second; 

= Mass in Kg 

= velocity of the particle (meters per second). 

It is impossible to determine all of the properties that are distinctive of a wave 

or a particle simultaneously, since the energy to do so changes one of the 

properties being determined. 


The quantum theory and the electromagnetic wave theory provide an 

explanation of radiant energy that is appropriate for the purposes of 

nondestructive testing. Whether it behaves like a wave or like a particle, light 

is radiation produced by atomic or molecular processes. That is, in an 

incandescent body, a gas discharge or a solid state device, light is produced 

when excited electrons have just reverted to more stable positions in their 

respective atoms, thereby releasing energy. 


1.2 Light and the Energy Spectrum 

The wave theory permits a convenient representation of radiant energy in an 

arrangement based on the energy's wavelength or frequency. This 

arrangement is called a spectrum and is useful for indicating the relationship 

between various radiant energy wavelength regions. Such a representation 

should not be taken to mean that each region of the spectrum is physically 

divided from the others- actually there is a small but discrete transition from 

one region to the next. 

The general limits of the radiant energy spectrum extend over a range of 

wavelengths varying from 10 -16 to over 10 5 m. Radiant energy in the visible 

spectrum has wavelengths between 380 x 10 -9 and 770 x 10 -9 m. In the SI 

system, the nanometer nm (10 -9 m) and the micrometer μm (10 -6 m) are the 

commonly used units of wavelength in the visible region. In the cgs system, 

the angstrom A (10 -10 m) was used to denote wavelength. 


All forms of radiant energy are transmitted at the same speed in a vacuum: 

299,793 km•s -1 (186,282 mil•s -1 ). However, each form of energy differs in 

wavelength and therefore in frequency. The wavelength and velocity may be 

altered by the medium through which it passes but the frequency is fixed 

independently of the medium. Equation 2 shows the relationship between 

radiation velocity, frequency and wavelength. 

v = λ v/ n 

(Eq.2) 

Where: 

v = velocity of waves in the medium (meters per second); 

n = the medium's index of refraction; 

λ = wavelength in a vacuum (meters); and 

v = frequency (hertz). 


Keywords: 

• The velocity of light change in different medium with different refractive 

index 

• The wavelength and velocity may be altered by the medium through which 

it passes but the frequency is fixed independently of the medium. 

When light travelling in different medium 

• Velocity change 

• Wavelength change 

• Frequency always remain the same in all mediums 


Planck and the Quanta 

In 1900, Max Planck was working on the 

problem of how the radiation an object 

emits is related to its temperature. He came 

up with a formula that agreed very closely 

with experimental data, but the formula only 

made sense if he assumed that the energy 

of a vibrating molecule was quantized--that 

is, it could only take on certain values. The 

energy would have to be proportional to the 

frequency of vibration, and it seemed to 

come in little "chunks" of the frequency 

multiplied by a certain constant. This 

constant came to be known as Planck's 

constant, or h, and it has the value 

6.626x10 -34 J x s 

http://web2.uwindsor.ca/courses/physics/high_schools/2005/Photoelectric_effect/planck.html 


Table 1 gives the speed of light in different media for a frequency 

corresponding to a wavelength of 589 nm in air. 

TABLE 1. Speed of light for a wavelength of 589 nanometers (sodium D-lines1 

Medium 

Vacuum 

Air 100 kilopascals 

[760 mm Hg] at 0° C 

Crown glass 

Water 

Speed 10 6 meters per second 

299.793 

299.724 

198.223 

224.915 


1.3 Blackbody Radiation 

The light from common sources, particularly light from incandescent lamps, is 

often compared with light from a theoretical source known as a blackbody. 

For equal area, a blackbody radiates more total power and more power at any 

wavelength than any other source operating at the same temperature. 

For experimental purposes, laboratory radiation sources have been built to 

approximate closely a blackbody. Designs of these sources are based on the 

fact that a hole in the wall of a closed chamber, small in size compared with 

the size of the enclosure, exhibits blackbody characteristics. This can be 

understood with the help of Fig. 1. At each reflection, some energy is 

absorbed. In time, all incoming energy is absorbed by the walls. Conversely, 

a blackbody can be a perfect radiator. If the interior walls of the blackbody are 

uniformly heated, the radiation which leaves the small opening will he that of 

a perfect radiator for a specific temperature and its emission energy and 

wavelength spectrum will be independent of the nature of the enclosure. From 

1948 to 1979, the international reference standard for the unit of luminous 

intensity was taken to be the luminance of a blackbody operating at the 

temperature of freezing platinum. 


FIGURE 1. Small aperture in an enclosure exhibits blackbody characteristics 


Black-body radiation is the type of electromagnetic radiation within or 

surrounding a body in thermodynamic equilibrium with its environment, or 

emitted by a black body (an opaque and non-reflective body) held at constant, 

uniform temperature. The radiation has a specific spectrum and intensity that 

depends only on the temperature of the body. 

The thermal radiation spontaneously emitted by many ordinary objects can be 

approximated as blackbody radiation. A perfectly insulated enclosure that is 

in thermal equilibrium internally contains black-body radiation and will emit it 

through a hole made in its wall, provided the hole is small enough to have 

negligible effect upon the equilibrium. 

A black-body at room temperature appears black, as most of the energy it 

radiates is infra-red and cannot be perceived by the human eye. Because of 

the specific colour responsiveness of the human eye, a black body, viewed in 

the dark at the lowest just faintly visible temperature, subjectively appears 

grey, even though its objective physical spectrum peaks in the red range. 

When it becomes a little hotter, it appears dull red. As its temperature 

increases further it eventually becomes blindingly brilliant blue-white. 


Although planets and stars are neither in thermal equilibrium with their 

surroundings nor perfect black bodies, black-body radiation is used as a first 

approximation for the energy they emit. Black holes are near-perfect black 

bodies, in the sense that they absorb all the radiation that falls on them. It has 

been proposed that they emit black-body radiation (called Hawking radiation), 

with a temperature that depends on the mass of the black hole. 

The term black body was introduced by Gustav Kirchhoff in 1860. When used 

as a compound adjective, the term is typically written as hyphenated, for 

example, black-body radiation, but sometimes also as one word, as in 

blackbody radiation. Black-body radiation is also called complete radiation or 

temperature radiation or thermal radiation. 

http://en.wikipedia.org/wiki/Black-body_radiation 


1.3.1 Planck Radiation Law 

Data describing blackbody radiation curves have been obtained using a 

specially constructed and uniformly heated tube as the source. Planck, 

introducing the concept of discrete quanta of energy, developed an equation 

depicting these curves. It gives the spectral radiance of a blackbody as a 

function of the wavelength and temperature. Figure 2 shows the spectral 

radiance of a blackbody as a function of wavelength for several values of 

absolute temperature, plotted on a logarithmic scale. 


FIGURE 2. Blackbody radiation curves showing Wien displacement of peaks for operating 

temperatures between 500 and 20,000 K 


FIGURE 2. Blackbody radiation curves 


FIGURE 2. Blackbody radiation curves 


FIGURE 2. Blackbody radiation curves- Peak Shifts 


FIGURE 2. Blackbody radiation curves- Intensity & Peak Shifts 


The color (chromaticity) 

of black-body radiation 

depends on the 

temperature of the black 

body; the locus of such 

colors, shown here in 

CIE 1931 x,y space, is 

known as the Planckian 

locus. 

http://en.wikipedia.org/wiki/Black-body_radiation 


1.3.2 Wien Radiation Law 

In the temperature range of incandescent filament lamps (2,000 to 3,400 K) 

and in the visible wavelength region (380 to 770 nm), a simplification of the 

Planck equation, known as the Wien radiation law, gives a good 

representation of the blackbody distribution of spectral radiance. The Wien 

displacement law gives the relationship between the wavelength at which a 

blackbody at temperature T in degrees Kelvin emits maximum power per unit 

wavelength and the temperature T. In fact the product of absolute 

temperature T and the peak wavelength is a constant. It gives the relationship 

between blackbody distributions at various temperatures only with this 

important limitation. 


Wien Radiation Law 


1.3.3 Stefan-Boltzmann Law 

The Stefan-Boltzmann law is obtained by integrating Planck's expression for 

the spectral radiant excitance from zero to infinite wavelength. The law states 

that the total radiant power per unit area of a blackbody varies as the fourth 

power of the absolute temperature. The Stefan-Boltzmann law is explained in 

introductory physics texts. Note that this law applies to the total power (the 

whole spectrum) and cannot be used to estimate the power in the visible 

portion of the spectrum alone. 


1.3.4 Spectral Emissivity 

No known radiator has the same emissive power as a blackbody. The ratio of 

a radiator's output at any wavelength to that of a blackbody at the same 

temperature and the same wavelength is known as the radiator's spectral 

emissivity ε(λ). 

1.3.5 Graybody Radiation 

When the spectral emissivity is uniform for all wavelengths, the radiator is 

known as a graybody. No known radiator has a uniform spectral emissivity for 

all visible, infrared and ultraviolet wavelengths. In the visible region, a carbon 

filament exhibits very nearly uniform emissivity and is nearly a graybody. 


1.3.6 Selective Radiators 

The emissivity of all known materials varies with wavelength. In Fig. 3, the 

radiation curves for a blackbody, a graybody and a selective radiator 

(tungsten), all operating at 3,000 K, are plotted on the same logarithmic scale 

to show the characteristic differences in output. 


FIGURE 3. Radiation curves for blackbody, graybody and selective radiators operating at 

3,000 K 


FIGURE 3. Radiation curves for blackbody, graybody and selective radiators operating at 

3,000 K ~ 6,000 K 


http://www.webexhibits.org/causesofcolor/3.html

1.3.7 Color Temperature 

The radiation characteristics of a blackbody of unknown area may be 

specified with the aid of the radiation equations by modifying two quantities: 

the magnitude of the radiation at any given wavelength and the absolute 

temperature. The same type of specification may be used with reasonable 

accuracy in the visible region of the spectrum for tungsten filaments and other 

incandescent sources. However, the temperature used in the case of 

selective radiators is not that of the filament but a value called the color 

temperature. The color temperature of a selective radiator is that temperature 

at which a blackbody must be operated so that its output is the closest 

approximation to a perfect color match with the output of the selective radiator. 

While the match is never actually perfect, the small deviations that occur in 

the case of incandescent filaments are not of practical importance. 


The apertures between coils of the filaments used in many tungsten lamps 

act something like a blackbody because of the interreflections that occur at 

the inner surfaces of the helix formed by the coil. For this reason, the 

distribution from coiled filaments exhibits a combination of the characteristics 

of the straight filament and of a blackbody operating at the same temperature. 

The use of the color temperature method to deduce the spectral distribution 

from other than incandescent sources, even in the visible region, usually 

results in appreciable error. Color temperature values associated with light 

sources other than incandescent are known as correlated color temperatures 

and are not true color temperatures. 


1.4 Atomic Structure and Radiation 

The atomic theories first proposed in 1913 have been expanded and 

confirmed by much experimental evidence. The atom consists of a central 

positively charged nucleus about which revolve negatively charged electrons. 

In the normal state, these electrons remain in specific orbits or energy levels 

and radiation is not emitted by the atom. The orbit described by a specific 

electron revolving about the nucleus is determined by the energy of the 

electron (there is a particular energy associated with each orbit). An atom's 

system of orbits or energy levels is characteristic of each element and 

remains stable until disturbed by external forces. 

The electrons of an atom can he divided into two classes. The first includes 

the inner shell electrons which are removed or excited only by high energy 

radiation. The second class includes the outer shell or valence electrons 

which cause chemical bonding into molecules. Valence electrons are readily 

excited by ultraviolet or visible radiation or by electron impact and can be 

removed completely with relative ease. The valence electrons of an atom in a 

solid when removed from their associated nucleus enter the so-called 

conduction band and give the material the property of electrical conductivity. 


After absorption of sufficient energy by an atom, the valence electron is 

pushed to a higher energy level further from the nucleus. Eventually, the 

electron returns to the normal orbit or to an intermediate orbit. In so doing, the 

energy that the atom loses is emitted as a quantum of radiation and this is the 

source of light. The wavelength (or frequency) of the radiation is determined 

by Planck's equation: 

E 1 –E 2 = hv (Eq. 3) 

Where: 

E 1 

E 2 

h 

v 

= energy associated with the excited orbit; 

= energy associated with the normal orbit; 

= Planck's constant; and 

= frequency of the emitted radiation. 


Plank’s Equation 


This equation can he converted to a more practical form: 

λ = 1239.76/ V d 

Where: 

λ 

V d 

= wavelength (nanometers); 

= the potential difference between two energy levels through which 

the displaced electron has fallen in one transition (electron volts). 


1.5 Luminous Efficiency of Radiant Energy 

Many apparent differences in intensity between radiant energy of various 

wavelengths are in fact differences in the ability of various sensing devices to 

detect them. The reception characteristics of the human eye have been 

subject to extensive investigations and the results may be summarized as 

follows. 

1. The spectral response characteristic of the human eye varies between 

individuals, with time and with the age and health of an individual, to the 

extent that the selection of any individual to act as a standard observer is 

not scientifically feasible. 

2. However, from the available data, a luminous efficiency curve has been 

selected to represent a typical human observer. This curve may be applied 

mathematically to the solution of photometric problems. 


The standard spectral luminous efficiency curve for photopic (light adapted) 

vision represents a typical characteristic, adopted arbitrarily to give unique 

solutions to photometric problems, from which the characteristics of any 

individual may be expected to vary. Some data indicate that most human 

observers are capable of experiencing a visual sensation on exposure to 

radiant energy of wavelengths longer than 770 nm, called infrared under most 

circumstances, provided the radiant energy reaches the eye at a sufficiently 

high rate. It also is known that ultraviolet radiation (wavelengths less than 380 

nm) under most circumstances can be seen if it reaches the retina even at a 

moderate rate. 


Most observers yield only a slight response to ultraviolet radiation at the 

nearly visible wavelengths because the lens of the eye absorbs nearly all of it. 

Typically, human observers have a response that under normal lighting 

conditions extends from 380 to 770 manometers but some individuals have 

greater sensitivity at the blue and/or red ends of this range. Of course, at 

lower lighting levels even the average observer experiences a shift of the 

visible spectrum to shorter wavelengths and vice versa at higher lighting 

levels. The spectral range of visible response is therefore not static but 

greatly dependent on the lighting conditions. 


1.6 Luminous Efficiency of Light Sources 

The luminous efficiency of a light source is defined as the ratio of the total 

luminous flux (lumens) to the total power input (watts or equivalent). 

The maximum luminous efficiency of an ideal white source (defined as a 

radiator with constant output over the visible spectrum and no radiation in 

other parts of the spectrum) is about 220 lm•W -1 


PART 2: MEASUREMENT OF THE PROPERTIES OF LIGHT 

2.0 General: 

The most widely used detector of light is the human eye. Other common, 

mechanical detectors are photovoltaic cells, photoconductive cells, 

photoelectric tubes, photodiodes, phototransistors and photographic film. 


2.1 Photovoltaic Cells 

Photovoltaic cells typically include selenium barrier layer cells and silicon or 

gallium arsenide, photodiodes operated in the photovoltaic or unbiased mode. 

These devices depend on the generation of a current resulting from the 

absorption of a photon. The cell is comprised of 

1. a p-type material, typically a metal plate coated with a semiconductor, 

such as selenium on iron; and 

2. a semitransparent n-type material such as cadmium oxide. 

Unless there is an external circuit, electrons liberated from the semiconductor 

are trapped at the p-n junction after exposure to light. The device thereby 

converts radiant energy to electric energy, which can be used directly or 

amplified to drive a micro-ammeter (see Fig. 4). Photovoltaic cells can be 

filtered to correct their spectral response so that the micro-ammeter can be 

calibrated in units of illuminance. Factors such as response time, fatigue, 

temperature effects, linearity stability, noise and magnitude of current 

influence the choice of cell and circuit for a given application. 


FIGURE 4. Cross section of a barrier layer photovoltaic cell showing motion of photoelectrons 

through a micro-ammeter circuit 











2.2 Photoconductor Cells 

Photoconductor cells depend on the resistance of the cell changing directly 

as a result of photon absorption. These detectors use materials such as 

cadmium sulfide, cadmium selenide and selenium. Cadmium sulfide and 

cadmium selenide are available in transparent resin or glass envelopes and 

are suitable for low illuminance levels less than 10 -4 lx (10 -5 ftc). 


Photoconductor Cells 


2.3 Photoelectric Tubes 

The photoelectric effect is the emission of electrons from a surface 

bombarded by sufficiently energetic photons. If the surface is connected as a 

cathode in an electric field (see Fig. 5), the liberated electrons flow to the 

anode, creating a photoelectric current. An arrangement of this sort may be 

used as an illuminance meter and can be calibrated in lux or footcandles. 

The photoelectric current in vacuum varies directly with the illuminance level 

over a wide range (spectral distribution, polarization and cathode potential 

remain the same). In gas filled tubes, the response is linear only over a 

limited range. If the radiant energy is polarized, the photoelectric current 

varies as the orientation of the polarization is changed (except at normal 

incidence). 


FIGURE 5. By the photoelectric effect, electrons may be liberated from an Illuminated metal 

surface, flowing to an anode and creating an electric current that may be detected by a 

galvanometer (see Eq. 1 and 2) 


Photoelectric Effects 


Photoelectric Effects & Secondary radiation 


Photoelectric Effects 

http://whs.wsd.wednet.edu/faculty/busse/mathhomepage/busseclasses/radiationphysics/lecturenotes/chapter12/chapter12.html 


2.4 Photodiodes and Phototransistors 

Photodiodes or junction photocells are based on solid state p-n junctions that 

react to external stimuli such as light. Conversely, if properly constructed, 

they can emit radiant energy (light emitting diodes). In a photosensitive diode, 

the reverse saturation current of the junction increases in proportion to the 

illuminance. Such a diode can therefore be used as a sensitive detector of 

light and is particularly suitable for indicating extremely short pulses of 

radiation because of its very fast response time. Phototransistors operate in a 

manner similar to photodiodes but, because they provide an additional 

amplifier effect, they are many times more sensitive than simple photodiodes. 


Photodiode 


Schematic cross section of an integrated CMOS single-photon-counting 

avalanche diode (SPAD) device.2HV: High-voltage. p, n: Semiconductor 

materials. 

http://spie.org/x93517.xml 


Photodiode 


Phototransistors 


Phototransistors 


2.5 Photometry 

Progress in the sciences is often dependent on our ability to measure the 

physical quantities associated with the technology- each advance in 

measurement ability or accuracy provides a broadening of the science's 

knowledge base. The measurement of light and its properties is called 

photometry. The basic measuring instrument is known as a photometer. The 

earliest photometers depended on visual appraisal by the operator as the 

means of measurement and such meters are rarely used now. They have 

been replaced by nonvisual meters that are sensitive to light's physical 

properties, measuring radiant energy incident on a receiver, producing 

measurable electrical quantities. Physical photometers are more accurate 

and simpler to operate than their earlier counterparts. 


2.5.1 Observer Response Curves 

Light measurements by physical photometers are useful only if they indicate 

reliably how the eye reacts to a certain stimulus. In other words, the 

photometer should be sensitive to the spectral power distribution of light in the 

same way that the eye is. Because of the substantial differences between 

individual eyes, standard observer response curves or eye sensitivity curves 

have been established. The sensitivity characteristics of a physical 

photometer should be equivalent to the standard observer. The required 

match is typically achieved by adding filters between the sensitive elements 

of the meter and the light source. 


2.5.2 Photopic and Scotopic Vision 

The human eye contains two basic types of retinal receptors known as rods 

and cones. They differ not only in relative spectral response and other 

properties but by orders of magnitude in responsivity. The rods are the most 

sensitive and spectrally respond more to the blue and less to the red end of 

the spectrum. However, they do not actually give the sensation of color as the 

cones do. Luminance is measured in candelas (cd). When the eye has been 

subjected to a field luminance of more than 3.0 cd•m -2 (0.27 cd•ft -2 ) for more 

than a few minutes, the eye is said to he in a light adapted state in which only 

the cones are responsible for vision; the state is also known as photopic 

vision or fovea/ vision. At light levels five or more orders of magnitude below 

this, at or below 3.0 x 10 -5 cd.m -2 (2.7 x 10 -6 cd•ft -2 ), the cones no longer 

function and the responsivity is that of the rods. This is known as dark 

adapted, or scotopic vision or parafoveal vision. After being light adapted, the 

eye usually requires a considerable time to become dark adapted when the 

light level is lowered. The time needed depends on the initial luminance of the 

starting condition but is usually achieved in 30 to 45 minutes. 


Photopic and Scotopic Vision 


http://www.solarlightaustralia.com.au/2013/05/30/photopic-scotopic-and-mesotopic-lumens/



Between the levels at which the eye exhibits photopic and scotopic responses 

the spectral and other responses of the eye are continuously variable; this is 

known as the mesopic state, in which properties of both cone and rod 

receptors contribute. Many visual tests are made under photopic conditions 

but most measurements of fluorescent and phosphorescent materials are 

made under scotopic and mesopic conditions. Because of the changes in the 

eye's spectral response at these levels it is necessary to take luminance into 

account when evaluating the results of such measurements. 




http://lumenistics.com/consider-photopic-scotopic-mesopic-vision-before-specifying-lumen-requirements/

2.5.3 Measurable Quantities 

As indicated in Table 2, many characteristics of light, light sources, lighting 

materials and lighting installations may be measured, including 

1. illuminance, 

2. luminance, 

3. luminous intensity, 

4. luminous flux, 

5. contrast, 

6. color (appearance and rendering), 

7. spectral distribution, 

8. electrical characteristics and 

9. radiant energy. 


TABLE 2. Measurable characteristics of light, light sources and lighting materials 


2.6 Principles of Photometry 

2.6.0 General 

Photometric measurements frequently involve a consideration of the cosine 

law and the inverse square law (strictly applicable only for point sources). 

2.6.1 Inverse Square Law 

The inverse square law (see Fig. 6a) states that the illumination E (in lux) at a 

point on a surface varies directly with the luminous intensity I of the source 

and inversely as the square of the distance d between the source and the 

point. If the surface at the point is normal to the direction of the incident light, 

the law may be expressed as: 

E= I/d 2 (Eq. 5) 

This equation is accurate within 0.5 percent when d is at least five times the 

maximum dimension of the source, as viewed from the point on the surface. 


Inverse Square Law 

http://pondscienceinstitute.on-rev.com/svpwiki/tiki-index.php?page=Square%20Law 


2.6.2 Lambert Cosine Law 

The Lambert cosine law (see Fig. 6b) states that the illuminance of any 

surface varies as the cosine of the angle of incidence. The angle of incidence 

0 is the angle between the normal to the surface and the direction of the 

incident light. The inverse square law and the cosine law can be combined 

to yield the following relationship (in lux): 

E = I/d 2 Cos ϴ (Eq. 6) 


Lambert Cosine Law 


http://webx.ubi.pt/~hgil/FotoMetria/HandBook/ch06.html

2.6.3 Photometric Reference Standards 

Reference standards for candlepower, luminous flux and color are 

established by national standard laboratories. A primary reference standard is 

reproducible from specifications and is typically found only in a national 

laboratory. Secondary reference standards are usually derived directly from 

primary standards and must be prepared using precise electrical and 

photometric equipment. Preservation of the reference standard's rating is very 

important. Accordingly, a standard is used as seldom as possible and is 

handled and stored with care. Photometric reference lamps are used when 

accuracy warrants the highest attainable precision. Because of the cost of 

reference standards, so-called working standards are prepared for frequent 

use A laboratory can prepare its own working standards for use in calibrating 

photometers. The working standard is not used to conduct a test, except 

where a direct comparison is necessary. 


2.6.4 Photometric Applications 

Photometric measurements make use of the basic laws of photometry, 

applied to readings from visual photometric comparison or photoelectric 

instruments. Various procedures are discussed below Direct photometry is 

the simultaneous comparison of a standard lamp and an unknown light 

source. Substitution photometry is the sequential evaluation of the desired 

photometric characteristics of a standard lamp and an unknown light source 

in terms of an arbitrary reference. 

To avoid the use of standard lamps, relative photometry is often used. 

Relative photometry is the evaluation of a desired photometric characteristic 

based on an assumed lumen output of the test lamp. Alternately, relative 

photometry refers to the measurement of one uncalibrated light source to 

another uncalibrated light source. It is sometimes necessary to measure the 

output of sources that are nonsteady or cyclic and, in such cases, extreme 

care should be taken. 


2.6.5 Means of Achieving Attenuation 

During photometric measurement, it often becomes necessary to reduce the 

luminous intensity of a source in a known ratio to bring it within the range of 

the measuring instrument. A rotating sector disk with one or more angular 

apertures is one means of doing this. If such a disk is placed between a 

source and a surface and is rotated at such speed that the eye perceives no 

flicker, the effective luminance of the surface is reduced in the ratio of the 

time of exposure to the total time (Talbot's law). The reduction is by the factor 

ϴ/360 degrees. The sector disk has advantages over many filters: (1) it is not 

affected by a change of characteristics over time and (2) it reduces luminous 

flux without changing its spectral composition. Sector disks should not be 

used with light sources having cyclical variation in output. 


Various types of neutral filters of known transmittance are also used for 

attenuation. Wire mesh or perforated metal filters are perfectly neutral but 

have a limited range. Partially silvered mirrors have high reflectance but the 

reflected light must be controlled to avoid errors in the photometer. When a 

mirror filter is perpendicular to the light source photometer axis, serious errors 

may be caused by multiple reflections between the filter and receiver surface. 

This can be avoided by mounting the filter at a small angle (not over 3 

degrees) from perpendicular at a sufficient distance from the receiver surface 

to throw reflections away from the photometric axis. In this canted position, 

care must be taken not to reflect light from adjacent surfaces on to the 

receiver. Also, it is difficult to secure completely uniform transmission over all 

parts of the surface. So-called neutral glass filters are seldom neutral and 

transmission characteristics should be checked before use. In general, they 

have a characteristic high transmittance in the red and low transmittance in 

the blue, so that spectral correction filters may be required. However, this 

type of filter varies in transmittance with ambient temperature, as do many 

other optical filters. 


Neutral gelatin filters are satisfactory, although not entirely neutral. Some 

have a small seasoning effect (losing neutrality over a period of time). They 

must be protected by mounting between two pieces of glass and must be 

watched carefully for loss of contact between the glass and gelatin. Filters 

should not be stacked unless cemented, because of errors that may be 

created by interference between surfaces. With modem metering techniques, 

electronic alterations can be accomplished to keep the output of a receiver 

and amplifier combination in range of linearity and readability. 


2.7 Photometers 

A photometer is a device for measuring radiant energy in the visible spectrum. 

Various types of physical instruments consist of an element sensitive to 

radiant energy and appropriate measuring equipment and are used to 

measure ultraviolet and infrared energy. When used with a filter to correct 

their response to the standard observer, such devices can measure visible 

light. In general, photometers may be divided into two types: 

1. laboratory photometers are usually fixed in position and yield results of 

highest accuracy, 

2. portable photometers are of lower accuracy for making measurements in 

the field. 


Both types of meters may be grouped according to function, such as the 

photometers used to measure luminous intensity (candlepower), luminous 

flux, illuminance, luminance, light distribution, light reflectance and 

transmittance, color, spectral distribution and visibility Illuminance 

Photometers. Visual photometric methods have largely been supplanted 

by physical methods. Because of their simplicity, vision based photometry 

methods are still used in educational laboratories for demonstrating 

photometric principles and for less routine types of photometry. 

Photoelectric photometers' may be divided into two classes: 

1. those employing solid state devices such as photovoltaic and 

photoconductive cells and 

2. those employing photoemissive tubes. 


Photometry 


http://safety.fhwa.dot.gov/roadway_dept/night_visib/lighting_handbook/

2.8 Photovoltaic Cell Meters 

2.8.0 General 

A photovoltaic cell is one that directly converts radiant energy into electrical 

energy. It provides a small current that is about proportional to the incident 

illumination and also produces a small electromotive force capable of forcing 

this current throtigh a low resistance circuit. Photovoltaic cells provide much 

larger currents than photoemissive cells and can directly operate a sensitive 

instrument such as a microammeter or galvanometer. However, as the 

resistance of their circuit increases, photovoltaic cells depart from linearity of 

response at higher levels of incident illumination. Therefore, for precise 

results, the external circuitry and metering should apply nearly zero 

impedance across the photocell. 


Some portable illuminance meters consist of a photovoltaic cell or cells, 

connected to a meter calibrated directly in lux or footcandles. However, with 

solid state electronic devices, operational amplifiers have been used to 

amplify the output of photovoltaic cells. The condition that produces the most 

favorable linearity between cell output and incident light is automatically 

achieved by reducing the potential difference across the cell to zero. The 

amplifier power requirements are small and easily supplied by small batteries. 

In addition, digital readouts may be conveniently used to eliminate the 

ambiguities inherent in deflection instruments. 


2.8.1 Spectral Response 

The spectral response of photovoltaic cells is quite different from that of the 

human eye and color correcting filters are usually needed.."-" As an example, 

Fig. 7 illustrates the degree to which a typical commercially corrected 

selenium photovoltaic cell commonly used in illuminance meters 

approximates the standard spectral luminous efficiency curve. Cells vary 

considerably in this respect and for precise laboratory photometry each cell 

should be individually color corrected. 

The importance of color correction can be illustrated by comparing the human 

eye match under illumination generated by a monochromatic source. For 

example, if a predominant blue light source is used, the majority of the 

visible energy is concentrated near 465 nm. It can be seen in Fig. 7 that the 

relative eye response and the filtered receptor response are about 10 and 15 

percent. This represents a 50 percent differential and indicates that the 

photoreceptor could read as much as 50 percent high under the blue light 

source. Care should be taken to correct for this difference. 


FIGURE 7. Average spectral sensitivity characteristics of selenium photovoltaic cells, compared 

with relative eye response (luminous efficiency curve) 


2.8.2 Transient Effects 

When exposed to constant illumination, the output of photovoltaic cells 

requires a short finite rise time to reach a stable output. The output may 

decrease slightly over time because of fatigue. 4U-42 Rise times for silicon 

cells often are considerably shorter than for selenium cells. 


2.8.3 Effect of Incidence Angle 

At high incidence angles, part of the light reaching a photovoltaic 

cell is reflected by the cell surface and the cover glass and some may be 

obstructed by the rim of the case. The resulting error increases with angle of 

incidence. When an appreciable portion of the flux comes at wide angles, an 

uncorrected meter may read illuminance as much as 25 percent below the 

true value. The cells used in most illuminance meters are provided with 

diffusing covers or some other means of correcting the light sensitive surface 

to approximate the true cosine response. 

The component of illuminance contributed by single sources at wide angles of 

incidence may be determined by positioning the cell perpendicular to the 

direction of the light and multiplying the reading by the cosine of the incidence 

angle. 


The possibility of cosine error must be taken into consideration for some 

laboratory applications of photovoltaic cells. One satisfactory solution to the 

problem consists of placing a nonfluorescent opal diffusing acrylic plastic disk 

with a matte surface over the cell. At high angles of incidence, the disk 

reflects the light specularly so that the readings are too low. This can be 

compensated by allowing light to reach the cell through the edges of the 

plastic. The readings at very high angles are then too high but can be 

corrected using a screening ring. In general it is important that the opal 

diffusing plastic disk with a matte surface should be nonfluorescent or 

erroneous values of illuminance may be obtained in the presence of blueviolet 

and ultraviolet radiations; such a situation is common in fluorescent 

penetrant and magnetic particle testing applications in which measurements 

of the ambient visible light, in he presence of the blacklight are required by 

certain industrial and military specifications. Certain photometers are actually 

provided with fluorescent diffusers and should be avoided in such situations. 


2.8.4 Effect of Temperature. 

Wide temperature variations affect the performance of photovoltaic cells, 

particularly when the external resistance of the circuit is high. Prolonged 

exposure to temperatures above 50 °C (120 °F) permanently damages 

selenium cells. Silicon cells are considerably less affected by temperature. 

Measurements at high temperatures and at high illuminance levels should 

therefore be made rapidly to avoid overheating the cell. Hermetically sealed 

cells provide greater protection from the effects of temperature and humidity. 

When using photovoltaic cells at other than their calibrated temperature, 

conversion factors may be used or means may be provided to maintain cell 

temperatures near 25 °C (77 °F). 


2.8.5 Effect of Cyclical Variation of Light 

When electric discharge sources are operated on alternating current power 

supplies, precautions should be taken with regard to the effect of frequency 

on photocell response. In some cases, these light sources may be modulated 

at several kilohertz. Consideration should then be given to whether the 

response of the cell is exactly equivalent to the Talbot's law response of the 

eye for cyclic varying light. Because of the internal capacitance of the cell, it 

cannot always be assumed that its dynamic response exactly corresponds to 

the mean value of the illuminance. It has been found that a low or zero 

resistance circuit is the most satisfactory for determining the average intensity 

of modulated or steady state light sources with which photovoltaic cell 

instruments are generally calibrated. Although a microammeter or 

galvanometer appears to register a steady photocell current, it may not be 

receiving such a current. The meter actually may be receiving a pulsating 

current which it integrates because its natural period of oscillation is long 

compared to the pulses. Meters are available that can average over a period 

of time, eliminating the effect of cyclic variation. 


2.8.6 Photometer Zeroing 

It is important to check photometer zeroing before taking measurements. If an 

analog meter is used, this requires manual positioning of the indicator to zero. 

For any type of equipment using an amplifier, it may be necessary to zero 

both the amplifier and the dark current (current flowing through the device 

while it is in absolute darkness). When possible, it should be verified that the 

meter remains correctly zeroed when the photometer scale is changed. 

Alternately, any deviation from zero under dark current conditions may be 

measured and subtracted from the light readings. 


2.8.7 Electrical interference 

With electronic meters, care should be taken to eliminate interference induced 

in the leads between the cell and the instrumentation. This can be achieved 

by filter networks, shielding, grounding or combinations of the above. 


2.9 Meters Using Photomultiplier Tubes 

2.9.0 General 

Photoelectric tubes produce current when radiant energy is received on a 

photoemissive surface and then amplified by a phenomenon known as 

secondary emission. These tubes require a high voltage to operate (2 to 5 kV) 

and an amplifier to provide a measurable signal. The resulting current may 

be measured by a deflection meter, oscillograph or a digital output device. 

Dark current (current flowing through the device while it is in absolute 

darkness) must be compensated for in the circuitry or subtracted from the 

lighted tube output. Meters using this device are often extremely sensitive. 

Photomultiplier tubes can he damaged by shock and the calibration of the 

meter can be altered by strong magnetic fields. In addition, the device is 

temperature sensitive and should be operated at or below room temperature. 

As with other photoelectric devices, the photomultiplier spectral response 

curve does not match the human eye and color correcting filters are required. 


Because of the large number of photomultiplier types available, 

manufacturers commonly supply the proper optical filter for their design. 

When a photomultiplier tube is used in conjunction with an optical lens 

system, the resulting luminance meter can be of high sensitivity and broad 

range. 

Keywords: 

Secondary emmision 


2.9.1 Luminance Photometers 

The basic principles for the measurement of illuminance apply equally well for 

the measurement of luminance. Luminance meters are essentially a 

photoreceptor in front of a focusing mechanism. By suitable optics, the 

luminance of a certain size spot, when cast onto the receptor, generates an 

electrical signal that is dependent on the object luminance. This signal can be 

measured and, assuming that the necessary calibration has been performed, 

a reading is produced that directly measures luminance. Usually an eyepiece 

is provided so that the user is able to see the general field of view through the 

instrument. By changing the lens system in front of the photoreceptor, 

different fields of view and therefore different sizes of measurement area may 

be achieved. This can vary from areas subtending a few minutes of arc up to 

several degrees. Photoreceptors may be selenium but are usually silicon 

cells or photomultipliers. The meter reading may be analog or digital and 

either built into the meter or remote. Amplifier controls for zeroing and scale 

selection are usually provided. Other options include optical filters for color 

work or scale selection by means of neutral density filters. 


Luminance Photometers 


2.9.1 Brightness Spot Meter 

The brightness spot meter is a photoelectric device for measuring the 

luminance of small areas, typically 0.25, 0.5 or 1 degree field of view. A beam 

splitter allows a portion of the light from the objective lens to reach a reticule 

viewed by the eyepiece. 

The remainder of the light is reflected in front of the photomultiplier 

tube. The output of the tube after amplification is read on a microammeter 

with a scale calibrated in candelas per square meter or footlamberts. One of 

the filters provided for such instruments corrects the response of the 

photomultiplier to the standard spectral luminous efficiency curve. Full scale 

deflection is produced by 10 -1 to 10 -7 cd•m -2 


2.9.3 High Sensitivity Photometer 

One version of the photomultiplier photometer has interchangeable field 

apertures covering fields from arc minutes to 3 degrees in diameter. Full scale 

sensitivity ranges are from 10 -4 to 10 8 cd•m -2 . In this meter, readings of the 

measured light are free from the effects of polarization because there are no 

internal reflections of the beam. The spectral response of each photometer 

is individually measured. The filters to match it best to the standard spectral 

luminous efficiency curve are then inserted. Filters are also included to permit 

evaluation of polarization and color factors. 


High Sensitivity Photometer 


High Sensitivity Photometer 


2.10 Equivalent Sphere Illumination 

2.10.1 Photometers 

Equivalent sphere illumination (ESI) may be used as a tool as part of the 

evaluation of lighting systems. The equivalent sphere illumination of a visual 

task at a specific location in a room illuminated with a specific lighting system 

is defined as that level of perfectly diffuse (spherical) illuminance that makes 

the visual task as visible in the sphere as it is in the real lighting environment. 

Measurements may be made visually and/or physically. In the visual method 

the measurement is made by comparison between a task viewed in the 

measured (actual) environment and the task viewed in a luminous sphere by 

using a visibility meter. The physical method, however, is based on certain 

algorithms and requires only measurements in situ of background illuminance 

L b and task illuminance L t . All physical equivalent sphere illumination devices 

measure L b and L t in one way or another. Measuring devices are discussed 

below in chronological order of development. 


2.10.2 Visual Task Photometer 

The visual task photometer is a basic, reference instrument against which 

others are often compared. Equivalent sphere illumination is not measured 

directly: L b and L t are measured and equivalent sphere illumination is 

subsequently calculated. The task to be evaluated is mounted on a target 

shifter and a telephotometer is aimed at it from the desired viewing angle. 

The task and telephotometer (usually mounted on a cart) are then positioned 

so that: 

■ 

■ 

the task is in the location where the measurement is to he made and 

the telephotometer is facing the proper direction of view. 

The standard body shadow (attached to the telephotometer) shades the task 

in a manner similar to an actual observer. The L b value is measured, then the 

shifter is activated to bring the target into view of the telephotometer. The L t 

value is then measured. 


2.10.3 Visual Equivalent Sphere Illumination Meter 

The visual equivalent sphere illumination meter' consists of an optical system, 

variable luminous veil, target carrier, luminous sphere, illuminance meter 

(inside the sphere) and a body shadow. A task is placed on the target carrier 

and viewed through the optical system. The contrast of the task is then 

reduced to threshold by adjusting a variable luminous veil. Field luminance is 

automatically kept constant so as not to alter the adaptation luminance of the 

observer's eye. The task is then carried inside the sphere and the optical 

system is adjusted until the target is again at threshold (task visibility is the 

same inside the sphere as it was outside). The illuminance in the sphere is 

measured directly to determine equivalent sphere illumination. 


Visual Equivalent Sphere Illumination Meter 










2.10.4 Physical Equivalent Sphere Illumination Meters 

Two devices are available that do not rely on the actual presence of a task for 

their precision. Instead, they use numerical data that represents the task's 

reflectance characteristics: bidirectional reflectance distribution functions. 

One meter uses cylinders that represent an optical analog of the visual task 

photometer." There are two cylinders used per measurement: one 

representing a task's Lb is called the background cylinder and one 

representing L b -L t is called the difference cylinder. These two parameters 

can be used to calculate equivalent sphere illumination in place of L b and 

L t alone. Each cylinder has its own body shadow. A cosine corrected 

illuminance probe is placed where the measurement is desired. The 

background cylinder is placed atop the probe and oriented in the appropriate 

viewing direction. 


Background illuminance is then recorded. The background cylinder is 

replaced by the difference cylinder, oriented in the same direction and the 

difference illuminance is recorded. Equivalent sphere illumination is then 

calculated from the background and difference illuminance readings. 

A second meter using bidirectional reflectance distribution functions is a 

scanning luminance meter." This instrument contains a narrow field 

luminance probe attached to a motorized scanning apparatus and a 

minicomputer to control scanning, store the distribution function data and 

perform calculations. To use this device for measuring equivalent sphere 

illumination, the probe is positioned at the desired location and the 

minicomputer is instructed to begin scanning. Luminances are multiplied by 

their appropriate bidirectional reflectance distribution functions to determine 

L b and L t . The minicomputer then calculates equivalent sphere illumination 

directly. The scanning luminance meter has the capabilities of rotating 

the task in any viewing direction and of determining the equivalent sphere 

illumination on different tasks with only one set of scanning measurements. 


2.10.5 Visual Task Photometers 

The bidirectional reflectance distribution functions used with physical meters 

are obtained by illuminating a task from a particular direction and by viewing 

the task from some other unique direction. A visual task photometer is used to 

perform these measurements. The visual task photometer is the same as that 

used for equivalent sphere illumination measurements except that it includes 

a collimated light source that can be positioned anywhere on a hemisphere 

over the task. The task is illuminated from each azimuth and declination angle 

(usually in 5 degree increments) and the reflectance is measured at each 

angle. The collection of bidirectional reflectance data for the task and its 

background form the distribution function. 


2.11 Reflectometers 

Reflectometers are photometers used to measure reflectance of materials or 

surfaces in specialized ways. The reflectometer measures diffuse, specular 

and total reflectance. Those instruments designed to determine specular 

reflectance are known as glossmeters. One popular reflectometer uses a 

collimated beam and a photovoltaic cell. The beam source and cell are 

mounted in a fixed relationship in the same housing. The housing has an 

aperture through which the beam travels. This head or sensor is set on a 

standard reflectance reference with the aperture against the standard. The 

collimated beam strikes the standard at a 45 degree angle. The photovoltaic 

cell is constructed so that it measures the light reflected at 0 degrees from the 

standard. The instrument is then adjusted to read the value stated on the 

standard. The sensor is placed on the test surface and the reading is 

recorded. Two cautions are recommended for use of reflectometers. The 

reference standard should be in the range of the value expected for the 

surface to be measured. Also, if the area to be considered is large, several 

measurements should he taken and averaged to obtain a representative 

value. 


Another type of reflectometer (see Fig. 8) measures both total reflectance and 

diffuse transmittance."' The instrument consists of two spheres, two light 

sources and two photovoltaic cells. The upper sphere is used alone for the 

measurement of reflectance. The test object is placed over an opening at the 

bottom of the sphere and a collimated beam of light is directed on it at about 

30 degrees from normal. The total reflected light, integrated by the sphere, is 

measured by two cells mounted in the sphere wall. The tube carrying the light 

source and the collimating lenses is then rotated so that the light is incident 

on the sphere wall and a second reading is taken. The test object is in place 

during both measurements, so that the effect on both readings of the small 

area of the sphere surface it occupies is the same. The ratio of the first 

reading to the second reading is the reflectance of the object for the 

conditions of the test. Test objects made of translucent materials should be 

backed by a nonreflecting diffuse material. Transmittance for diffuse incident 

light is measured by using the light source in the lower sphere and taking 

readings with and without the test object in the opening between the two 

spheres. The introduction of the test object changes the characteristics of the 

upper sphere. Correction must be made to compensate for the introduced 

error. 


Various instruments are available for measuring such properties as specular 

reflectance and the gloss characteristics of materials. For example, an 

instrument similar to that described above for the measurement of diffuse 

reflectance may be used, except that the cell is fixed at 45 degrees on the 

side of the test object opposite to the light source, thus measuring the 

specularly reflected beam. The angle subtended by the photocell to the test 

object affects the reading and appropriate compensations are recommended. 


FIGURE 8. A light cell reflectometer In an arrangement for transmittance measurement 


2.12 Radiometers 

Radiometers are sometimes called radiometric photometers and are used to 

measure radiant power over a wide range of wavelengths, including the 

ultraviolet, visible or infrared spectral regions. Radiometers may use detectors 

that are nonselective in wavelength response or that give adequate 

response in the desired wavelength band. Nonselective detectors (response 

varies little with wavelength) include thermocouples, thermopiles, bolometers 

and pyroelectric detectors. One class of wavelength selective detectors is 

photoelectric and includes photoconductors, photoemissive tubes, 

photovoltaic cells and solid state sensors such as photodiodes, 

phototransistors and other junction devices. The overall response of such 

detectors can he modified by using appropriate filters to approximate some 

desired function. For example, these detectors can be color corrected by 

means of a filter to duplicate the standard luminous efficiency curve in the 

visible range or to level a detector's response to radiant power over some 

hand of wavelengths. The corrections must compensate for any selectivity in 

the spectral response of the optical system. Care must be exercised to 

eliminate a detector's response to radiation lying outside the range of interest. 


When a monochromator is used to disperse the incoming radiation, the 

radiant power can be determined in a very small band of wavelengths. Such 

an instrument is called a spectroradiometer and is used to determine the 

spectral power distribution (the radiant power per unit wavelength as a 

function of wavelength) of the radiation in question. The spectral power 

distribution is fundamental; from it radiometric, photometric and colorimetric 

properties of the radiation can be determined. The use of digital processing 

has greatly facilitated both the measurement and the use of the spectral 

power distribution. The range of spectral response generally depends on the 

nature of the detector. Photomultiplier tubes extend wavelength sensitivity 

from 125 to 1,100 nm. 


Various types of silicon photodiodes cover the range from 200 to 1,200 nm. In 

the infrared range are intrinsic germanium (0.9 to 1.5 μm), lead sulfide (1.0 to 

4.0 μm), indium arsenide (1.0 to 3.6 μm), indium antimonide (2.0 to 5.4 μm), 

mercury cadmium telluride (1.0 to 13 μm) and germanium doped with various 

substances such as zinc (2.0 to 40 μm). The response of nonselective 

detectors ranges from near ultraviolet to 30μm (300,000 A) and beyond. The 

electrical output of detectors (voltage, current or charge) is very small and 

special precautions are often required to achieve acceptable signal levels, 

signal-to-noise ratios and response times (for rapidly varying signals). Photon 

counting and charge integration techniques are used for extremely low 

radiation level. 


In all radiometric work, it is important to avoid stray radiation and care must 

be taken to ensure its exclusion. This is difficult because stray radiation is not 

visible and a surface seen as black may actually be an excellent reflector of 

radiant energy outside the visible spectrum. Often, unwanted radiation can be 

absorbed by an appropriate filter. Sometimes such a high flux must be 

removed to avoid the absorption filter's heating to the point of breaking or its 

transmittance for other desired wavelengths is altering. Because radiated flux 

of some wavelengths is dispersed or absorbed by a layer of air between the 

radiator and the detector, consideration must be given to the placement of the 

source and the detector and to the medium surrounding them. 


2.13 Spectrophotometers 

Photometry is the measurement of power in the visible spectrum, weighted 

according to the visual response curve of the eye. When the power is 

measured as a function of wavelength, the measurement is referred to as 

spectrophotometry. Its applications extend from precise quantitative chemical 

analysis to the exact determination of the physical properties of matter. 

Spectrophotometry is important for the determination of spectral 

transmittance and spectral reflectance. It is also applied to the measurement 

of the spectral emittance of lamps, in which case it is known as 

spectroradiometry. This form of measurement commonly covers the visible 

portion of the spectrum, the ultraviolet and near infrared wavelengths. 

Instruments used for performing such measurements are called 

spectrophotometers and spectroradiometers. 


These devices consist basically of a monochromator (separates or disperses 

the wavelengths of the spectrum using prisms or gratings) and a receptor 

(measures the power contained within a certain wavelength range of the 

dispersed light). If 

the spectrum is examined visually rather than by a photoreceptor, 

the instrument is known to as a spectroscope. 

In the visible spectrum, the only fundamental means of 

examining a color for analysis, standardization and specification 

is by spectrophotometry. In addition, this is the only 

means of color standardization that is independent of material 

color standards (always of questionable permanence) and 

independent of the abnormalities of color vision existing 

among so-called normal observers. 

Commercial development of spectrophotometers has 

extended the wavelength range from about 200 to 2,500 nm, 

made them automatically record and added tristimulus integration. 

Self scanned silicon photodiode arrays provide 

nearly instantaneous determination of spectral power 

distributions. 


Spectrophotometers 












http://www.nature.com/nature/journal/v512/n7512/full/nature13382.html

2.14 Types of Photometers 

2.14.1 Optical Bench Photometers 

Optical bench photometers are used for the calibration of instruments for 

illumination measurement. They provide a means for mounting light sources 

and photocells in proper alignment and a means for easily determining the 

distances between them. If the source is of known luminous intensity 

(candlepower), the inverse square law is used to compute illuminance, 

provided that the source-to-detector distance is at least five times the 

maximum source dimension. 


2.14.2 Distribution Photometers 

Luminous intensity (candlepower) measurements are made on a distribution 

photometer which may be one of the following types: 

(1) goniometer and single cell, (2) fixed multiple cell, (3) moving cell and (4) 

moving mirror. 

All types of photometers have advantages and disadvantages. The 

significance attached to each advantage or disadvantage depends on factors 

such as available space and facilities, polarization requirements and 

economic considerations. 


2.14.3 Goniometer and Single Cell 

The light source is mounted on a goniometer, which allows the source to 

rotate about horizontal and vertical axes. The candlepower is measured by a 

single fixed cell. There are several kinds of goniometers, each related to the 

type of source being photometered and the facilities in which it is located. 

With the use of computers, the coordinate system of one goniometer system 

can be easily changed to another coordinate system and the compatibility of 

data reporting becomes practical. Figure 9 shows two types of goniometer 

systems. 


FIGURE 9. Goniometer variations: (a J the projector turns about a fixed horizontal axis and about 

a second axis which, in the position of rest, is vertical and, on rotation, follows the movement of 

the horizontal axis; and (b) the light source turns about a fixed vertical axis and also about a 

horizontal axis following the movement of the vertical axis; the grid lines shown represent the loci 

traced by the photocell as the goniometer axes are rotated 


2.14.4 Fixed Multiple Cell Photometer 

In a multiple cell photometer, many individual photocells are positioned at 

various angles around the light source under test. Readings are taken on 

each photocell to determine the light intensity or candlepower distribution 

(see Fig. 10). 


FIGURE 10. Schematic side elevation of a fixed multiple cell photometer 


2.14.5 Moving Cell Photometer 

The moving cell photometer (Fig. 11) has a photocell that rides on a rotating 

boom or an arc shaped track. The light source is centered in the arc traced by 

the cell. Readings are collected with the cell positioned at the desired angular 

settings. Sometimes a mirror is placed on a boom to extend the test distance. 


FIGURE 11. Schematic side elevation of a moving cell photometer 


2.14.6 Moving Mirror Photometer 

In the moving mirror photometer, a mirror rotates around the light source, 

reflecting the candlepower to a single photocell. Readings are taken at each 

angle as the mirror moves to that location. 


2.14.7 Integrating Sphere Photometer 

The total luminous flux from a source can be measured by a form of integrator 

sphere. Other geometric forms are sometimes used. The theory of the 

integrating sphere assumes an empty sphere whose inner surface is perfectly 

diffusing and of uniform nonselective reflectance. Every point on the inner 

surface reflects to every other point and the illuminance at any point is made 

up of two components: the flux coming directly from the source and that 

reflected from other parts of the sphere wall. With these assumptions, it 

follows that, for any part of the wall, the illuminance and the luminance from 

reflected light only is proportional to the total flux from the source, regardless 

of its distribution. The luminance of a small area of the wall or the luminance 

of the outer surface of a diffusely transmitting window in the wall, carefully 

screened from direct light from the source but receiving light from other 

portions of the sphere, is therefore a relative measurement of the flux output 

of the source. 


The presence of a finite source, its supports, electrical connections, 

the necessary shield and the aperture or window, are all departures from the 

assumptions of the integrating sphere theory. The various elements entering 

into the considerations of a sphere, as an integrator, make it undesirable 

to use a sphere for absolute measurement of flux unless various correction 

factors are applied. This does not detract from its use when a substitution 

method is employed 


Integrating Sphere Photometer 


Integrating Sphere Photometer 






2014-August 



2014-August 


At works 


Reading 4 




2014-August 



Fion Zhang 

2014/August/15

SECTION 3 

THE VISUAL AND OPTICAL TESTING 

ENVIRONMENT 


SECTION 3: THE VISUAL AND OPTICAL TESTING ENVIRONMENT 

PART 1: EFFECT OF DESIGN CRITERIA ON VISUAL AND OPTICAL 

TESTS 

1.1 Visual Testing in Product Design 

1.2 Designing for Quality Assurance 

PART 2: ENVIRONMENTAL FACTORS 

2.1 Cleanliness 

2.2 Texture and Reflectance 

2.3 Lighting for Visual Tests 

2.4 Light Intensities 

2.5 Vision in the Testing Environment 


PART 3: PHYSIOLOGICAL FACTORS 

3.1 The Lens 

3.2 The Fovea 

3.3 Rods and Cones 

3.4 Receptors 

3.5 Perception 

3.6 Physiology of Vision 

3.7 Mechanism of Vision 

3.8 Color and Color Vision 

3.9 Observer Differences 


PART 4: VISUAL WELD TESTING PERFORMANCE STANDARDS 

4.1 Near Vision Acuity 

4.2 Color Perception 

4.3 Target Detection 

4.4 Acuity Variables 

4.5 Reserve Vision Acuity and Visual Efficiency 

4.6 Performance Standards for Visual Weld Testing 

4.7 Use of Visual Reference Standards 

4.8 Knowledge of Crack Pattern Recognition 

4.9 Scanning Techniques 

4.10 Lighting 

4.11 Practical Qualification Requirements 

4.12 Remote Visual Tests 

4.13 Vision Hardware 

4.14 Other Factors Affecting Perception 

4.15 Recommendations for Visual Weld Testing 

4.16 Conclusion 


PART 1: 

EFFECT OF DESIGN CRITERIA ON VISUAL AND OPTICAL TESTS 

1.0 General 

To use visual testing effectively for quality control of a manufactured 

component, the test method's capabilities must be considered early in the 

product's design phase (see Table 1). Realistic accept and reject criteria must 

be established as a first step in designing for process control but these 

realistic criteria are not always obvious. For example, what is the distribution 

of voids in nonstructural composite honeycomb that can be tolerated for 

satisfactory service life? What quality of surface finish must be achieved to 

make a product acceptable? Or to make a product marketable? What level 

and type of material anomalies can be reliably detected by visual testing? 

How must the product design be changed to accommodate visual testing 

procedures? If correct controls are to be established, these and similar 

questions must be considered and answered as early as possible. One of the 

most complex problems is determining when, during a fabrication or 

assembly process, visual testing is most effective and least expensive. 


TABLE 1. Summary of the visual and optical testing method 

Method 

Key process and basic result 

Principles 

Probe medium or energy source 

Nature of signal or signature 

Detection or sensing method 

Indication or recording method 

Interpretation basis 

Objectives 

Discontinuities and separations 

Structure 

Dimensions and metrology 

Physical and mechanical properties 

Composition and chemical analyses 

Stress and dynamic responses 

Signature analysis 

Direct visual and optically aided testing is applied to 

object surfaces for indications of unacceptable 

conditions 

Visible natural or artificial light 

Reflected or transmitted photons 

Eyes, optical aids, magnifiers, borescopes, video and 

film cameras 

Visual image, video and film 

Direct, used with other methods for direct interpretation 

(liquid penetrant, magnetic particle) 

Cracks, voids, pores and inclusions 

Roughness, grain and film 

Mechanically aided measurements 

None 

None 

Visible responses to stress 

None 


TABLE 1. Summary of the visual and optical testing method 

Applications 

Applicable materials 

Applicable features and forms 

Process control applications 

in situ or diagnostic applications 

Typical structures and 

components 

All 

Surfaces, layers, films, coatings, entire objects 

On-line and off-line monitoring or control 

All forms of nondestructive testing 

Machined parts, internal surfaces, indefinite range of test objects, 

materials, components assemblies. 

Limitations 

Access, contact or preparation 

Probe and object limits 

Sensitivity or resolution 

Interpretation limits 

Related techniques 

Visual access 

Specialized optical aids often required 

Various degrees of magnification 

May require supplementation with other nondestructive test 

methods for discontinuity 

detection and measurement 

Borescopy, refractometry, diffractometry, interferometry, 

microscopy. telescopy, light 

radiometry, phase-contrast and Schlieren techniques 


1.1 Visual Testing in Product Design 

Product design typically comprises four steps: conceptual, preliminary, layout 

and detail. During the concept phase, compatibility with visual and other 

nondestructive testing procedures must be ensured. In the preliminary design 

phase, performance criteria and material selection should be made 

compatible with nondestructive testing. During layout, inspectability of the 

product must be determined. It is important that the efforts of qualified 

engineering, manufacturing and nondestructive testing personnel he closely 

coordinated during this determination. Producibility and quality should receive 

the greatest attention in the detail design phase but all disciplines must be 

considered. 


Complex structures may not be inspectable because of geometric constraints 

or accessibility. It is necessary either for 

■ 

■ 

such components must be redesigned or 

for the approval of the design to take un-inspectability into account. 

Nondestructive testing is an added cost but, when properly applied, it can 

substantially reduce total life-cycle costs. 

The visual testing specialist participates in the design process by providing 

knowledge of the visual testing function. This can best be accomplished by: 

■ 

■ 

■ 

providing qualified NDT support during design, 

revising design handbook data to cover nondestructive testing and 

establishing nondestructive testing guidelines to govern testing as part of 

overall quality procedures. 


1.2 Designing for Quality Assurance 

Quality assurance is the establishment of a program to guarantee the desired 

quality level of a product from raw materials through fabrication, assembly 

and delivery. Quality control is the physical and administrative actions 

required to ensure compliance with the quality assurance program. Quality 

control includes nondestructive testing at appropriate points in the 

manufacturing cycle. A quality assurance program consists of five basic 

elements. 


1. Prevention: a formalized plan for designing, for inspectability and costeffectiveness. 

2. Control: documented workmanship standards and compatible procedures 

for training of and use by production and quality control personnel. 

3. Assurance: establishing quality control check points and a rapid 

information feedback system. 

4. Corrective action: implementation of the feedback system and necessary 

corrective action. 

5. Audit: unbiased third party review of all aspects of the program, including 

vendor materials. 


Management must decide what quality level it will produce and support. Once 

this is established, production and testing personnel aim to maintain this level 

and not to depart from it either toward lower or higher quality. For example, 

when drawing a component, the designer sets tolerances on dimensions and 

finish. If a drawing specifies a certain dimension as 32 mm (1.25 in.) but fails 

to specify the tolerance, the machine shop supervisor could: 

■ 

■ 

reject the drawing as incomplete or 

assume a standard tolerance. 

In nondestructive testing, a quality tolerance (the acceptable limits on the 

characteristic of interest) must also be specified. For example, no defects is 

an unworkable quality acceptance criteria. The lack of this single requirement 

has caused much misunderstanding of nondestructive testing in general and 

visual tests in particular. 


PART 2: 

ENVIRONMENTAL FACTORS 

2.0 General 

An important environmental factor affecting visual tests is lighting. Often, 

emphasis is placed on equipment variables such as borescope view angle or 

degree of magnification. But if the lighting is incorrect, no magnification is 

going to improve the image. Other working conditions are also important 

including factors causing operator discomfort and fatigue. 


2.1 Cleanliness 

The act of seeing depends on the amount of light reaching the eye. In visual 

tests, the amount of light may be affected by distance, reflectance, brightness, 

contrast or the cleanliness, texture, size and shape of the test object. 

Cleanliness is a basic requirement for a good visual test- it is impossible to 

gather visual data through layers of opaque dirt unless cleanliness itself is 

being examined. 

In addition to obstructing vision, dirt on the test surface can mask actual 

discontinuities with false indications. Cleaning typically may be done by 

mechanical or chemical means or both. Cleaning avoids the hazards of 

undetected discontinuities and improves customer product satisfaction. 


2.2 Texture and Reflectance 

Vision is dependent on reflected light entering the eye. The easiest way to 

ensure adequate lighting is by placing the light source and eye as close to the 

test surface as the focal distance allows. Similarly, a magnifier should be held 

as close to the eye as possible, ensuring that the maximum amount of 

light from the target area reaches the eye. Reflectance and surface texture 

are related characteristics. It is important for lighting to enhance a target area, 

but glare should not be allowed to mask the test surface. A highly reflective 

surface or a roughly textured surface may require special lighting to illuminate 

without masking. Supplementary lighting must be shielded to prevent glare 

from interfering with the inspector's view. Reflected or direct glare can be a 

major problem that is not easily corrected. Glare can be minimized by 

decreasing the amount of light reaching the eye. This is done by increasing 

the angle between the glare source and line of vision by increasing the 

background light in the area surrounding the 


This is done by increasing the angle between the glare source and line of 

vision by increasing the background light in the area surrounding the glare 

source or by dimming the light source. Such solutions are simple to 

implement for direct glare from a supplemental light or the reflected glare from 

a small test object. Glare from permanent lighting fixtures is more difficult to 

control. Ceiling fixtures should be mounted as far above the line of sight as 

possible and must be shielded to eliminate light at an angle greater than 45 

degrees to the field of vision. Task lighting should be shielded to at least 25 

degrees from horizontal. Such shielding must allow a sufficient amount of 

light to reach the test area. 


Glare 


2.3 Lighting for Visual Tests 

The amount of light required for a visual test is dependent on several factors, 

including the type of test, the importance of speed or accuracy, reflections 

from backgrounds and inspector variables. Physiological processes, 

psychological state, experience, health and fatigue all contribute to the 

accuracy of a visual inspection. 

The reflections and shadows from walls, ceiling, furniture and equipment 

must also be considered. Some reflectance from the environment must occur 

or the room will be too dark to be practical. Recommended reflectance values 

are: ceiling, 80 to 90 percent; walls, 40 to 60 percent; floors, not less than 20 

percent; desks, benches and equipment, 25 to 45 percent. For visual and 

other nondestructive testing applications, a ratio of 3:1 between the test 

object and darker background is recommended. A 1:3 ratio is recommended 

for a test object and lighter surroundings. 

Keywords: 

3: 1 ratio 


Certain psychological factors can also affect a visual inspector's performance. 

Wall colors and patterns have been shown to have a measurable effect on 

attitude and this is especially important when visually inspecting critical or 

small components. In general, a visual inspector's optimum attitude is relaxed 

but not bored, alert but not restless. To complement the illumination needed 

for visual testing, all colors in a room should be light tones. Otherwise, up to 

50 percent of the available light can be absorbed by dark walls and flooring. A 

strong contrast of pattern or color can cause restlessness and eventually 

fatigue. Cool (blue) colors are recommended for work areas with high noise 

levels and heavy physical exertion. 


2.4 Light Intensities 

The nanometer (tip), equal to 10 -9 meters, has replaced the angstrom unit (A) 

as the preferred unit for measuring radiation wavelengths. There are ten 

angstrom units in a nanometer. 

To perform a visual test, there must be a source of natural or artificial light 

adequate in both intensity and spectral distribution. Even under optimum 

conditions the human eye can be stimulated by only a small part of the 

electromagnetic spectrum. The limits of this visible portion are ill defined, 

depending on the amount of energy available, its wavelength and the health 

of the eye. For most practical purposes, the visible spectrum may be 

considered to be between about 380 nm at the beginning of the violet and 

770 nm at the end of the red, However, with especially intense sources and 

with a completely dark adapted eye, the shorter wavelength boundary may be 

extended down to 350 nm or shorter, with a corresponding reduction in the 

longest wavelength perceived. Similarly, with an especially intense longer 

wavelength source and an eye adapted to a higher level of light, the longer 

wavelength boundary may extend up to 900 nm. These ranges together are 

only a small part of the electromagnetic spectrum. 


Brightness is an important factor in visual test environments. The brightness 

of a test surface depends on its reflectivity and the intensity of the incident 

light. Excessive or insufficient brightness interferes with the ability to see 

clearly and so obstructs critical observation and judgment. For this reason, 

light intensity must be tightly controlled. A minimum intensity of 160 lx (15 ftc) 

of illumination should be used for general visual testing. A minimum of 500 lx 

(50 ftc) should be used for critical or finely detailed tests. According to the 

Illuminating Engineering Society, visual testing requires light at 1,100 to 3,200 

lx (100 to 300 ftc) for critical work.' A commercially available light meter can 

be used to determine if the working environment meets this standard. 

To ensure compliance with the minimum intensity requirement, a known light 

source held within a specified maximum distance must be used. Alternatively, 

a light measuring device such as a photocell or phototube must be used. 

Examples of known light sources are shown in Table 2. 


Keywords: 

• A minimum intensity of 160 lx (15 ftc) of illumination should be used for 

general visual testing. 

• A minimum of 500 lx (50 ftc) should be used for critical or finely detailed 

tests. 

• According to the Illuminating Engineering Society, visual testing requires 

light at 1,100 to 3,200 lx (100 to 300 ftc) for critical work 


TABLE 2. Distances for minimum 500 Ix (50 ftc) illumination 

Light 

Maximum Source-to-Object Distance 

Source millimeters (inches) 

2 D cell flashlight 250 (10) 

60 W incandescent bulb 250 (10) 




Light measuring device- Photometer 


2.5 Vision in the Testing Environment 

2.5.0 General 

The eye is a critical variable in visual tests because of variations in the eye 

itself as well as variations in the brain and nervous system. For this reason, 

visual inspectors must be examined to ensure natural or corrected vision 

acuity. The frequency of such examinations is determined by code, standard 

specification, recommended practice or company policy and yearly 

examinations are common. The Jaeger eye chart is widely used in the United 

States for near vision acuity examinations. The chart is a 125 x 200 mm (5 x 8 

in.) off-white or grayish card with an English language text arranged into 

groups of gradually increasing size. Each group is a few lines long and the 

lettering is black. In a vision examination using this chart, visual testing 

personnel may be required to read, for example, the smallest letters at 

a distance of 30 cm (12 in.). 


More clinically precise ways of measuring vision acuity involve recognition of 

Roman capital letters of various sizes from controlled distances. More on the 

determination of vision acuity may be found in the discussion of the 

physiology of sight. The exact requirements for near vision acuity examination 

are specified by the employer. If prescription lenses are required to pass a 

vision examination, then the subject must wear them during subsequent 

visual testing. Photograying lenses can be a problem where ultraviolet light is 

high, e.g., under some fluorescent lights. 


2.5.1 Visual Angle and Distance 

The angle of vision and the distance of the eye from the test surface 

determine the minimum angular separation of two points resolvable by the 

eye. This is known as the eye's resolving power. For the average eye, the 

minimum resolvable angular separation of two points on an object is about 

one minute of arc (or 0.0167 degrees). This means that at 300 mm (12 in.) 

from a test surface, the best resolution to be expected is about 0.09 mm (3.5 

mil). At 600 mm (24 in.), the best anticipated resolution is about 0.18 mm 

(0.007 in.). To complete a visual test, the eye is brought close to the test 

object to obtain a large visual angle. However, the eye cannot sharply focus 

on an object if it is nearer than 250 mm (10 in.). Therefore, a direct visual test 

should be performed at a distance of 250 to 600 mm (10 to 24 in.). Also of 

importance is the angle the eye makes with the test surface. For most 

indications, this should not be less than 30 degrees (see Fig. 1). 


FIGURE 1. Minimum angle for typical visual testing 


PART 3: 

PHYSIOLOGICAL FACTORS 

3.1 The Lens 

The human eye is a roughly spherical organ, set in a socket where it is free to 

rotate in all forward directions. (Refer to figure showing eye elsewhere in this 

book.) At the front, a compound lens (including the cornea) is set into an 

opening through which light enters the eye. This lens is of variable focal 

length and changes without conscious effort to focus objects at varying 

distances, forming images at the back of the eye. With aging, focusing 

becomes sluggish. Immediately in front of the lens is the iris, a circular 

pigmented membrane, perforated by an aperture known as the pupil. The iris, 

analogous to the diaphragm of a camera, adjusts spontaneously the area of 

the pupil to change the amount of light entering the eye by a maximum factor 

of about 16: 1. The pupil tends to be wider at low light intensities and smaller 

at higher intensities. It plays little part in color perception. 


The lens does not pass light of the shortest wavelengths and is largely 

responsible for the termination of response at the low end of the spectrum. As 

age increases, the lens yellows, increasing the absorption in the blue region 

and tending to increase the shortest wavelength that can be seen. This can 

be a factor in color differences reported between observers of different age, 

especially for tasks involving shorter wavelength perceptions. 


3.2 The Fovea 

The photographic plate used in the camera is represented in the eye by the 

retina, which contains the end plates of the optic nerve. These receptors are 

extremely complicated structures called rods and cones. Figures showing the 

approximate location of these microscopic receptors are in the introductory 

discussion on the physiology of sight. Nerve impulses stimulated by light arise 

in these structures and are conducted along the visual pathways to the 

occipital region of the brain. 

When the eye looks directly at a small area in the field of view, the images 

impinge on a region called the foven centrails (see Fig. 2). This is the region 

of sharpest vision and the retina component most important for visual testing. 

It is convenient to consider the cone and rod distributions and their 

dependence on increasing distance from the fovea centralis. The central part 

of the fovea consists almost entirely 


of color sensitive cones, nearly all of which are connected individually to optic 

nerve fibers. The foveal cones are packed more tightly together and the 

structure above them is much thinner, forming a depression in the retina in 

this region. 

There is a sound physiological basis for the superiority of detail perception in 

the fovea centralis. This rod-free area extends outward to around 2 or 3 

degrees as measured by the area's angular subtense in the external field; it is 

notably insensitive to shorter visible wavelengths. This may aid detail vision 

by offsetting chromatic aberrations of the eye's lens. Broadly speaking, no 

other part of the eye is used to perceive a momentary object of interest. At a 

distance of 500 mm (20 in.), the 2 degrees of rod-free surface corresponds to 

an object size of about 19 mm (0.75 in.). Visual testing of a component larger 

than 19 mm (0.75 in.) then becomes a series of quick and successive 

fixations along the object with occasional returns for verification. 


FIGURE 2. Cross section of the eye, showing fovea centralis (see also figures in discussion of 

the physiology of eyesight) 


FIGURE 2. Cross section of the eye, showing fovea centralis (see also figures in discussion of 

the physiology of eyesight) 


3.3 Rods and Cones 

Proceeding outward from the fovea centralis, rods are found mingled with 

cones. With distance from the fovea, the percentage of cones decreases 

exponentially and the percentage of rods increases exponentially. At the 

same time, both rods and cones show a tendency to connect in groups to 

single nerve fibers. This tendency is much stronger for rods and these groups 

become larger with increased distance from the fovea. Vision for detail 

therefore decreases steadily but color perception persists, at normal light 

intensity levels. Partly overlapping the fovea and surrounding it out to around 

10 degrees in the visual field is an irregular, diffuse ring of yellow pigment 

known as the macular lutae. Its importance in perception comes from its 

absorption of blue light, thus changing the spectral energy distribution of the 

light reaching receptors that are under it. 


Rods and cones differ in the minimum intensity of light to which they can 

respond. This difference is caused in rods by the presence of a 

photosensitive pigment called rhodopsin. This material is very easily bleached 

by light at low levels and is assumed to produce an electrochemical response 

in the rods. This visual response is essentially without color sensation and the 

sensitivity of the eye as a function of wavelength at these intensity levels 

corresponds to the wavelength absorption curve of rhodopsin. It is distinctly 

different from the wavelength response curve of the whole eye at higher 

intensity levels, which is representative of the sensitivity of the cones. 

Keywords: 

Rod- Rhodopsin 


Three classes of human cones have been identified, with a sensitivity peaking 

at 445 nm, 535 nm and 570 nm. It is known that the blue absorbing cones are 

relatively sparse in the fovea centralis, thereby explaining its insensitivity to 

shorter wavelengths. Because of the absence of rods in the fovea, there is 

DO response for low level light, even if the chromatic sensitivity is at its 

highest level and the iris is fully dilated. It is the level of the stimulus that is 

inadequate to elicit a chromatic response. To obtain any response at all, it is 

necessary to look off to one side of the stimulus so that at least some rods 

participate in the perception. However, any sufficient stimulus in the central 

field of view (a distant light source, for example, or a discontinuity filled with 

fluorescent penetrant irradiated with ultraviolet radiation) produces a 

chromatic response while everything else remains colorless. 


3.4 Receptors 

Because receptors are grouped and the size of the groups increases rapidly 

with increasing distance from the fovea, peripheral vision is very indistinct and 

largely serves purposes of orientation and the detection of motion. The 

mechanism appears to play little if any part in perception of stationary objects 

at normal room and daylight intensities. The fibers from the various receptors 

cross the inner (vitreous humor) side of the retina and pass through it 

together in the optic nerve bundle. This transitional area is called the optic 

disk and is completely blind. Its surface area is comparable to that of the 

fovea. The optic disk lies about 16 degrees toward the nose from the fovea 

(outward in the visual field) so that corresponding parts of the visual field 

cannot fall on both disks simultaneously. An observer is not aware of this 

blind spot except when consciously arranging for an image to fall wholly 

within the optic disk. As mentioned above, the retina does not detect light 

uniformly over its area. The importance of this for perception is not so much 

the details overlooked because of the nonuniformity as the fact that even a 

rather keen observer is not normally aware of the nonuniformity unless an 

instance is pointed out. 


As we look about a scene (rather than at a fixed point), the image in the eye 

moves across the region of sharpest vision as well as all the other regions. 

This voluntary, though not usually conscious, movement corresponds to 

shifting focus of attention on details. During each pause, there is also a fairly 

rapid tremor of the eyes called saccadic movement. Both movements 

encourage contours in the image to cross the receptor elements of the retina. 

It is believed that this effect plays a role in contour perception and even 

appears to be a necessary condition for vision. If the center of such a field 

is rigidly fixated and viewed without blinking (both difficult), there is a gradual 

loss of both brightness and saturation over the whole area and this can 

eventually make the stimulus disappear. The progressive change can be 

interrupted at any point by either blinking or moving the eye quickly (changing 

the fixation point) from side to side. Together with other data, it is apparent 

that there has been a loss in sensitivity in the area of the retina covered by 

the image. 


3.5 Perception 

3.5.0 General 

In terms of its visible response, the sensitivity of the eye to light is not 

constant. The eye tends to respond more to differences in the field of view 

than to absolute values. It appears to do this automatically by adjusting 

sensitivities to something approaching the average of the stimuli. Sensitivity 

is also affected laterally by stimuli lying near the primary object. These are 

time-dependent factors, with the time scale being determined largely by the 

magnitude of change from the previous stimulation. 

Adaptation is essentially independent in the two eyes so that they may have 

quite different sensitivity levels at the same time. For gross light level changes, 

adaptation occurs as (1) the familiar and painful glare of a bright light after a 

long period in relative darkness, with adjustment sometimes taking as long as 

a minute and (2) the blindness after entering a normally lighted room from full 

sunlight, with adjustment taking as long as thirty minutes. Adaptation time 

increases with age. 


3.5.1 Influences on Perception 

The science of perception is the study of (1) how ideas and other mental 

events become organized to yield impressions of objects and (2) the 

influence of the observer's mental and physical states. It is known that the 

perceived qualities of a viewed object may change with the state of the 

observer, based on knowledge of or assumptions about the causes of a 

stimulus. For example, under certain conditions, two lines of the same length 

can be perceived as different lengths, as in the well known Muller-Lyer 

illusion with double ended arrows (see Fig. 3). 


FIGURE 3. In the MUller-Lyer illusion, the shafts of two arrows are the same length- contrary to 

appearances 



appearances 



appearances 


For the purposes of visual and optical testing, it is important to know why 

physical reality may differ from perception and what are the effects of the 

observer's knowledge, fatigue, health and attitude. Perception is an active 

process in which the observer uses vision in combination with experience 

to maximize the wanted details and minimize the unwanted details. Most 

visual inspectors recognize that test objects exist, that they emit or reflect light, 

that this light causes neural activity and that the brain then synthesizes some 

representation of the original object. Other assumptions are specific to the 

application and may even be erroneous for example, what sort of defects are 

likely to occur or are of concern to the employer. The following discussion 

emphasizes somatic conditions that can impair the inspector's judgment. 

Sluggishness of the iris or of the muscles adjusting the lens can be caused by 

age, fatigue, drugs, disease or emotions. Such sluggishness in turn can affect 

what the observer sees and does not see. 


3.5.2 Effects of Fatigue 

Seeing is not the passive formation of an image. It is an active process in 

which the observer keeps track of personal actions through a kind of 

feedback loop in which the perceived thing may be altered by the observer's 

actions. As one of the first steps in this complex feedback system, the image 

is formed by the lens of the eye on 100 million or so rods and cones in the 

retina. There are only about 1 million fibers that can carry the responses of 

these elements out of the eye through the optic nerve. Clearly, there must be 

grouping of these sensitive elements into single channels. 

Both this grouping and the distribution of rods and cones change 

systematically over the retina. In common with other psychological subjects, 

but unlike the physical sciences, the end result of seeing cannot be measured. 

It can only be described or compared to the effect of a previous, similar 

experience. In common with all other processes that require active 

participation, fatigue reduces the observer's efficiency for accurately 

interpreting visual data. 


3.5.3 Effect of Observer's Health 

There are many somatic conditions that can directly or indirectly affect an 

individual's ability to see. Glaucoma is one such disease, characterized by 

increased intraocular tension which can cause vision impairments ranging 

from slight abnormalities to absolute blindness. In many cases, the cause of 

visual impairment is not known and not easily discovered. Some problems of 

perception are secondary effects supplemented by predispositions of heredity, 

emotional state or circulatory factors. In other cases, impairment can result 

directly from disease of the ocular structures, including intraocular tumors, 

enlarged cataracts or intraocular hemorrhage. Presbyopia is a condition in 

which the lens stiffens with age and so loses its ability to focus. 


Diabetic retinopathy is another condition that impairs normal vision. It can 

occur eight years after the onset of diabetes, with effects ranging from minor 

to severe. Diabetes can also lead to degenerative changes in a normally 

developed lens, characterized by gradual loss of transparency. Well 

developed, diffuse cataracts sometimes result from diabetes as well as other 

causes. The condition can reduce vision until only light perception remains. 

Sometimes myopia develops in the early stages of nuclear cataracts so that 

someone whose vision is presbyopic may be able to read without corrective 

lenses. 

Gradual loss of vision in middle age is characteristic of both cataracts and 

glaucoma. Prolonged use of the eyes with defective illumination and a 

strained position should always he avoided. It is also important to avoid 

fatigue of the eye muscles particularly when caused by errors of refraction. 

Inability to concentrate on the subject and a rhythmic oscillation of the eye 

and eyelids may occur as a result of eye muscle fatigue, leading to ineffective 

visual tests. 


Corrective lenses and rest often relieve simple forms of eye strain. Because 

of physiological changes in the lens with age, the lens is rendered less 

responsive to the process of accommodation and the resulting presbyopic 

individual is unable to focus well for near vision. Blurring, increasing 

awareness of photophobia, too-watery eyes, throbbing pains in the eyeballs, 

burning, eyeball tenderness, a feeling of discomfort in the eyes and sluggish 

reaction of the iris are some of the signs that a thorough eye examination is 

needed. Color blindness is discussed in the part of this book on the 

physiology of eyesight. It is the initial stage of impairment that commonly 

causes the most problems for the unknowing visual inspector. Because vision 

impairment typically progresses slowly, individuals may not be aware of a 

problem until it impairs performance. Any individual who needs frequent 

changes of corrective lenses, who notes diminished vision acuity, has mild 

headaches, sees halos around light sources or has impaired dark adaptation 

should have an eye examination as soon as the condition is discerned. This 

is especially true for individuals over the age of 40. 


3.5.4 Effect of Observer's Attitude 

A complete representation of the visual field probably is not present in the 

brain at any one time. The brain must contain electrochemical activity 

representing some major aspects of a scene but such a picture typically does 

not correspond to how the observer describes the scene. This occurs 

because the observer adds experience and prejudices that are not 

themselves part of the visual field. Such sensory experience may reflect 

physical reality or may not. Sensory data entering through the eye are 

irretrievably transformed by their contexts—an image on the retina is 

perceived differently if its background or context changes. Perceptually, 

the image might be a dark patch in a bright background that can, in turn, 

appear to be a white patch if displayed against a dark background. No single 

sensation corresponds uniquely to the original retinal area of excitation. 


The context of a viewed object can affect perception and, in addition, the 

intention of the viewer may also affect perception. The number of visible 

objects in a scene far exceeds the typical description of the scene. And a 

great deal of information is potentially available to the observer immediately 

after viewing. If an observer has the intention of looking for certain aspects of 

a scene, only certain visual information enters the awareness, yet the total 

picture is certainly imaged on the retina. if a scene or an object is viewed a 

second time, many new characteristics can be discerned. This new 

nformation directly influences perception of the object, yet such information 

might not be available to the viewer without a second viewing. The selective 

nature of vision is apparent in many common situations. An individual can 

walk into a room full of people and effectively see only the face of an 

expected friend. The same individual can walk right by another friend without 

recognition because of the unexpected nature of the encounter. Vision is 

strongly selective and guided almost entirely by what the observer wants and 

does not want to see. Any additional details beyond the very broadest have 

been built up by successive viewing. Both the details and the broad image 

are retained for as long as they are needed and then they are quickly erased. 


The optical image on the retina is constantly changing and moving as the eye 

moves rapidly from one point to another- the sensing rods and cones are 

stimulated in ways that vary widely from one moment to the next. The mental 

image is stationary for stationary objects regardless of the motion of the 

optical image or, for that matter, the motion of the observer's head. It is very 

difficult to determine how a unique configuration of brain activity can be the 

result of a particular set of sensory experiences. A unique visual configuration 

must be a many-to-one relationship requiring complex interpretation. If an 

observer does not apply experience and the intellect, it is likely that a 

nondestructive visual test will be inadequate. 


3.6 Physiology of Vision 

3.6.1 Visual Functions 

Vision comprises a number of factors, including perception of light, form, color, 

depth and distance. Form perception occurs when light from an object is 

focused in the eye. This visual image is affected by the lens system in almost 

the same way that any inorganic lens brings rays of light to a focus and forms 

an image. The focus of the lens system in the eye can be changed like that of 

a camera. A diaphragm (the iris) regulates the quantity of light admitted. The 

retina is a light sensitive plate on which the image is formed. Adjustments 

of focus are made by changing the thickness and curvature (the focusing 

power) of the lens. Increasing the lens thickness is called accommodation. 

This is done by the action of tiny muscles attached to the lens. 


3.6.2 Refractivity and Binocular Vision 

In the normal eye, the length of the eyeball and the refractive power of the 

cornea and lens are such that images of objects at a distance of 6 m (20 ft) or 

more are sharply focused on the retina when the muscles of accommodation 

are relaxed. Errors in these relationships require correction with specially 

prepared lenses. In a farsighted individual, the situation can be corrected 

with convex lenses. These bring light from distant objects to a focus without 

contracting the accommodation muscles which make the lens more convex. 

In the nearsighted person, light rays from distant objects come to a focus in 

front of the retina. This causes blurring of all objects located beyond a critical 

distance from the eye. By use of concave lenses, distant objects can be seen 

clearly by the nearsighted individual. 

Keywords: In the normal eye, the length of the eyeball and the refractive 

power of the cornea and lens are such that images of objects at a distance of 

6 m (20 ft) or more are sharply focused on the retina when the muscles of 

accommodation are relaxed 


Myopia 


Myopia 


Hyperopia 

http://www.tedmontgomery.com/the_eye/eyephotos/Farsightedness-grphc.html 


Hyperopia 


Hyperopia 


3.6.3 Distance Judgment 

Binocular vision is an important aid in accurate judgment of distance. 

Distance judgment is the basis for depth perception or stereoscopic vision. 

Stereoscopic vision depends, at least in part, on the fact that each eye gets a 

slightly different view of close objects. The right eye sees a little more of the 

right hand surface of the object. The left eye sees a little less of this surface 

but more of the left hand surface. When the images on the two retinas differ in 

this way, the object is perceived as three-dimensional. 


3.7 Mechanism of Vision 

3.7.0 General 

The photographic plate used in the camera is represented in the eye by the 

retina, which contains the end plates of the optic nerve. These receptors are 

the rods and cones. Nerve impulses stimulated by light arise in these 

structures and are conducted along the visual pathways to the occipital region 

of the brain. 


3.7.1 Photochemical Processes 

The mechanism of converting light energy into nerve impulses is a 

photochemical process in the retina. The so called visual purple, a 

chromoprotein called rhodopsin, is the photosensitive pigment of rod vision. It 

is transformed by the action of radiant energy into a succession of products, 

finally yielding the protein called opsin plus the carotenoid known as retinene. 

This process occurs by the action on the visual purple of a small number of 

quanta of radiant energy in the visible range of wavelengths. It has been 

shown that the peak and slope of the curve of scotopic (night vision) 

luminosity sensation are almost identical with the absorption curve of 

rhodopsin. 


Chromoprotein 


3.7.2 Light Receptors 

The two kinds of light receptors in the retina, the rods and the cones, differ in 

shape as well as function. At the point where the optic nerve enters the retina, 

there are no rods and cones. This portion of the retina, called the blind spot, 

is insensitive to light. At the other extreme, the maximum vision acuity at high 

brightness levels exists only for that small portion of the image formed on the 

center of the retina. This is the fovea centralis discussed in detail earlier. Here, 

the layer of blood vessels, nerve fibers and cells above the rods and cones is 

far thinner than in peripheral regions of the retina. 


Light Receptors 

http://www.sciencephoto.com/media/308752/enlarge 


Light Receptors 

http://ncifrederick.cancer.gov/atp/imaging-and-nanotechnology/electron-microscopylaboratory/eml-protocols-and-resources/eml-image-gallery/retina3/ 


3.7.3 Daylight Vision 

Daylight vision, which gives color and detail, is performed by the cones, 

mainly in the fovea centralis. There are at least three different kinds of cones, 

each of which is in some way activated by one of the three fundamental 

colors, as discussed earlier in this section. 


Cones 

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html 


3.8 Color and Color Vision 

3.8.0 General 

Color vision is one of the most interesting aspects of the function of the 

human eye. Color vision occurs only in the light-adapted eye and is 

dependent on the acuity of the cones. Light is the specific stimulus for the eye 

but the eye is sensitive only to rays of certain wavelengths. Within those 

wavelengths, the stimulus must have a certain minimum intensity. The 

sensation of color varies according to the intensity of the light, the wavelength 

of the different radiations and the combinations of different wavelengths. In 

daylight vision, yellow is the brightest color. 


3.8.1 Color Characteristics 

Every color has three characteristics: (1) tone or hue, (2) saturation or purity 

and (3) brightness or luminosity. Hue is associated with a range of 

wavelengths in the spectrum and is usually what an observer means when 

describing a color (red or blue, for instance). An estimated seven million or 

more colors can be discriminated but, because the transition from one hue to 

the next is gradual, the demarcations are ill defined and to some extent a 

matter of opinion. For practical purposes only a few main colors are 

commonly distinguished, with the following approximate wavelengths: violet, 

380 to 450 nm; blue, 450 to 480 nm; blue-green, 480 to 510 nm; green, 510 

to 550 nm; yellow-green, 550 to 570 nm; yellow, 570 to 590 nm; orange, 590 

to 630 nm; and red, 630 to 730 nm. Light from a limited part of the spectrum 

is called monochromatic. 


A hue may also vary in brightness, according to the intensity of its 

predominant radiation. Indigo, with wavelengths approximately from 425 to 

455 nm, is sometimes included between violet and blue, perhaps because of 

the name Roy G. Biv, a mnemonic comprising initials of the colors of the 

rainbow. Another color characteristic is saturation. This is a relative or 

comparative characteristic and may be described as a hue's dilution with 

white light. 


3.8.2 Color Changes 

The critical evaluation of color and color change is one of the basic principles 

of most visual tests. Corrosion or oxidation of metals or deterioration of 

organic materials is often accompanied by a change in color imperceptible to 

the eye itself. For example, minute color changes on the surface of meat may 

not be detectible by the human eye but can be detected with photoelectric 

devices designed for the automatic inspection of meat before canning. 


3.8.3 Brightness Characteristics 

Brightness contrast is generally considered the most important factor in 

seeing. The brightness of a diffusely reflecting colored surface depends on its 

reflection factor and the quantity of incident light (lux or footcandles of 

illumination). Excessive brightness (or brightness within the field of view 

varying by more than 10:1) causes an unpleasant sensation called glare. 

Glare interferes with the ability of clear vision, critical observation and 

judgment. Glare can be avoided by using polarized light or other polarizing 

devices. 

Keywords: 

■ 

■ 

Glare 10:1 brightness differences 

Polarizing devices as a mean to reduce glare. 


3.9 Observer Differences 

The visibility of an object is never independent of the human observer. 

Human beings differ inherently in the speed, accuracy and certainty of seeing, 

even though they may possess average or normal vision. Individuals vary 

particularly in threshold measurements and in their interpretations of visual 

sensations. Their psychological state, tensions and emotions influence their 

appraisals of the visibility of objects and influence their performance of visual 

tasks under many conditions. 

The importance of an inspector's attitude cannot be overemphasized. 

Because many visual testing decisions may involve marginal material, all 

interpretations must be impartial and consistent. A defined policy of test 

procedure and standards should be adopted and followed faithfully. 


PART 4: 

VISUAL WELD TESTING PERFORMANCE STANDARDS 

4.0 General 

The text below focuses on direct and remote visual weld testing with 

emphasis on crack detection. Visual weld testing for cracks is one of the most 

prevalent nondestructive tests. However, of all the nondestructive methods, 

visual testing has the least defined performance procedures for qualifying 

or quantifying minimum test performance. Three procedures are used to 

verify a visual inspector's performance or sensitivity: (1) near vision acuity, (2) 

color recognition and (3) target detection. The validity of the procedure 

for verifying field reliability can be improved by understanding the use and 

limitations of performance oriented tests. 


4.1 Near Vision Acuity 

The majority of recommended practices or standards require 20/30 

uncorrected or corrected vision in one eye. Section V of AS ME's Boiler and 

Pressure Vessel Code requires 20/30 near vision acuity and Section XI 

requires 20/20. 5 While this provides a baseline for vision performance, 

it does not measure stereo vision or other vision problems such as 

astigmatism that can significantly affect detection reliability. Measurement of 

physiological vision capability involves several tests that can have complex 

interactive variables. As a screening standard, 20/20 or 20/30 near vision 

acuity in both eyes is a reasonable beginning for the vision requirements of 

weld testing. However, near vision acuity measurement alone is not sufficient 

for predicting probability of detection for fine discontinuities. 


4.2 Color Perception 

Although color recognition is not part of typical visual testing specifications, it 

is a part of the requirements for inspector qualification. Color recognition 

screening is usually a requirement for nondestructive tests that are distinctly 

color based. For example, magnetic particle testing in many cases requires 

the ability to see red or green (fluorescent) indications. 

Individuals with good vision acuity and red/green color deficiency can often 

pass practical tests based only on contrast recognition. Color can be a 

significant factor for pattern recognition of color based information during weld 

testing. However, it is difficult to qualify or quantify visual weld testing 

performance criteria. The fundamental question of what degree of color 

deficiency disqualifies a visual weld inspector is not well defined. The 

American Welding Society's Certified Welding Inspector Programs states that 

color vision acuity is desirable in some specific applications but is not 

considered essential for all inspections. 


4.3 Target Detection 

A critical performance standard for some visual tests is the detection of a line 

to verify a system's sensitivity. This procedure is often called a resolution test. 

Detection may be defined as the task of perceiving the absence or presence 

of an object. In vision physiology and psychology,' resolution is the ability of a 

vision system to discriminate between the critical elements of a stimulus 

pattern. Detection of a single line does not fulfill the standard definition of 

resolution. Single line detection for a direct visual examination is usually 

performed using a 750 μm (30 mil) width black line on an 18 percent neutral 

gray flat uniform background. Some performance criteria" require detection of 

a 25 μm (1 mil) black line for remote visual tests in critical applications. 

Because simple line detection is a relatively gross task, it can be a poor 

performance standard, allowing detection of a highly blurred image. This does 

not emulate sharpness quality recognition for evaluation of weld 

discontinuities. 


A 750 μm (30 mil) black line can be reliably detected by individuals 

classified as legally blind (20/200 corrected both eyes). The 750 μm (30 mil) 

and even the smaller 25 μm (1 mil) widths should not be used as 

performance standards because they do not determine image sharpness. 

Image sharpness is critical to discontinuity recognition and is a key feature for 

pattern recognition of welding discontinuities. Figures 4 to 6 show a 750 μm 

(30 mil) line detection in which detection occurs with both 20/20 and 20/200 

near vision acuity One means of simulating the effect of 20/200 near vision 

acuity is to observe objects underwater with the naked eye (assuming 20/20 

near vision acuity as a baseline). 


FIGURE 4. General view of 20/20 near visual acuity card and line card of 18 percent neutral gray 

background 


FIGURE 5. Photograph of 0.75 mm (0.033 in.) line on 18 percent neutral gray card taken with an 

equivalent 20/20 near vision acuity 


FIGURE 6. Photograph of 0.75 mm (0.033 in.) line on 18 percent neutral gray card taken with an 

equivalent 20/200 near vision acuity (line is detectable but blurred) 


4.4 Acuity Variables 

Several variables affect vision acuity including target movement, lighting, 

target angle, target knowledge and psychophysics. Information about these 

variables is helpful for quantifying visual performance standards for 

measuring test system sensitivity. 

4.4.1 Kinetic Vision Acuity 

Near vision acuity examinations are performed with the eye chart (target) in a 

stable position. Performing visual testing tasks that require the object to be 

scanned results in a dynamic observer- or video camera- to target movement. 

The term kinetic vision acuity' is used for acuity measurements with a moving 

target. Studies indicate that 10 to 20 percent of visual efficiency can be lost by 

target movement. 


4.4.2 Lighting and Target Angle 

Near vision acuity tests are performed under uniform lighting and on targets 

that do not cast shadows. Because the target characters have a uniform 

luminance contrast for both the figures and the background, near vision acuity 

tests are not designed to measure detection of detail in rough surface 

topographies such as welds. While a visual testing specification may specify 

a viewing angle, near vision acuity charts are made with the eye or video 

camera perpendicular to the target, resulting in optimum vision acuity 


4.4.3 Target Knowledge 

Target knowledge is the key feature for detection and recognition. Targets 

such as letters, numbers and straight lines are simple for human recognition, 

especially on a uniform background. Such targets have little transferability for 

the discontinuities of interest during a visual weld test. In fact, there are 

several different near vision acuity tests based on varying targets. One of the 

problems with using well known patterns such as letters is that the individual 

may be responding to visual clues and filling in a partially visible pattern 

deduced from letter shape. This is known as closure. Optometrists score and 

measure vision acuity based on both the number of errors and response time. 

Additionally, vision acuity is a function of the observer's acuity for a given 

time. For example, acuity may be diminished if the observer has been 

performing strenuous detailed vision tasks. Because of these variables, near 

vision acuity is not a precise quantified measurement but one having the 

accuracy required to fit a high probability of eyesight correction to the 20/20 

standard. Variability in eyesight measurement is such that a 10 percent 

difference in measurement is possible based on the type of acuity test and 

the individual's performance for that time. 


4.4.4 Psychophysics 

Psychophysics is the interaction between vision performance and physical or 

psychological factors. One example is the so-called vigilance decrement, the 

degradation of reliability based on performing visual tasks over a period of 

time. If not identified as a significant variable and controlled, vigilance 

decrement can result in diminishing visual performance 

Keywords: 

vigilance decrement, the degradation of reliability based on performing visual 

tasks over a period of time 


4.5 Reserve Vision Acuity and Visual Efficiency 

The 20/20 standard for near vision acuity is a baseline designed in the late 

1800s as a means for standardizing eyesight relative to the ability to read fine 

print and to provide a means for prescribing corrective lenses. The standard 

was not intended to identify vision acuity relative to the detectability of fine 

lines such as cracks but measurements of near vision acuity are transferable 

to the ability to detect cracks. Visual systems with a near vision acuity of 

20/20 can detect cracks with widths of 10 μm (0.4 mil) on polished surfaces. 

Such systems can detect hairline weld fractures with widths near 25 μm (1 mil) 

in the toe of a weld. The term reserve vision acuity refers to the ability of an 

individual to maintain acuity under poor viewing conditions. u An individual 

with 20/20 near vision acuity observing under degraded viewing conditions 

has considerable reserve vision acuity compared to an individual with 20/70 

near vision acuity. The term visual efficiency uses 20/20 near vision acuity as 

a baseline for 100 percent visual efficiency. The concept is useful for defining 

the reliability of a visual system based on detection relative to visual efficiency. 


4.6 Performance Standards for Visual Weld Testing 

In addition to specific calibration or verification standards, the majority of 

nondestructive testing specifications include use of test objects with known 

discontinuities. These reference standards have intentionally fabricated 

discontinuities or discontinuities from production cutouts. Reference 

standards with known discontinuities have three disadvantages: 

(1) procurement of the test object, (2) validity of the test 

object and (3) standardization of discontinuity sizes. 

Fabrication of tight visual cracks is controllable and such standards can be 

manufactured or purchased for a reasonable cost. The simplest method for 

creating a tight visual crack is to butt two highly machined plates together with 

a surface weld head minimally joining the faying surface. The weld is then 

broken and the plates reassembled mechanically or by tack welding (see Figs. 

7 to 9). Another method for creating toe cracks is to fabricate a highly 

restrained welded object with an invisible crack that becomes visible as the 

plate cools. These two methods produce toe weld cracks that are 

representative of the most prevalent inservice welding discontinuity. 


FIGURE 7. Two plates machined to a 4 (100 nm) rms finish and bolted together to appear as one 

plate 


FIGURE 8. Same plate as in Figure 8 with 0.025 mm (0.001 in.) width separation achieved by 

spacing with a feeler gage 


FIGURE 9. Same plate as in Figure 8 with a purposely cracked surface weld bead at the faying 

surface; can be used to show the effects of line of sight and width opening; separation here is 0.5 

mm (0.02 in.) to show fabrication method 


Transverse cracks can be fabricated by grinding a notch transverse to the 

weld and then filling the notch with copper. When a stringer bead is run over 

the copper, a tight visual transverse crack is produced. Transverse cracks 

can also be produced by restraining high tensile low alloy steel such as 

A514 or A517 and welding with 10018, 11018 or 12018 weld electrode. The 

amount of restraint and other variables, such as the moisture content of the 

weld electrode flux and inadequate preheat or post heat, will determine the 

size of the cracks created. The use of the weld discontinuity reference 

standards has two significant advantages: (1) they represent actual conditions 

that cannot be accurately simulated by vision acuity eye tests or line detection 

and (2) reference standards are critical for training inspectors in pattern 

recognition as well as proper detection and evaluation methodology Plastic 

reference standards replicated from actual cracks are sometimes used in 

training programs. These plastic standards are convenient and transportable 

but they often lack the realism essential for effective training. 


4.7 Use of Visual Reference Standards 

Visual testing inspectors with 20/20 or 20/30 near vision acuity in one eye 

should reliably pass the 8 μm (0.32 mil) detection test (based on transferring 

predicted acuity to detection). Therefore, for sensitivity verification, the line 

test does not provide a means for determining which visual inspectors cannot 

detect actual cracks. In other nondestructive testing techniques, verification 

and practical tests are designed to determine sensitivity and can result in 

some personnel failures (the pass/fail rate is dependent on the testing 

technique and the application). For example, there is typically a greater pass 

rate for ultrasonic discontinuity detection. Likewise, the pass rate for an 

intergranular stress corrosion crack detection program can produce a lower 

pass rate than typical ultrasonic detection methods. A greater pass rate can 

be expected for generic magnetic particle testing than for ultrasonic tests. 


Therefore, a reliable method must be established for qualifying visual 

inspectors for crack detection- one solution is to use valid visual reference 

standards containing the types of cracks predicted and required to be 

detected. Since 1985, major oil and gas companies have used performance 

demonstration programs to test underwater inspectors for nondestructive 

testing qualification. These programs are strongly based on practical 

demonstration for proficiency. The reference standards typically contain non 

visual magnetic particle testing indications. In 1990, oil and gas company 

operators placed additional emphasis on visual testing and instituted a 

program using visual reference standards for performance demonstration. 

Magnetic particle and visual testing trials were carried out concurrently 

because the two methods are complementary for underwater weld tests. 


Before visual and magnetic particle testing, personnel were given basic near 

vision acuity and color recognition screening tests. Near vision 

measurements were recorded for each eye and for both eyes. The majority of 

diving personnel fell into the 20/20 to 20/30 range with a small percentage 

in the 20/40 to 20/50 category. In some cases, diving personnel with minor 

near vision acuity deficiencies do not wear corrective lenses (wearing bifocals 

in the diving helmet is somewhat tedious and uncomfortable). Contact lenses 

are not recommended for diving. 

The initial qualification program indicated that personnel with near vision 

efficiency less than 20/20 did not perform detection of cracks as well as those 

near 20/20. The initial observation was based on a small sample population 

but was noted as a potential problem. Subsequent testing of a larger 

population indicates that individuals with less than near 20/20 have 

significantly poorer detection ability. 


There are several integrated visual testing variables beyond equating near 

vision acuity to performance, including 

1. knowledge of crack pattern recognition, 

2. knowledge of scanning techniques, 

3. lighting, 

4. orientation of the test objects, 

5. test instructions, 

6. feedback from the test administrator and 

7. psychophysical factors. 


4.8 Knowledge of Crack Pattern Recognition 

The tour main weld crack categories are weld toe, transverse, face and heataffected 

zone cracks. Most nondestructive tests require the observer to focus 

attention on reasonably well defined targets and patterns. In visual tests for 

weld discontinuities, detection is constrained by a lack of knowledge about 

the patterns to be detected. Tight hairline weld cracks are not well defined 

targets and can be discrete, based on their position in the weld. If reference 

standards or photographic examples are not used, the inspectors' reliability of 

detection is determined almost solely by experience. For inservice weld tests, 

the frequency of fine cracks is small and does not provide a high degree of 

pattern recognition based on experience. Detection of relatively gross cracks 

with tight ends may supply the inspector with some knowledge of hairline 

crack recognition. Training programs should include discontinuities that have 

crack like appearances but cannot be fully evaluated without supplemental 

nondestructive testing. This results in lowering the false positive alarm rate 

(there are some weld conditions that have suspect crack like appearances). 

Although visual testing is often considered a stand alone test, knowledge of 

penetrant testing or magnetic particle testing is essential. 


4.9 Scanning Techniques 

Detection is a function of both scanning coverage and speed. Other 

nondestructive testing techniques define these parameters by physically 

positioning a probe or the measurement material. Visual testing is primarily 

noncontact and controls for scanning and coverage are difficult to quantify. 

Coverage and scanning rate are determined by the type of discontinuity to be 

detected. Fine crack detection requires considerably more control than 

detection of gross cracks, to ensure 100 percent area coverage at a 

reasonable speed. While the human eye and machine vision have required 

sensitivity to detect extremely fine cracks, the vision system must be 

positioned in the proper orientation to detect the discontinuity. If a hairline 

crack is present in the weld toe opposite from the primary viewing angle, 

there is a high probability that the crack will be missed. 


Because most visual tasks do not require a high degree of angular probing, 

there is a tendency for inspectors to view the weld from a single position, 

resulting in a measurable loss of test sensitivity. This problem can be 

minimized by specifying that the inspector view the weld from several different 

angles. Visual testing is analogous to ultrasonic crack detection, in which the 

probability of detection is increased by using different angle probes and 

defining maximum scanning speeds. 


Scanning coverage requires a visual mapping plan to compensate 

for the fact that the human memory is not optimally equipped for scanning 

tasks. While the brain is processing the area under test, little data can he 

retained about past coverage and no hard copy documentation is produced to 

show the coverage area. 

In some qualification tests, crack reference standards are usually placed with 

the cracks opposite the observer's initial viewing angle. If the inspector does 

not visually scan the test object from more than one angle, the crack is 

usually missed. When the reference standard is placed so that the crack is in 

direct line of sight, crack detection significantly increases. 


4.10 Lighting 

Lighting for visual testing has two functions: (1) providing luminance contrast 

for discontinuity detection and (2) illuminating the object to assist in scanning 

guidance. There are subtle lighting thresholds at which cracks become 

detectable and it may accurately be said that optimal lighting conditions 

increase visual sensitivity. Some visual testing specifications require 500 Ix 

(50 ftc) at the test site but light characteristics (hue, for example) are not 

given. Many specifications refer only to light levels adequate for test 

inspection. 

Other nondestructive testing specifications require visual detection of physical 

indications, as in magnetic particle and liquid penetrant testing." Such 

techniques typically require 1,000 to 2,000 lx (100 to 200 ftc) on the viewing 

area. In addition, these surface techniques often make use of high 

contrast backgrounds to maximize indication detection. The 500 lx (50 ftc) 

minimum does not give optimum visual detection. 


The other key feature of artificial lighting is that the lighted area provides a 

guidance system for the visual testing and aids the mapping required for 

coverage. With video systems, use of side lighting can further optimize visual 

testing when component geometry creates shadows that degrade visual 

performance. Simple empirical tests on the effects of lighting can be 

performed using reference standards. In qualification tests, lighting is an 

important variable that has a measurable influence on performance. 


Florescence MPI 


Dye Penetrant Testing 


4.11 Practical Qualification Requirements 

Performance verification can be achieved using three distinct methodologies. 

1. Use a completely quantified test regime in which performance is measured 

with no prior cues regarding test object information (sometimes referred to 

as a blind test). 

2. Use a written review to provide guidelines on means to optimize visual 

testing techniques and basic test object characteristics without giving 

knowledge of the discontinuities. 

3. Provide the test candidate with some degree of real-time performance 

feedback. 


The blind test regime represents the most severe environment for visual 

testing. For quality assurance reasons, the blind test may be required but 

there are drawbacks to exclusive use of blind testing programs. Many visual 

testing qualification programs benefit by providing test candidates with some 

knowledge of required detection criteria. The real time feedback process 

allows the test administrator to determine if lack of detection is an eyesight 

acuity problem or a matter of poor technique. In some cases, when the 

candidate is given the exact location of a hairline crack, the individual still 

cannot trace the crack. This strongly indicates that lack of detection is a vision 

acuity problem that can he remedied with corrective lenses and retesting. If 

the missed crack can be traced once the candidate has knowledge of the 

location, it can be possible that improper technique was a factor. 


Detection may be such that the candidate's recognition is at a threshold level 

and a range of crack sizes should be used. The larger crack widths are first 

used to separate technique problems from vision acuity deficiencies. 

One unique feature of underwater testing is the presence of an oral nasal 

mask in the diving helmet. This oral nasal mask is in the field of view when 

performing close visual tasks and can create potential problems with visual 

disturbance and binocular vision. This is especially true when visual tasks 

must he performed for long periods of time. 


Expert Diver at works 






4.12 Remote Visual Tests 

Remote visual testing is used in hostile environments unsafe for human 

intervention or in areas of inaccessibility. All of the variables that apply to 

direct visual testing can be applied to remote visual testing. The main 

differences are: 

(1) some loss of depth cues caused by the two-dimensional medium, (2) more 

difficulty in scanning the test site with full coverage line of sight and (3) 

inability to easily implement supplemental nondestructive tests. 

Of these, the inability to use supplemental nondestructive testing is the most 

severe constraint. This is critical because a percentage of visual targets that 

appear as crack-like discontinuities cannot be separated into non relevant or 

relevant indications without additional nondestructive testing. Although the 

number of suspect crack targets may be low, the inability to provide 

evaluation with a high confidence level is a significant limitation of the remote 

visual testing method. The potential for false positive alarms must be critically 

evaluated before effecting a remote visual testing plan over a direct visual 

testing plan. 


Keywords: 

• some loss of depth cues caused by the two-dimensional medium, 

• more difficulty in scanning the test site with full coverage line of sight and 

• inability to easily implement supplemental nondestructive tests. 

Of these, the inability to use supplemental nondestructive testing is the most 

severe constraint. This is critical because a percentage of visual targets that 

appear as crack-like discontinuities cannot be separated into non relevant or 

relevant indications without additional nondestructive testing. 


The line detection procedure is often used to qualify video systems. This is a 

poor test because the line can be detected at acuity levels much lower than 

the optimal 20/20. Image size should be consistent with magnification allowed 

only to aid in acuity. Magnification levels should be set so that features 

necessary for recognition are still identifiable. Other tests cited in the use of 

video quantification are a variety of resolution tests. Video cameras are often 

stated to be high resolution. The term high resolution can be misleading if 

thought to be based on actual image quality using a specific system. 

Resolution is a function of the complete system's ability to resolve minimum 

line pairs. As a visual standard, line pair resolution can be quantified with 

greater precision than simple vision acuity tests. However, line pair resolution 

is a relatively cumbersome concept. It must be determined if 400 horizontal 

television lines are adequate for detection and with what ease the visual 

inspector can identify resolution with crack detection. 


The few visual standards that reference remote visual testing state that 

remote visual tests must be equivalent to direct visual requirements. This 

implies that the video system should have a near vision acuity equivalent to 

that of direct visual testing. In 1990, the term equivalent 20/20 near vision 

acuity was introduced to resolve this inconsistency for remote video testing 

systems. 16 Most near vision acuity cards are designed to be read at a 

defined distance. Using the equivalent 20/20 near vision acuity criterion, the 

observer reads the near vision acuity 20/20 characters on a video monitor. 

Camera distance from the object is not critical but field of view and depth of 

field are set according to the needs of the test. For example, a specification 

may require equivalent 20/20 near vision acuity for a 100 cm 2 (16 in.2 ) 

viewing area with a depth of field of 25 mm (1 in.). If the depth of field is 

extremely shallow, constant focusing is required and this produces operator 

fatigue. Medium wide angle lenses with high f-stops (achievable with high 

intensity lights) usually produce equivalent 20/20 near vision acuity. The 

20/20 standard is better for remote than 20/30 because of the inherent loss of 

visual sensitivity caused by some lost depth cues. In fact, 20/10 is a 

preferable acuity but magnification should not be so great as to remove key 

pattern recognition features. 


As stated, 20/20 near vision acuity is a good guideline for hairline crack 

detectability. However, use of actual cracked test objects is preferred for 

performance testing and training. In remote visual testing, both 20/20 near 

vision acuity characters and reference standards with specified discontinuity 

sizes can often be mounted on the robotic system for performance checks. 

On steel structures, where remote video is performed using manipulators, 

20/20 near vision acuity characters can be magnetically positioned at test 

sites to verify visual performance. 


Remote Visual Tests 








4.13 Other Factors Affecting Perception 

Most nondestructive testing techniques require specific learning regimes 

while visual testing is wrongly assumed to be an innate human process. 

There are several factors that make individuals good observers and these 

factors are sometimes counterintuitive. In one case, for example, the best 

observer had poor near vision acuity but was able to find objects more rapidly 

than observers with good eyesight. A plausible explanation lies in the fact that 

the test objects were more discernible from the background when slightly 

blurred. This is not the case for detection of hairline cracks but it does confirm 

the complexity of using the human vision system. 


Visual inspectors are highly encouraged to discuss vision considerations and 

problems with an optometrist. However, it should be recognized that 

optometrists are trained to examine eyes and not welded test objects. 

Interesting results are sometimes obtained when optometrists are given near 

vision acuity tests using unfamiliar crack reference standards. In one 

experiment, an optometrist with 20/20 near vision acuity was unable to detect 

a hairline crack and unable to trace the crack after given its location. The 

crack reference standard was validated as detectable when the optometrist's 

six year old daughter was able to detect the crack without prior knowledge of 

its location. 


4.14 Recommendations for Visual Weld Testing 

There is a need to improve training and standards for visual weld testing. 

Personnel qualification and certification in visual testing were first formalized 

by ASNT in 1988 with completion of a training outline but there is no ASNT 

certification as such for visual inspectors. The American Welding Society's 

Certified Welding Inspector Certification is a broad program of which visual 

testing for cracks is a small part. The majority of visual testing specifications 

are typically short (one or two pages) and provide minimum performance 

criteria that can be applied to practical conditions. The following 

recommendations have proved to be effective. 

1. Adopt a policy of using valid crack reference standards for training and 

verification of a system's performance sensitivity. 

2. Use a 1,070 lx (100 ftc) minimum for lighting on the test site. 


3. Require scanning based on predicted line of sight for the discontinuity 

4. Require yearly eye tests with a minimum of 20/30 near vision acuity in 

both eyes. The eye tests should be performed by an optometrist. 

5. Remove the detection of a 8 μm (0.32 mil) wide line as a visual testing 

performance standard. 

6. Develop courses and specifications especially designed for weld testing. 



Improved training and specifications for visual weld testing which address 

vision acuity, lighting and line of sight requirements will measurably increase 

visual testing sensitivity. More attention must be placed on detection 

methodology- present training tends to focus on categorizing discontinuity 

types. Because the training issues are academically fundamental, visual weld 

testing methods will be easy to implement. 

Enhancement and miniaturization of video hardware will allow equivalent 

20/20 near vision acuity testing using remote techniques. Career testing 

personnel should have yearly check-ups by optometrists to ensure there are 

no vision problems which can affect visual testing. Without yearly testing, 

vision degradation may go unnoticed and may be more difficult to correct. 





Self Study Notes Reading 4 Section 4A 

2014-August 



2014-August 


At works 


Reading 4 


Visual & Optical testing- Section 4A 


2014-August 



Fion Zhang 

2014/August/15

SECTION 4 

BASIC AIDS AND ACCESSORIES FOR 

VISUAL TESTING 


SECTION 4: BASIC AIDS AND ACCESSORIES FOR VISUAL TESTING 

PART 1: BASIC VISUAL AIDS 

1.1 Effects of the Test Object 

PART 2: MAGNIFIERS 

2.1 Range of Characteristics 

2.2 Low Power Microscopes 

2.3 Medium Power Systems 

2.4 High Power Systems 


PART 3: BORE SCOPES 

3.1 Fiber Optic Borescopes 

3.2 Rigid Borescopes 

3.3 Special Purpose Borescopes 

3.4 Typical Industrial Borescope Applications 

3.5 Borescope Optical Systems 

3.6 Borescope Construction 


PART 4: MACHINE VISION TECHNOLOGY 

4.1 Lighting Techniques 

4.2 Optical Filtering ' 

4.3 Image Sensors 

4.4 Image Processing 

4.5 Mathematical Morphology 

4.6 Image Segmentation 

4.7 Optical Feature Extraction for High Speed Optical Tests 



PART 5: REPLICATION 

5.1 Cellulose Acetate Replication 

5.2 Silicon Rubber Replicas 



PART 6: TEMPERATURE INDICATING MATERIALS 

6.1 Other Temperature Indicators 

6.2 Certification of Temperature Indicators 

6.3 Applications for Temperature Indicators 


PART 7: CHEMICAL AIDS 

7.1 Test Object Selection 

7.2 Surface Preparation 

7.3 Etching 

7.4 Using Etchants 




1.0 General 

The human eye is an important component for performing visual 

nondestructive tests. However, there are situations where the eye is not 

sensitive enough or cannot access the test site. In these cases mechanical 

and optical devices can be used to supplement the eye to achieve a complete 

visual test. 

Visual tests comprise five basic elements: the inspector, the test object, an 

optical instrument, illumination and a recording method. Each of these 

elements interacts with the others and affects the test results. Training and 

vision acuity are the two most important factors affecting the visual inspector. 

According to the American Society of Mechanical Engineers' Boiler and 

Pressure Vessel Code, Section XI, visual inspectors must be qualified 

through formal training programs for certification to ensure competency. 


Keywords: 

1. Training and vision acuity are the two most important factors affecting the 

visual inspector. 

2. According to the American Society of Mechanical Engineers' Boiler and 

Pressure Vessel Code, Section XI, visual inspectors must be qualified 

through formal training programs for certification to ensure competency. 


Inspector’s Factors 






Levels of vision acuity are determined by eye examination. Approximately 50 

percent of Americans over the age of twenty need corrective eyeglasses. In 

early stages of eyesight deficiency, many people are unaware of their 

condition- some simply do not want to wear glasses. It is important that 

borescopes be designed to allow diopter adjustments on the eyepiece. 

Frequently, wearing glasses is an inconvenience when using a borescope it is 

difficult to place the eye at the ideal distance from the eyepiece and the view 

is distorted by external glare and reflections. Rubber eye shields on 

borescopes are designed to shut out external light but are not as effective 

when glasses are worn. For these reasons, it is critical that the inspector be 

able to adjust the instrument without wearing glasses to compensate for 

variations in vision acuity. 



The test object determines the specifications for (1) the instrument used 

during the visual test and (2) the required illumination. Objective distance, 

object size, discontinuity size, reflectivity, entry port size, object depth and 

direction of view are all critical aspects of the test object that affect the visual 

test. Objective distance (see Fig. 1) is important in determining the 

illumination source, as well as the required objective focal distance for the 

maximum power and magnification. 

Object size, combined with distance, determines what lens angle or field of 

view is required to observe an entire test surface (see Fig. 2). Discontinuity 

size determines the magnification and resolution required for visual testing. 

For example, greater resolution is required to detect hairline cracks than to 

detect undercut (see Fig. 3). Reflectivity is another factor affecting illumination. 

Dark surfaces such as those coated with carbon deposits require higher 

levels of illumination than light surfaces do (see Fig. 4). 


FIGURE 1. Objective distance (arrows, for direct and side viewing borescopes 


FIGURE 2. Arrows indicate portion of object failing within the field of view for side viewing 

borescope 


FIGURE 3. Discontinuity size affects resolution limits and magnification requirements 


FIGURE 4. Reflectivity helps determine levels of illumination 


Entry port size determines the maximum diameter of the instrument that can 

he used for the visual test (see Fig. 5). Object depth affects focusing. If 

portions of the object are in different planes, then the borescope must have 

sufficient focus adjustment or depth of field to visualize these different planes 

sharply (see Fig. 6). Direction of view determines positioning of the 

borescope, especially with rigid borescopes. Viewing direction also 

contributes to the required length of the borescope. 

Some of the factors affecting visual tests with borescopes are in conflict and 

compromise is often needed. For example, a wide field of view reduces 

magnification but has greater depth of field (see Fig. 7). A narrow field of view 

produces higher magnification but results in shallow depth of field. Interaction 

of these effects must be considered in determining the optimum setup for 

detection and evaluation of discontinuities in the test object. 


FIGURE 6. Object depth (arrows) is a critical factor affecting focus 


FIGURE 7. Effects of viewing angle on other test parameters: (a) narrow angle with high 

magnification and shorter depth of field and (b) wide angle with low magnification and greater 

depth of field 


Keywords: 

1. Entry port size determines the maximum diameter of the instrument, 

2. If portions of the object are in different planes, then the borescope must 

have sufficient focus adjustment or depth of field to visualize these 

different planes sharply, 

3. Direction of view determines positioning of the borescope, especially with 

rigid borescopes, 

4. A wide field of view reduces magnification but has greater depth of field, 

5. A narrow field of view produces higher magnification but results in shallow 

depth of field. 


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At Works 


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At Works 



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At Works 




2.1.0 General 

Magnification as an aid to vision ranges in magnifying power from 1.5 x to 

2,000 x . Field coverage of conventional magnifiers ranges from 90 mm (3.5 

in.) down to 0.15 mm (0.006 in.) wide. Resolving powers range from 0.05 mm 

(0.002 in.) to 0.2 μm (0.008 mil). Powers of magnification refer to 

enlargement in one dimension only. A two-dimensional image magnified x 2, 

for example, doubles in width and in height though its area quadruples. 

The microscope is a typical magnifier. In its simplest form, it is a single 

biconvex lens in a housing adjustable for focus. Many forms of illumination 

are available, including bright field, dark field, oblique, polarized, phase 

contrast and interference. 


Bi-Convex Microscope 


Bi-Convex Microscope 


Bright Field Microscope 




http://e-materials.ensiacet.fr/domains/d07/doc02/tem.html





http://item.taobao.com/item.htm?spm=a230r.1.14.84.SgOIEN&id=15135391353&ns=1#detail

Dark Field Microscope 


Oblique Microscopy 


Oblique Microscopy 


Polarized Microscopy 


Polarized Microscopy 


http://micro.magnet.fsu.edu/primer/techniques/polarized/gallery/pages/glauconite1large.html

Interference Microscopy 









A picture from differential interference contrast microscopy showing A. 

cantonensis. This larva was obtained from a P. martensi slug collected in 

Hawaii. Infective, third-stage larvae measure 0.425 mm – 0.523 mm in length 


http://blogs.cdc.gov/publichealthmatters/2009/04/snails-slugs-and-semi-slugs-a-parasitic-disease-in-paradise/





Phase Contrast Microscopy 


http://en.wikipedia.org/wiki/Phase_contrast_microscopy




2.1.1 Conventional Magnifiers and Readers 

The major considerations for choosing a magnifier are: 

(1) power or magnification, (2) working distance, (3) field of 

view, (4) chromatic correction and (5) binocular or monocular vision. 

These magnifier attributes are interrelated. A high power magnifier, for 

example, has a short working distance, a small field of view and cannot he 

used for binocular observation. A low power magnifier, such as a rectangular 

reader lens, has a long working distance, a large field of view and can he 

used for binocular vision. To attain chromatic correction (to eliminate color 

fringing), the high power lens must be complex. It typically contains a 

cemented doublet or triplet of different optical glasses. By comparison, the 

low power reader lens is sufficiently achromatic as a simple lens. 


Chromatic correction (to eliminate color fringing) 


Chromatic correction (to eliminate color fringing) 


Table 1 shows the characteristics of a few typical magnifiers. These values 

are approximations because eye accommodation can cause each of the 

values to vary. Except for the reader lens, all magnifiers are used with the 

eye fairly close to the magnifier, giving the largest field of view. The reader 

lens is used binocularly and is normally held some distance away from the 

eyes. Because of its large diameter, the 3.5 x doublet magnifier has as large 

a field as the 2 x loupe. The double convex lens of the doublet magnifier with 

its central iris has a comparatively small field. The triplet is a three-element 

design having excellent optical correction for field coverage and reduction of 

color fringing. Its resolving power is the limit of detection for fine structures. In 

comparison, the doublet magnifier can barely differentiate two points 0.025 

mm (0.001 in.) apart. There are many variations of these characteristics. 

Commercial magnifiers can be as high as 30 x in power and there are many 

special mountings for particular applications. 


TABLE 1. Characteristics of typical magnifiers 

Magnifier Type 

Working 

Power 

Distance 

Resolving 

Field of View 

millimeters 

Power 

millimeters 

(inches) 

micrometers 

(inches) 

(mils) 

Reader lens 

90 x 40 (3.5 x 1.5) 

1.5 x 

100 (4) 

50 )2) 

Eyeglass loupe 

60 (2.375) 

2x 

90 (3.5) 

40 (1.5) 

Doublet magnifier 

60 (2.375) 

3.5 x 

75 (3) 

25 (1) 

Coddington magnifier 

19 (0.75) 

7x 

25 (I) 

10 (0.4) 

Triplet magnifier 

22 (0.875) 

10 x 

20 (0.75) 

7.5 (0.3) 


Reader Lens 


Eyeglass Loupe 








Doublet Magnifier 


Triplet Magnifier 


Coddington magnifier 


2.1.2 Surface Comparators 

The surface comparator is a magnifier that provides a means for comparing a 

test surface against a standard surface finish. The observer views the two 

surfaces side by side, as shown in Fig. 8. The surface comparator uses a 

small battery powered light source, a semitransparent beam divider and a 

10 x triplet. The light is divided between the reference surface and the 

standard surface. Flat and shiny surfaces reflect the filament image directly 

into the pupil of the eye so that these parts look bright. Sloping or rough 

surfaces reflect the light away from the pupil and such areas appear dark. 

This form of illumination sharply delineates surface pattern characteristics. 

The resolving power is about 7.5 p.m (0.3 mil). The field of view is about 1 

mm (0.4 in.) diameter. 


FIGURE 8. The surface comparator: (a) two surfaces magnified for comparison and (b) test 

setup 


Other Surface Comparators 


Other Surface Comparators 


2.1.3 Measuring Magnifier 

A measuring magnifier incorporates a measuring scale that is positioned 

against the test object to measure tiny details on its flat surfaces (see Fig. 9). 

A transparent housing permits light to fall on the measured surface. Scales 

are available for measurements in inches, millimeters and other units (see 

Fig. 10). The magnifier uses a 7 x triplet lens. The resolving power is about 1 

μm (0.04 mil). The diameter of the field of view is about 25 mm (1 in.). 


FIGURE 9. Measuring magnifier in transparent sleeve mount 


2.1.4 Illuminated Magnifiers 

Illuminated magnifiers range from large circular reader lenses, equipped with 

fluorescent lighting and an adjustable stand, to a small battery powered 10 x 

magnifier shaped like a pencil. Some illuminated magnifiers can be obtained 

in either a battery powered model or equipped for 115 V line operation. Such 

triplet magnifiers give about a 50 mm (2 in.) field of view. Resolving power is 

about 1.5 μm (0.06 mil). 


Illuminated Magnifiers 



2.2.0 General 

When magnifications above 10 x are required, the short working distance of 

the magnifier becomes a problem and a low power compound microscope is 

preferred. Two such magnifiers are described below. Their resolving powers 

are about 7.5 μm (0.3 mil). 


2.2.1 Wide Field Tubes 

The simplest form of compound microscope is a wide field tube, comprising 

an objective lens mounted in one end of a tube and an eyepiece in the other. 

This design is typically supplied either in a tripod sleeve mount or in a 

simplified microscope stand. Focusing is accomplished by a friction slide fit in 

a sleeve. The 10 x wide field tube covers a field of 25 mm (1 in.) and has a 

working distance (the clearance between the objective and test object) of 

about 80 mm (3.25 in.). The 40 x version has a field of about 6 mm (0.25 in.) 

and a working distance of about 40 mm (1.625 in.). The image from such a 

simple microscope is inverted and reversed and is not convenient for hand 

manipulation of the test object during observation. Wide field tubes are 

frequently equipped with eyepiece scales to permit measurements in the test 

object plane. 


Wide Field Tubes 


http://www.lmscope.com/produkt22/LM_Universal_DSLR_Weitfeld_Adapter_Tube30mm_en.shtml





2.2.2 Wide Field Macroscope 

The wide field macroscope is similar to a wide field tube, with the same 

magnification range (10 x to 40 x) and the same mounting and focusing 

devices. Unlike the wide field tube, the macroscope produces an image that 

is upright and not reversed, so that manipulation of the test object can be 

conveniently done during observation. 

The prism system that corrects the image also provides an inclined 

observation tube for more convenient prolonged viewing. The macroscope is 

often supplied with measuring scales for size determinations. 


FIGURE 10. Typical measuring scales and reticules (in inches) for the measuring magnifier 



2.3.0 General 

Typical medium power magnifiers range from 20 x to 100 x in a variety of 

designs. 


2.3.1 Wide Field Stereoscopic Microscopes 

As can be seen in Fig. 11, the wide field stereoscopic microscope is very 

complex. It is basically two erect image microscopes, one for each eye, 

comprising two objectives, two erecting prisms, two inclination prisms and two 

eyepieces. Furthermore, as shown in the figure, the stereoscopic microscope 

is usually supplied with several pairs of objectives in a nosepiece so that the 

power can be changed rapidly. It may also be provided with a glass stage and 

a substage mirror for transmitted illumination. 

The power range of the stereomicroscope is typically 7 x to 150 x , although 

its usefulness beyond 60 x is limited. The resolving power is about 5 μm (0.2 

mil). Field coverage is approximately inverse to the power: at 10 x field 

coverage is about 25 mm (1 in.). The instrument provides binocular vision, 

which makes possible its prolonged use for visual testing. Like the 

macroscopes, manual manipulation during observation is practical. 


The stereoscopic microscope provides a true view of depth, so that test 

objects may be inspected in three dimensions. There are many variations in 

the construction of the stereoscopic microscope. They are sometimes built on 

stands having long universal joint arms, permitting vertical as well as lateral, 

horizontal and angular movements, for scanning extended regions of the test 

object. A single pair or several paired objectives may he supplied and 

stereoscopic 


FIGURE 11. Wide field stereoscopic microscope 


Wide Field Stereoscopic Microscopes 


2.3.2 Shop Microscope 

The shop microscope is similar to a wide field tube. It is a simple tube with an 

objective near one end and an eyepiece at the other. It has a power of 40 x 

and contains a built-in light source that may be operated from a battery or 115 

V line current. The shop microscope contains a scale permitting direct 

measurement on the object plane to 0.025 mm (0.001 in.) or estimates to 6 

(0.25 mil), over a scale length of 4 mm (0.15 in.). The field of view is 5 mm 

(0.22 in.) and the resolving power is about 3.3 μm (0.13 mil). 

The instrument is extremely lightweight, only 500 g (18 oz) with dry cells. 

Applications of the shop microscope include on-site tests of plated, painted or 

polished surfaces; detection of cracks, blowholes and other discontinuities; 

and measurement of small holes in heading dies, 'gages and other machined 

components. It also provides a quick method for checking wear in mechanical 

components. Welding of machine tool frames, piping, structural members, 

pressure vessels, jigs and fixtures, can be quickly inspected. 


In finishing and electroplating operations, surface tests with the shop 

microscope can detect cracks, blister, irregular deposits, pitting and poor 

quality buffing or polishing. It can reveal slag inclusions and poor surfacing of 

base metals before plating. On painted surfaces it permits quick and accurate 

evaluation of quality, uniformity and pigment distribution. In the graphic arts, it 

is used to check halftones for size, shape and distribution of dots. 

Textile mills use shop microscopes for identification of fiber textures, 

distribution of coloring matter and test of weave, twist and other general 

characteristics. Fabric finishes, markings, lusters and dye transfers can be 

inspected for penetration and quality. In the paper industry, the shop 

microscope is used to check fiber uniformity, evenness of coating and wear of 

Fourdrinier wires. 


Shop Microscope 


Shop Microscope 


2.3.3 Brinell Microscope 

The Brinell microscope is similar to the shop microscope. It is specifically 

designed for measuring the diameter of an impression made by the ball of a 

Brinell hardness testing machine. Its magnification is 20 x, the field of view is 

8 mm (0.32 in.) and its resolving power is 3.5 μm (0.14 mil). The scale is 

calibrated to read (in tenths of millimeters) the actual size of the impression 

over a range of 6 mm. 

Focusing is accomplished by rotating the eyepiece in its spiral mount. 

Adequate illumination of the Brinell depression regardless of the color of the 

test object, is ensured by an annular mirror in the base of the microscope. 

The mirror reflects light on the viewing area and the outline of the Brinell 

impression stands out in contrast. Three types of illumination are available: 

integral battery in a side tube; 0.3 A, 3.8 V, with 115 V alternating current 

transformer; and daylight or ordinary room illumination. 


Brinell Microscope 




Brinell Tester 














Vicker Hardness Microscope 



2.4.0 General 

High power optical systems are used in laboratory, metallurgical, 

metallographic, polarizing, interference and phase contrast microscopes. The 

power of such systems ranges from 100 x to 2,000 x 


2.4.1 Laboratory Microscope 

The conventional compound microscope is often called a laboratory 

microscope. Inclined binocular eyepieces provide ease of vision over 

prolonged periods of use. Complexity of design for this type of microscope 

ranges from a simple straight monocular model for student use to elaborate 

systems for combined visual and photo-micrographic use. A great range of 

magnification, resolution and field coverage is available, depending on the 

objective design (see Table 2). 

The field coverage, magnification and resolving power given for the laboratory 

microscope may be roughly applied to other types of high power microscopes. 

The laboratory microscope is designed principally for transmitted light, so that 

it is largely useful on transparent or semitransparent materials. It is normally 

supplied with means for illuminating the test object under controlled 

conditions to provide the optimum balance between contrast and resolution. 


Among many available accessories are graduated mechanical stages, 

eyepiece and stage micrometer scales, filar micrometer eyepieces, 

comparison eyepieces for viewing two objects under separate microscopes, 

cross-line eyepieces and various cross-ruled slides for particle counting. 




TABLE 2. Ranges of magnification, resolution and field coverage based on objective design 

Objective (Numerical 

Resolving Power 

Approximate Real 

Approximate 

Aperture] 

Micrometers (mills) 

Field Diameter 

Useful Power Range 

Millimeters (Inches) 

3.5 x (0.09) 

3 (0.12) 

4.3 (0.17) 

20 to 50 x 

10.0 x (0 09) 

I (0.044) 

1.5 (0.06) 

50 to 100x 

21.0 x (0.50) 

0.6 (0.022) 

0.75 (0.029) 

100 to 250 x 

43.0 x (0.65) 

0.4 (0.017) 

0.35 (0.014) 

250 to 750 x 

97.0x (1.25) 

0.2 (0 009) 

0.15 (0.006) 

750 to 1,500x 

90.0 x (1.40) 

0.2 (0.008) 

0.15 (0.006) 

1,000 to 2,000 x 


2.4.2 Metallurgical Microscope 

The metallurgical microscope is similar to a laboratory microscope with the 

addition of top or vertical illumination to permit viewing of opaque materials. 

The vertical illuminator, located directly above the objective, is a semi 

reflecting, thin, transparent plate. It directs light down through the objective 

onto the test object. The microscope is normally equipped with a built-in light 

source and has field and aperture iris controls in the illuminating arm. 

Because thick preparations are common in opaque test objects, the stage 

may be focused. This also permits the use of an intense external light source, 

so that focusing can be carried out without upsetting the illumination centering. 

Although this microscope finds its principal applications in metallurgy, it can 

he used on almost any opaque material having a reasonably high reflectivity. 

When test objects are dark by nature (dark plastics, paints, minerals) or have 

excessive light scattering (fabrics, paper, wood, or biological specimens), 

a form of incident dark field illumination is superior to regular vertical 

illumination. 


2.4.3 Metallographic Microscope 

When a camera is built into a metallurgical microscope, it is called a 

metallographic microscope or a metallograph. In general, the increase in 

design complexity for a typical metallograph goes far beyond the simple 

addition of a camera. Most metallographic microscopes also have the 

following features. 

1. They are built on a stand with concealed shock absorbers. 

2. They use an intense light source, often an automatic carbon arc. 

3. They use an inverted stand so that the test object need not be plane 

parallel (the test object is face down on the stage). 

4. They have viewing screens for prolonged visual tasks such as dirt count or 

grain size measurements. 

5. They have bright field, dark field and polarized light illumination for diverse 

applications. 


Metallographic Microscope 


2.4.4 Polarizing Microscope 

The addition of two polarizing elements and a circular stage converts a 

laboratory microscope into an elementary polarizing microscope. A polarizing 

element is a device that restricts light vibration to a single plane. This form of 

light is useful for studying most materials with directional optical properties, 

including fibers, crystals, sheet plastic and materials under strain. As such 

materials are rotated between crossed polarizers on the microscope stage, 

they change color and intensity in a way that is related to their directional 

properties. 

The polarizing microscope normally has other added features, 

beyond the polarizing elements and circular stage. Much work, for example, 

requires study of crystal properties or minerals in three dimensions. The 

simplest of these accessories is the Bertrand lens, which focuses an image of 

the objective aperture in the eyepiece. In the aperture is a chart of crystal 

properties in many directions. For more quantitative work, a universal stage is 

used, on which the crystal can be rotated around one of five axes through its 

center. The amount of rotation is then measured. 


Polarizing Microscope on liquids 


Polarizing Microscope 


Polarizing Microscope on Liquid Crystals 


http://news.science360.gov/obj/pic-day/6a45e444-9b2d-45b6-83ca-b9043397384c/polarization-microscope-image-liquid-crystals


http://www.ebay.com/itm/Binocular-Polarizing-Microscope-40x-640x-/140927225743 




2.4.5 Interference Microscope 

The interference microscope is a tool using the wavelength of light as a unit of 

measure for surface contour and other characteristics. In one form of 

interference microscope, the stage is inverted and the test object is placed 

face downward. The image appears as a contour map, with a separation 

of one half-wave or about 0.25 μm (0.01 mil) between contour lines. 

Extremely precise measurements can be made with such equipment. 

Applications of the interference microscope include the measurement, testing 

and control of very fine finishes, including highly polished or glossy finished 

surfaces, where the degree of surface roughness is within a few wavelengths 

of light. With coarser surfaces, the contour lines are close together and 

interpretation is difficult. An advantage of the interference microscope is that 

the test object is not moved manually during inspection. A considerably less 

elaborate device called an interference 


objective is also available as an accessory to the metallurgical microscope. 

This objective has a small, metallized glass mounted in contact with the test 

object and adjustable for tilt to control fringe spacing. The disadvantage of the 

interference objective is that the test surface must be moved manually during 

inspection. Otherwise, its test results are virtually the same as those from an 

interference microscope. 


Interference Microscope 

Interference microscopes are used in interference microscopy. They are a 

variation of phase contrast microscopes and use a prism to split a beam in 

two. These beams allow a specimen to be seen through the difference in the 

fields caused by the two beams. Inference microscopes are more sensitive 

than phase contrast microscopes, which helps to avoid extra light. Inference 

microscopes and their different types have applications in biology, 

crystallography, mineralogy and chemistry 

Read more : http://www.ehow.com/about_5876329_interference-microscope_.html 








2.4.6 Phase Contrast Microscope 

Completely transparent materials with refractive index discontinuities can be 

only faintly seen in a normal microscope. Such index discontinuities are 

readily visible in a phase contrast microscope. Figure 12 shows the two 

additional optical elements needed to convert a normal microscope to a 

phase contrast microscope. An annular diaphragm located below the 

condenser is imaged into an annular phase shifting element in the objective. 

The combined effect of the diffracted and un-diffracted light transmitted by 

this phase shifting element produces contrast in a completely transparent 

object. The phase contrast microscope is limited to uses with transparent 

materials having very small index discontinuities. If the index discontinuities 

are gross, a normal microscope is used for visual inspection. Extensive work 

with living tissues and cells has been done with phase contrast devices. 


FIGURE 12. Arrangement of elements in a phase contrast microscope 








Phase contrast microscopy is an optical microscopy technique that 

converts phase shifts in light passing through a transparent specimen to 

brightness changes in the image. Phase shifts themselves are invisible, but 

become visible when shown as brightness variations. When light waves 

travels through a medium other than vacuum, interaction with the medium 

causes the wave amplitude and phase to change in a manner dependent on 

properties of the medium. Changes in amplitude (brightness) arise from the 

scattering and absorption of light, which is often wavelength dependent and 

may give rise to colors. Photographic equipment and the human eye are only 

sensitive to amplitude variations. Without special arrangements, phase 

changes are therefore invisible. Yet, phase changes often carry important 

information. 

Keywords: 

Phase Changes 



Phase contrast microscopy is particularly important in biology. It reveals many 

cellular structures that are not visible with a simpler bright field microscope, 

as exemplified in Figure 1. These structures were made visible to earlier 

microscopists by staining, but this required additional preparation and killed 

the cells. The phase contrast microscope made it possible for biologists to 

study living cells and how they proliferate through cell division.[1] After its 

invention in the early 1930s,[2] phase contrast microscopy proved to be such 

an advancement in microscopy, that its inventor Frits Zernike was awarded 

the Nobel prize (physics) in 1953. 


Phase contrast microscopy 


Phase contrast microscopy 


More Reading on Phase Contrast Microscopy: 

To understand the phase contrast microscope, it is first necessary to review 

the ordinary compound light microscope. It features a set of components that 

function together to transmit light from the subject being studied to the 

observer's eye. From the bottom up, these are the light source, the substage 

condenser, the mechanical stage, the glass slide holding the subject matter, 

the objective lens and the eyepiece lens. 

Viewing a specimen through the ordinary compound light microscope---called 

the bright field microscope---involves illuminating the object from below 

through an opening in the stage. The user selects an objective lens of the 

desired power by turning the nosepiece just above the microscope's stage. 

Peering through the microscope's eyepiece, the operator then adjusts the 

focus controls to bring the specimen into clear view. 


As the name suggests, the difference between the ordinary light microscope 

(the bright field microscope) and the phase contrast microscope concerns 

contrast. Living organisms or living cells tend to have low contrast, making 

details of their structure difficult to see. Some structures in a specimen, like 

the cilia of a paramecium or the flagellum of a euglena for example, may not 

have very much contrast compared to the surrounding medium. They would 

therefore not show up very well under regular bright field microscopy. 

Treating that specimen with certain stains can compensate for this. But the 

tradeoff is that this will kill the cells, preventing the user from observing details 

of a living cell. 

A phase contrast microscope, on the other hand, would correct for this by 

producing the desired contrast without killing the living cells. It seems to 

magically transform the view of a specimen showing very little contrast or 

detail in the bright field microscope into a dramatic image. 


The phase contrast microscope is essentially an ordinary light microscope 

with all the same basic components. The difference is that the phase contrast 

microscope features something called a phase plate. This plate is situated in 

the light path between the subject matter being viewed and the viewer's eye 

(actually in the objective lens housing). Additionally, a so-called phase ring or 

annulus is installed onto the substage condenser (the device that focuses 

light onto the specimen). 

The optical physics involved in the process can be a bit difficult. But suffice it 

to say that the phase contrast device alters the wavelengths of light that are 

traveling through the medium containing the specimen and the specimen 

itself. This wavelength alteration produced by the combination of the phase 

plate and the phase ring or annulus generates the desired contrast. Regular 

bright field light microscopes can be converted to phase contrast microscopes. 

This is done by installing the phase ring and objective lenses with the 

requisite phase plates manufactured into them. 

Read more : http://www.ehow.com/how-does_5150971_phase-contrast-microscope-work.html 










PART 3: BORESCOPES 


3.1.0 General 

The industrial fiber optic borescope is a flexible, layered sheath protecting two 

fiber optic bundles, each comprising thousands of glass fibers. One bundle 

serves as the image guide and the other bundle helps illuminate the test 

object. Light travels only in straight lines but optical glass fibers bend light by 

internal reflection and so can carry light around corners (see Fig. 13). Such 

fibers are 9 to 30 μm (0.4 to 1.2 mil) in diameter or roughly one-tenth the 

thickness of a human hair. 

A single fiber transmits very little light, but thousands of fibers may be 

bundled for transmission of light and images. To prevent the light from 

diffusing, each fiber consists of a central core of high quality optical glass 

coated with a thin layer of another glass with a different refractive index (Fig. 

14). 


This cladding acts as a mirror- all light entering the end of the fiber is reflected 

internally as it travels (Fig. 13) and cannot escape by passing through the 

sides to an adjacent fiber in the bundle. 

Although the light is effectively trapped within each fiber, not all of it emerges 

from the opposite end. Some of the light is absorbed by the fiber itself and the 

amount of absorption depends on the length of the fiber and its optical quality. 

For example, plastic fiber can transmit light and is less expensive to produce 

than optical glass but plastic is less efficient in its transmission and unsuitable 

for use in fiber optic borescopes. 

Keywords: 

Plastid fibers 

Optical glass fibers 


Fiber Optic 


FIGURE 13. Internal reflection of light in an optic fiber can be used to move the light 

path in a curve 


FIGURE 14. Light paths in fiber bundles: (a) uncoated fibers allow light to travel laterally through 

the bundle and (b) coated fibers restrict the light's path to its original fiber 


3.1.1 Fiber Image Guides 

The fiber bundle used as an image guide (see Fig. 15) carries the image 

formed by the objective lens at the distal end or tip of the borescope hack to 

the eyepiece. The image guide must be a coherent bundle: the individual 

fibers must be precisely aligned so that they are in identical relative positions 

at their terminations. Image guide fibers range from 9 to 17 μm (0.35 to 0.67 

mil) in diameter. Their size is one of the factors affecting resolution, although 

the preciseness of alignment is far more important. Note that a real image is 

formed on both highly polished 

faces of the image guide. Therefore, to focus a fiber optic borescope for 

different distances, the objective lens at the tip must be moved in or out, 

usually by remote control at the eyepiece section. A separate diopter 

adjustment at the eyepiece is necessary to compensate for differences in 

eyesight. 


Fiber Image Guides 






FIGURE 15. Optical fiber bundle used as an image guide 


3.1.2 Fiber Light Guides 

Another fiber bundle carries light from the an external high intensity source to 

illuminate the test object. This is called the light guide bundle and is 

noncoherent (see Fig. 16). These fibers are about 30 μm (1.2 mil) in 

diameter 

and the size of the bundle is determined by the diameter of the scope. 

Fiber optic borescopes usually have a controllable bending section near the 

tip so that the inspector can direct the borescope during testing and can scan 

an area inside the test object. Fiber optic borescopes are made in a variety of 

diameters, some as small as 3.7 mm (0.15 in.), in lengths up to 10 m (30 ft), 

and with a choice of viewing directions at the tip. 


FIGURE 16. Diagram of a typical fiber optic borescope 


Fiber Image 



The rigid borescope (see Fig. 17) was invented to inspect the bore of rifles 

and cannons. It was a thin telescope with a small lamp at the top for 

illumination. Most rigid borescopes now use a fiber optic light guide system as 

an illumination source. 

The image is brought to the eyepiece by an optical train consisting of an 

objective lens, sometimes a prism, relay lenses and an eyepiece lens. The 

image is not a real image but an aerial image: it is formed in the air between 

the lenses. This means that it is possible to both provide diopter correction for 

the observer and to control the objective focus with a single adjustment to the 

focusing ring at the eyepiece. 


Rigid Borescopes 


FIGURE 17. Typical lens system in a rigid borescope 


3.2.1 Focusing a Rigid Borescope 

The focus control in a rigid borescope greatly expands the depth of field over 

nonfocusing or fixed focus designs. At the same time, focusing can help 

compensate for the wide variations in eyesight among inspectors. Figures 18 

and 19 emphasize the importance of focus adjustment for expanding the 

depth of field. Figure 18 was taken at a variety of distances with fixed focus. 

Figure 19 was taken at the same distances as in Fig. 18 but with a variable 

focus, producing much sharper images. 


FIGURE 18. Borescope images for a variety of distances with fixed focus (see Fig. 19): (a) at 75 

mm (3 in.), (b) at 200 mm (8 in.) and (c) at 300 mm (12 in.) 


FIGURE 19. Borescope images with variable focus (see Fig. 18): (a) 75 mm (3 in.), (b) 200 mm 

(8 in.) and (c) 300 mm (12 in.) 


Borescopic Inspection 


3.2.2 Need for Specifications 

Because rigid borescopes lack flexibility and the ability to scan areas, 

specifications regarding length, direction of view and field of view become 

more critical for achieving a valid visual test. For example, the direction of 

view should always be specified in degrees rather than in letters or words 

such as north, up, forward, or left. Tolerances should also be specified. 

Some manufacturers consider the eyepiece to be zero degrees and therefore 

a direct view rigid borescope (Fig. 20a) is 180 degrees. Other manufacturers 

start with the borescope tip as zero degrees and then count back toward the 

eyepiece, making a direct-view 0 degrees. 


FIGURE 20. Borescope direction of view: (a) direct, (b) side, (c) forward oblique and (di 

retrospective 


3.2.3 Setup of a Rigid Borescope 

To find the direction and field of view during visual testing with a rigid 

borescope, place a protractor scale on a board or worktable. Position the 

borescope carefully so it is parallel to the zero line, with the lens directly over 

the center mark on the protractor. Remember that the optical center of a 

borescope is usually 25 to 50 mm (1 to 2 in.) behind the lens window. 

By sighting through the borescope, stick pins into the board at the edge of the 

protractor to mark the center and both the left and right edges of the view field. 

This simple procedure gives both the direction of view and the field of view 

(see Figs. 21 and 22). 


FIGURE 21. Field of view for a rigid borescope 


FIGURE 22. Field of view width for varying distances 


3.2.4 Mini-borescope 

One variation of the rigid borescope is called the mini-borescope (see Fig. 23). 

In this design, the relay lens train is replaced with a single, solid fiber. The 

fiber diffuses ions in a parabola from the center to the periphery of the 

housing, giving a graded index of refraction. Light passes through the fiber 

and at specific intervals an image is formed. The solid fiber is about 1 mm 

(0.4 in.) in diameter, making it possible to produce high quality and thin rigid 

borescopes from 1.7 to 2.7 mm (0.07 to 0.11 in.) in diameter. The lens 

aperture is so small that the lens has an infinite depth of field (like a pinhole 

camera) and no focusing mechanism is needed. 


FIGURE 23. Mini-borescope wide angle lens: (a) general shape and (la) lens detail 


3.2.5 Accessories 

Many accessories are available for rigid borescopes. Instant cameras, 35 mm 

cameras, and video cameras can be added to provide a permanent record of 

a visual test. Closed circuit television displays, with or without video tape, are 

common as well. Also available are attachments at the eyepiece permitting 

dual viewing or right angle viewing for increased accessibility. 



Angulated borescopes are available with forward oblique, right angle or 

retrospective visual systems. These instruments usually consist of an 

objective section with provision for attaching an eyepiece at right angles to 

the objective section's axis. This permits inspection of shoulders or recesses 

in areas not accessible with standard borescopes. Calibrated borescopes are 

designed to meet specific test requirements. The external tubes of these 

instruments can be calibrated to indicate the depth of insertion during a test. 

Borescopes with calibrated reticles are used to determine angles or sizes of 

objects in the field when held at a predetermined working distance. 


Panoramic borescopes are built with special optical systems to permit rapid 

panoramic scanning of internal cylindrical surfaces of tubes or pipes. 

Wide field borescopes have rotating objective prisms to provide fields of view 

up to 120 degrees. One application of wide field borescopes is the 

observation of models in wind tunnels under difficult operating conditions. 

Ultraviolet borescopes are used during fluorescent magnetic particle and 

fluorescent penetrant tests. These borescopes are equipped with ultraviolet 

lamps, filters and special transformers to provide the necessary wavelengths. 

Waterproof and vapor proof borescopes are used for internal tests of liquid, 

gas or vapor environments. They are completely sealed and impervious to 

water or other types of liquid. Water cooled or gas cooled borescopes are 

used for tests of furnace cavities, jet engine test cells and for other high 

temperature applications. 


Panoramic borescopes 



3.4.1 Aviation Industry 

The use of borescopes for tests of airplane engines and other components 

without disassembly has resulted in substantial savings in costs and time. A 

borescope of 11 mm (0.44 in.) diameter by 380 mm (15 in.) working length 

can be used by maintenance and service departments for visual testing of 

engines through spark plug openings, without dismantling the engines. An 

excellent view of the cylinder wall, piston head, valves and valve seats is 

possible and several hundred hours of labor are saved for each engine test. 

Spare engines in storage can also be inspected for corrosion of cylinder 

wall surfaces. 


Aircraft propeller blades are visually tested during manufacture. The entire 

welded seam of a blade can be inspected internally for cracks and other 

discontinuities. Propeller hubs, reverse pitch gearing mechanisms, hydraulic 

cylinders, landing gear mechanisms and electrical components also can 

be inspected with borescopes. Aircraft wing spars and struts are inspected for 

evidence of fatigue cracks and rivets and wing sections cam be tested 

visually for corrosion. Borescopes used for tests of internal wing tank surfaces 

and wing corrugations subject to corrosion have saved airlines large sums of 

money by reducing the time aircraft are out of service. 


Aircraft Inspection Applications 










3.4.2 Automotive Industry 

Borescopes are widely used in the manufacturing and maintenance divisions 

of the automotive industry. Engine cylinders can be examined through spark 

plug holes without removing the cylinder head. The cylinder wall, valves and 

piston head can be visually tested for excess wear, carbon deposits and 

surface discontinuities. Crankcases and crankshafts are examined through 

wall plug openings without removing the crankcase. Transmissions and 

differentials are similarly inspected. 

Borescopes are also useful for locating discontinuities such as cracks or 

blowholes in castings and forgings. Machined components such as cross 

bored holes can be examined for internal discontinuities. Borescopes are 

used to inspect cylinders for internal surface finish after honing. Tapped holes, 

shoulders or recesses also can be observed. Inaccessible areas of hydraulic 

systems, small pumps, motors and mechanical or electrical assemblies can 

be visually tested without dismantling the engine. 


Automotive Industry 


3.4.3 Machine Shops 

Borescopes find applications in production machine shops, tool and die 

departments and in ferrous, nonferrous and alloy foundries. In production 

machine operations, horescopes of various sizes and angles of view are used 

to examine internal holes, cross bored holes, threads, internal surface 

finishes and various inaccessible areas encountered in machine and 

mechanical assembly operations. Specific examples are visual tests of 

machine gun barrels, rifle bores, cannon bores, machine equipment and 

hydraulic cylinders. In tool and die shops, borescopes are used to examine 

internal finishes, threads, shoulders, recesses, dies, jigs, fixtures, fittings and 

the internal mating of mechanical parts. In foundries, horescopes are widely 

used for internal inspections to locate discontinuities, cracks, porosity and 

blowholes. Borescopes are also used for tests of many types of defense 

materials, including the internal surface finish of rocket heads, rocket head 

seats and guided missile components. 


Machine Shops 


3.4.4 Power Plants 

In steam power plants, borescopes are used for visual tests of boiler tubes for 

pitting, corrosion, scaling or other discontinuities. Borescopes used for this 

type of work are usually made in 2 or 3 m (6 or 9 ft) sections. Each section is 

designed so that it can be attached to the preceding section, providing an 

instrument of any required length. Other borescopes are used to examine 

turbine blades, generators, motors, pumps, condensers, control panels and 

other electrical or mechanical components without dismantling. In nuclear 

plants, horescopes offer the advantage that the inspector can be in a low 

radiation field while the distal, or sensor, end is in a high radiation field. 


3.4.5 Chemical Industry 

Visual tests of high pressure distillation units are used to determine the 

internal condition of tubes or headers. Evaporation tubes, fractionation units, 

reaction chambers, cylinders, retorts, furnaces, combustion chambers, heat 

exchangers, pressure vessels and many other types of chemical process 

equipment are inspected with borescopes or extension borescopes 

Tank cars are inspected for internal rust, corrosion and the condition of outlet 

valves. Cylinders and drums can be examined for internal conditions such as 

corrosion, rust or other discontinuities. 


3.4.6 Petroleum Industry 

Borescopes are used for visual tests of high pressure catalytic cracking units, 

distillation equipment, fractionation units, hydrogenation equipment, pressure 

vessels, retorts, pumps and similar process equipment. Use of the borescope 

in the examination of such structures is doubly significant. Not only does it 

allow the examination of inaccessible areas without the lost time and expense 

incurred in dismantling, it avoids breakdown and the ensuing costly repair. 



Borescopes are precise optical devices containing a complex system of 

prisms, achromatic lenses and plain lenses that pass light to the observer 

with high efficiency. An integral light source is usually located at the objective 

end of the borescope to provide illumination for the test object. 


3.5.1 Angles of Vision 

To meet a wide range of visual testing applications, borescopes are available 

in various diameters and working lengths to provide various angles of vision 

for special requirements. The most common types of vision are: (1) right 

angle, (2) forward oblique, (3) direct and (4) retrospective (see Fig. 20). 

These types of vision are characterized by different angles of obliquity for the 

central ray of the visual field, with respect to the forward direction of the 

borescope axis (see Table 3). 


TABLE 3. Comparison of vision types and angles of obliquity 


3.5.2 General Characteristics 

Desirable properties of borescopic systems are large field of vision, no image 

distortion, accurate transmission of color values and adequate illumination. 

The brightest images are obtained with borescopes of large diameter and 

short length. As the length of the borescope is increased, the image becomes 

less brilliant because of light losses from additional lenses required to 

transmit the image. To minimize such losses, lenses are typically coated with 

anti reflecting layers to provide maximum light transmission. 


3.5.3 Optical Components 

The optical system of a borescope consists of an objective, a middle lens 

system, correcting prisms and an ocular section (see Fig. 24). The objective 

is an arrangement of prisms and lenses mounted closely together. Its design 

determines the angle of vision, the field of view and the amount of light 

gathered by the system. 

The middle lenses conserve the light entering the system and conduct it 

through the borescope tube to the eye with a minimum loss in transmission. 

Design of the middle lenses has an important effect on the character of the 

image. For this reason, the middle lenses are achromatic, each lens being 

composed of two elements with specific curvatures and indexes of refraction. 

This design preserves sharpness of the image and true color values. 

Depending on the length of the borescope, the image may need reversal or 

inversion or both, at the ocular. This is accomplished by a correcting prism 

within the ocular for borescopes of small diameter and by erecting lenses for 

larger designs. 


3.5.4 Depth of Focus, Field of View and Magnification 

The depth of focus for a borescopic system is inversely 

related to the numerical aperture N. 

N = n sin a (Eq. 1) 

Where: 

n = the refractive index of the object space; and 

a = the angle subtended by the half diameter of the entrance pupil of the 

optical system. 


FIGURE 24. Sectional view of a typical borescope, showing relationship of parts in its optical 

system 


The entrance pupil is that image of any of the lens apertures, imaged in the 

object space, which subtends the smallest angle at the object plane. Because 

the numerical aperture of borescope systems is usually very small compared 

with that of a microscope, the corresponding depth of focus is exceedingly 

large. This permits the use of fixed focus eyepieces in many small and 

moderately sized instruments. Field of view, on the other hand, is relatively 

large, generally on the order of 50 degrees of angular field. This corresponds 

to a visual working field of about 25 mm (1 in.) diameter at 25 mm (1 in.) from 

the objective lens. At different working distances, the diameter of the field of 

view varies almost directly with the working distance (see Fig. 22). 

Magnification of a borescope 's optical system is given by the relation: 

M = m 1 x m 2 x m 3 (Eq. 2) 


where m,, m, and m, are the magnifications of the objective, middle lenses 

and ocular. The total magnification of borescopes varies with diameter and 

length but generally ranges from about 2 x to 8 x in use. Note that the linear 

magnification of a given borescope changes with working distance and is 

about inversely proportional to the object distance. A borescope with 2 x 

magnification at 25 mm (1 in.) working distance therefore will magnify 4 x at 

13 mm (0.5 in.) distance. 



A borescopic system usually consists of one or more borescopes having 

integral or attached illumination, additional sections or extensions, a battery 

handle, battery box or transformer power supply and extra lamps, all 

designed to fit in a portable case (see Fig. 25). The parts of a fixed length 

borescope for right angle vision are shown in Fig. 26. Also shown is a lamp at 

the objective end of the device. In this configuration, insulated wires are 

located between the inner and outer tubes of the borescope and serve as 

electrical connections between the lamp and the contacts at the ocular 

end. A contact ring permits rotation of the borescope through 360 degrees for 

scanning the object space without entangling the electrical cord. In other 

models, a fixed contact post is provided for attachment to a battery or a 

transformer, or the illumination is provided by fiber optic light guides (see Fig. 

16). 


Borescopes with diameters under 37 mm (1.5 in.) are usually made in 

sections, with focusing eyepieces, interchangeable objectives and high power 

integral lamps. This kind of borescope typically consists of an eyepiece or 

ocular section, a 1 or 2 in (3 or 6 ft) objective section, with I, 2 or 3 m (3, 6 

or 9 ft) extension sections. The extensions are threaded for fitting and ring 

contacts are incorporated in the junctions for electrical connections. Special 

optics can be added to increase magnification when the object is viewed at a 

distance. Eyepiece extensions at right angles to the axis of the borescope can 

be supplied, with provision to rotate the borescope with respect to the 

eyepiece extension, for scanning the object field. 


FIGURE 25. Components of typical borescope system (case not shown) 


3.6.1 Right Angle Borescopes 

The right angle borescope is usually furnished with the light source positioned 

ahead of the objective lens (see Fig. 26). The optical system provides vision 

at right angles to the axis of the borescope and covers a working field of 

about 25 mm (1 in.) diameter at 25 mm (1 in.) from the objective lens. 

Applications of the right angle borescope are widespread. The instrument 

permits testing of inaccessible corners and internal surfaces. It is available in 

a wide range of lengths, in large diameters or for insertion into apertures as 

small as 2.3 mm (0.09 in.). It is the ideal instrument for visual tests of rifle and 

pistol barrels, walls of cylindrical or recessed holes and similar components. 


Another application of the right angle borescope is inspection of the internal 

entrance of cross holes, where it may be critical to detect and remove burrs 

and similar irregularities that interfere with correct service. Drilled oil leads in 

castings can be visually inspected, immediately following the drilling operation, 

for blowholes or other discontinuities that cause rejection of the component. 

Right angle borescopes can be equipped with fixtures to provide fast routine 

tests of parts in production. The device's portability allows occasional tests to 

be made at any point in a machining cycle 


FIGURE 26. A typical right angle borescope 


3.6.2 Forward Oblique Borescopes 

The forward oblique system is a design that permits the mounting of a light 

source at the end of the borescope yet also allows forward and oblique vision 

extending to an angle of about 55 degrees from the axis of the borescope. 

A unique feature of this optical system is that, by rotating the borescope, the 

working area of the visual field is greatly enlarged. 

3.6.3 Retrospective Borescope 

The retrospective borescope has an integral light source mounted slightly to 

the rear of the objective lens. For a bore with an internal shoulder whose 

surfaces must be accurately tooled, the retrospective borescope provides a 

unique method of accurate visual inspection. 


3.6.4 Direct Vision Borescope 

The direct vision instrument provides a view directly forward with a typical 

visual area of about 19 mm (0.75 in.) at 25 ram (1 in.) distance from the 

objective lens. The light carrier is removable so that the two parts can be 

passed successively through a small opening. 


3.6.5 Section Borescopes 

Borescopes under 38 mm (1.5 in.) diameter are often made in pieces, with 

the objective section 1 or 2m (3 or 6 ft) in length. The additional sections are 1, 

2 or 3m (3, 6 or 9 ft) long with threaded connections. These sections may he 

added to form borescopes with lengths up to 15m (45 ft) for diameters under 

37 mm (1.5 in.). Tables 4 through 7 list the diameters and working lengths of 

typical borescopes. For special applications, custom made sizes and designs 

are available. 


TABLE 4. Specifications of right angle borescopes 



Borescopes can be built to meet many special visual testing requirements. 

The factors affecting the need for custom designs include: (1) the length and 

position of test area, (2) its distance from the entry port, (3) the diameter and 

location of the entry port and (4) inspector distance from the entry port. 

Environmental conditions such as temperature, pressure, water immersion, 

chemical vapors or ionizing radiation are important design factors. The range 

of special applications is partly illustrated by the examples given below 


3.7.1 Miniature Borescopes 

Miniature borescopes are made in diameters as small as 1.75 mm (0.07 in.), 

including the light source. They are useful because they can go into small 

holes. Inspection of microwave guide tubing is a typical application. 

3.7.2 Periscopes 

A large periscopic instrument with a right angle eyepiece and a scanning 

prism at the objective end is shown in Fig. 27. This instrument is 125 mm (5 

in.) in diameter and 9 m (27 ft) long. It is sectioned and provides for visual or 

photographic study of models in wind tunnels. A field of view 70 degrees 

in azimuth by 115 degrees in elevation is covered by this design. The cave 

borescope is a multiangulated, periscopic instrument used for remote 

observation of otherwise inaccessible areas. 


Periscopes 


3.7.3 Indexing Borescope 

Butt welds in pipes or tubing 200 mm (8 in.) in diameter or larger can be 

visually tested with a special 90 degree indexing borescope. The instrument 

is inserted in extended form through a small hole drilled next to the weld 

seam and is then indexed to the 90 degree position by rotation of a knob at 

the eyepiece. The objective head is then centered within the tube for viewing 

the weld. A second knob at the eyepiece rotates the objective head through 

360 degrees for scanning the weld seam. Another application of this 

instrument is for inspecting the inside surface of cathode ray tubes. 


3.7.4 Panoramic Borescopes 

The panoramic borescope has a scanning mirror mounted in front of the 

objective lens system. Rotation of the mirror is accomplished by means of an 

adjusting knob at the ocular end of the instrument. This permits scanning in 

one plane to cover the ranges of forward oblique, right angle and 

retrospective vision (see Fig. 28). Another form of panoramic borescope 

permits rapid scanning of the internal cylindrical surfaces of tubes or pipes. 

This instrument has a unique objective system that simultaneously covers a 

cylindrical strip 30 degrees wide around the entire 360 degrees with respect 

to the axis of the borescope. The diameter of this instrument is 25 mm (1 in.) 

and the working length is 1 m (3 ft) or larger. 


TABLE 5. Specifications of section borescopes with working lengths of 1, 2 and 3 m (3, 6 and 9 ft) 

and extension sections of 1, 2 and 3 m (3, 6 and 9 ft) 


TABLE 6. Specifications of forward oblique borescopes 


FIGURE 27. Eyepiece end of large wind tunnel periscope 


3.7.5 Reading Borescopes 

Low power reading borescopes are used in plant or laboratory setups for 

viewing the scales of instruments such as cathetometers at moderately 

remote locations. The magnification is about 3 X at 1 m (3 ft) distance. 


TABLE 7. Specifications of borescopes with separate light carriers 


FIGURE 28. Panoramic borescope: (a) comparative ranges of vision and (b) panoramic system 

components 


3.7.6 Photographic Adaptations 

Many borescopes also include the ability to record with still photography, 

motion picture or video tape. For example, still pictures on 35 mm film can be 

taken with a borescope fitted with an adapter designed for the purpose. A 

telescopic system with a movable prism built into the adapter operates 

on the reflex principle, permitting observation of the visual field of the 

horescope up to the instant of photographic exposure. High intensity light 

sources incorporated into the borescope provide illumination for 16 mm 

circular pictures on 35 mm film. Motion pictures are possible with a fiber optic 

light source or a rod illuminator that eliminates electrical connections and the 

heat of a lamp from the objective end of the borescope. This is especially 

valuable where explosive vapors are present. 


Photography of the interiors of large power plant furnaces during operation 

has been done since the 1940s using a unit power periscope and camera.' 

The periscope extends through the furnace wall and relays the optical image 

to the camera. A water cooled jacket protects the optical system and the 

camera from the furnace's high temperatures. With this equipment, still and 

motion picture studies have been made of the movement of the fuel bed and 

the action of the powdered fuel burner in furnaces operating at full load. 


Wireless Bluetooth Borescope 


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Self Study Notes Reading 4 Section 4B 

2014-August 



2014-August 


http://www.cnoocengineering.com/en/single_news_content.aspx?news_id=12343

At works 


Reading 4 


Visual & Optical testing- Section 4B 


2014-August 



Fion Zhang 

2014/August/15

SECTION 4 

BASIC AIDS AND ACCESSORIES FOR 

VISUAL TESTING 


SECTION 4: BASIC AIDS AND ACCESSORIES FOR VISUAL TESTING 









PART 3: BORESCOPES 









4.1 Lighting Techniques 

4.2 Optical Filtering 

4.3 Image Sensors 

4.4 Image Processing 

4.5 Mathematical Morphology 

4.6 Image Segmentation 

4.7 Optical Feature Extraction for High Speed Optical Tests 





5.2 Silicon Rubber Replicas 











7.3 Etching 





1.0 General 

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5.0 General 

Replication is a valuable tool for the analysis of fracture surfaces and 

microstructures and for documentation of corrosion damage and wear. There 

is also potential for uses of replication in other forms of surface testing. 

Replication is a method used for copying the topography of a surface that 

cannot be moved or one that would be damaged in transferal. A police officer 

making a plaster cast of a tire print at an accident scene or a scientist malting 

a cast of a fossilized footprint are common examples of replication. These 

replicas produce a negative topographic image of the subject known as a 

single stage replica. A positive replica made from the first cast to produce a 

duplicate of the original surface is called a second stage replica. Many 

replicating mediums are commercially available. The types commonly used in 

nondestructive testing typically fall into one of two categories: cellulose 

acetate replicas and silicone rubber replicas. Both have advantages and 

limitations but both can also provide valuable information without altering the 

test object. 



Acetate replicating material is used for surface cleaning, removal and 

evaluation of surface debris, fracture surface microanalysis and for 

microstructural evaluation. Single stage replicas are typically made, creating 

a negative image of the test surface. A schematic diagram of microstructural 

replication is shown in Fig. 56. 

5.1.1 Cleaning and Debris Analysis 

Fracture surfaces should be cleaned only when necessary. Cleaning is 

required when the test surface holds loose debris that could hinder analysis 

and that cannot be removed with a thy air blast. Cleaning debris from fracture 

surfaces is useful when the test object is the debris itself or the fracture 

surface. Debris removed from a fracture can be coated with carbon and 

analyzed using energy dispersive spectroscopy. This provides a 

semiquantitative analysis when a particular element is suspected of 

contributing to the fracture. 


Removal of loose surface particles is usually done by wetting a piece of 

acetate tape on one side with acetone, allowing a short period for softening 

and applying the wet side of the tape to the area of interest. Thicker tapes of 

0.013 mm (0.005 in.) work best for such cleaning applications (thin tapes tend 

to tear). Following a short period, the tape hardens and is removed. This 

procedure is normally repeated several times until a final tape removes no 

debris from the surface. 


5.1.2 Fracture Surface Analysis 

The topography of fracture surfaces can be replicated and analyzed using an 

optical microscope, scanning electron microscope or transmission electron 

microscope. The maximum useful magnification obtained using optical 

microscopes depends on the roughness of the fracture but seldom exceeds 

100 x . The scanning electron microscope has good depth of field at high 

magnifications and is typically used for magnification of 10,000 x or less. The 

transmission electron microscope has been used to document microstructural 

details up to 50,000 x . In general, scanning electron microscope analysis of a 

replica provides information regarding mode of failure and, in most instances, 

is sufficient for completion of this kind of analysis. 


An example of a replicated fracture surface is shown in Fig. 57. The 

transmission electron microscope is used in instances where information 

regarding dislocations and crystallographic planes is needed. Both single 

stage (negative) and second stage (positive) replicas can be used for failure 

analysis. Some scanning electron microscope manufacturers offer a reverse 

imaging module that provides positive images from a negative replica. This 

eliminates the need to think and interpret in reverse. This feature has also 

proven valuable for evaluating microstructures through replication. 

As with the removal of surface debris, it has been found that the thicker 

replicas provide better results, for the same reasons. The procedure for 

replication of fracture surfaces is identical to that for debris removal. On rough 

surfaces, however, difficulty may be encountered when trying to remove the 

replica. This can cause replication material to remain on the fracture surface 

but this can easily be removed with acetone. 


Replicas, in the as-stripped condition, typically do not exhibit the contrast 

needed for resolution of fine microscopic features such as fatigue striations. 

To improve contrast, shadowing or vapor deposition of a metal is performed. 

The metal is deposited at an acute angle to the replica surface and collects at 

different thicknesses at different areas depending on the surface topography. 

This produces a shadowing that allows greater resolution at higher 

magnifications. Shadowing with gold or other high atomic number metals 

enhances the electron beam interaction with the sample and greatly improves 

the image in the scanning electron microscope by reducing the signal-tonoise 

ratio. 


5.1.3 Microstructural Interpretation 

To date, the greatest advances in the use of acetate replicas 

for nondestructive testing have come from their use in microstructural 

testing and interpretation. Replication is an integral part of visual tests in the 

power generation industries as well as in refining, chemical processing and 

pulp and paper plants. Replication, in conjunction with microstructural 

analysis, is used to quantify microstrain over time and to predict the remaining 

useful life of a component. Future applications are not limited by material type. 

In industry, tests are carried out at preselected intervals to assess the 

structural integrity of components in their systems. These components can be 

pressure vessels, piping systems or rotating equipment. Typically these 

components are exposed to stresses or an environment that limits their 

service life. Replication is used to evaluate such systems and to provide data 

regarding their metallurgical condition. 


Microstructural replication is done in two steps: surface preparation followed 

by the replication procedure. Surface preparation involves progressive 

grinding and polishing until the test surface is relatively free of scratches 

(metallurgical quality). Depending on the material type and hardness, this 

can be obtained by using a I to 0.05 p.m (0.04 to 0.002 mil) polishing 

compound as the final step. Electrolytic polishing can increase efficiency if 

many areas are being tested. Surfaces can be electropolished with a 320-400 

grit finish. The disadvantages of electropolishing are that (1) the equipment is 

costly, (2) with most systems only a small area can he polished at one time 

and (3) pitting has been known to occur with some alloy systems containing 

large amounts of carbides. 

Next, the polished surface is etched to provide microstructural topographic 

contrast which may be necessary for evaluation. Etchants vary with material 

type and can be applied electrolytically, by swabbing or spraying the etchant 

onto the surface. With some materials, a combination of etch-polish etch 

intervals yields the most favorable results. 


To replicate the surface microstructure, an area is wetted with acetone and a 

piece of acetate tape is laid on the surface. The tape is drawn by capillary 

action to the metal surface, producing an accurate negative image of the 

surface microstructure. Thin acetate tape at 0.025 mm (0.001 in.) provides 

excellent results and gives the best resolution at high magnifications. Thicker 

tapes must be pressed onto the test surface and, depending on the expertise 

of the inspector, smearing can result. Thicker tapes are more costly and the 

resolution of microscopic detail does not match thinner tapes. Studies of 

carbide morphology and creep damage mechanisms have been performed at 

magnifications as high as 10,000 x with thin tape replicas. Before removal of 

the tape from the test object, the back is coated with paint to provide a 

reflective surface that enhances microscopic viewing. The replica is removed 

and can be stored for future analysis. 


If analysis with the scanning electron microscope is needed, replicas should 

be coated to prevent electron charging. This is accomplished by evaporating 

or sputter coating a thin conductive film onto the replica surface. Carbon, gold, 

gold-palladium and other metals are used for coating. There are differences in 

the sputtering yield from different elements and this should he remembered 

when choosing an element or when attempting to calculate the thickness of 

the coating. The main advantage of sputter coating over evaporation 

techniques is that it provides a continuous coating layer. Complete coating is 

accomplished without rotating or tilting the replica. With evaporation, only line 

of sight areas are coated and certain areas typically are coated more than 

others. 


Some examples of replicated microstructures, documented with both a 

scanning electron microscope and with conventional optical microscopy, are 

shown in Figs. 58 to 61. Replication is used for detection of high temperature 

creep damage, stress corrosion cracking, hydrogen cracking mechanisms, 

as well as the precipitation of carbides, nitrides and second phase 

precipitates such as sigma or gamma prime. Replication is also used for 

distinguishing fabrication discontinuities from operational discontinuities. 


Sputter deposition 

is a physical vapor deposition (PVD) method of thin film deposition by 

sputtering. This involves ejecting material from a "target" that is a source onto 

a "substrate" such as a silicon wafer. Resputtering is re-emission of the 

deposited material during the deposition process by ion or atom 

bombardment. Sputtered atoms ejected from the target have a wide energy 

distribution, typically up to tens of eV (100,000 K). The sputtered ions 

(typically only a small fraction — order 1% — of the ejected particles are 

ionized) can ballistically fly from the target in straight lines and impact 

energetically on the substrates or vacuum chamber (causing resputtering). 

Alternatively, at higher gas pressures, the ions collide with the gas atoms that 

act as a moderator and move diffusively, reaching the substrates or vacuum 

chamber wall and condensing after undergoing a random walk. The entire 

range from high-energy ballistic impact to low-energy thermalized motion is 

accessible by changing the background gas pressure. The sputtering gas is 

often an inert gas such as argon. 


For efficient momentum transfer, the atomic weight of the sputtering gas 

should be close to the atomic weight of the target, so for sputtering light 

elements neon is preferable, while for heavy elements krypton or xenon are 

used. Reactive gases can also be used to sputter compounds. The 

compound can be formed on the target surface, in-flight or on the substrate 

depending on the process parameters. The availability of many parameters 

that control sputter deposition make it a complex process, but also allow 

experts a large degree of control over the growth and microstructure of the 

film. 


Sputtering 

http://en.wikipedia.org/wiki/Sputter_deposition 


Sputtering 

http://en.wikipedia.org/wiki/Sputter_deposition 


Sputtering 

http://clearmetalsinc.com/technology/ 


Brief Introduction to Coating Technology for Electron Microscopy 

Coating of samples is required in the field of electron microscopy to enable or 

improve the imaging of samples. Creating a conductive layer of metal on the 

sample inhibits charging, reduces thermal damage and improves the 

secondary electron signal required for topographic examination in the SEM. 

Fine carbon layers, being transparent to the electron beam but conductive, 

are needed for x-ray microanalysis, to support films on grids and back up 

replicas to be imaged in the TEM. The coating technique used depends on 

the resolution and application 


http://www.leica-microsystems.com/cn/science-lab/coating-technology-for-electron-microscopy/

Sputter Deposition 


Sputter deposition 


5.1.4 Strain Replication 

The replication technique can be used to evaluate and quantify the 

occurrence of localized strain in materials exposed to elevated temperatures 

and stresses over time (materials susceptible to high temperature creep), 

Replication allows monitoring for accumulated strain before detectable 

microstructural changes occur. Strain replication involves inscribing a grid 

pattern onto a previously polished surface. A reference grid pattern is 

replicated using material with a shrinkage factor that has been quantified 

through analysis. This known shrinkage factor is included in future 

numerical analysis of strain. The grid is then coated to prevent surface 

oxidation during use. After a predetermined period of operation, the coating is 

removed and the area is again replicated. The grid intersection points on the 

two replicas are compared for dimensional changes and the changes are 

then correlated to units of strain. This technique does not yield absolute 

values of strain but does provide the change in strain calculated over time. 

This gives the operator information that can help approximate where the 

component is in its service life. 


Strain replication is especially useful for materials that do not exhibit creep 

void formation until late in their service life. As long as the strain calculations 

indicate a linear relationship between strain and time, the material is still said 

to be in the second stage region on the creep curve (see Fig. 62 and Table 

14). When the relationship deviates from linearity, the material has begun 

third stage or tertiary creep, where the strain rate can become unstable. 


FIGURE 56. Principles of acetate tape replication producing a negative image of the surface: (a) 

microstructure cross section, In softened acetate tape applied, (cJ replica curing and (d) replica 

removal 


FIGURE 57. Fracture surface documentation using replication shows fatigue striations on the 

surface at magnifications originally of (a) 2,000 x and (b) 10,000 x 


Fatigue striations 


FIGURE 58. Comparison of optical microscopy to scanning electron microscopy in the 

documentation of a replicated microstructure; evidence of creep damage is visible in the grain 

boundaries; etchant is aqua regia; 100 x original magnification: (a) optical microscope image and 

(b) scanning electron microscope image 


FIGURE 59. Documentation of creep damage: (al a weld viewed originally at 500 x in an optical 

microscope; the microstructure consists of an austenitic matrix, precipitated nitrides and carbides; 

linked creep voids can be observed; and lb) the alloy in Fig. 58 viewed originally at 1,000 x in a 

scanning electron microscope; grain boundary carbides, creep voids and particles believed to be 

nitrides can be observed in the matrix 


Creep Void 


FIGURE 60. Documentation of stress corrosion cracking found in the welds of an anhydrous 

ammonia sphere; 3 percent nital etch at 200 x original magnification 


FIGURE 61. Documentation of heat affected zone cracking in A516 grade 70 steel; cracking 

associated with a nonstress relieved repair weld; the presence of this repair weld was not known 

until in-field metallography and replication were performed; 3 percent nital etch at 100 x original 

magnification 


TABLE 14. Action required for creep damage in typical stressed material (see Figure 62) 

Damage Parameter 

isolated cavities 

Oriented cavities 

Microcracks 

Macrocracks 

Action Required 

no action until next major 

scheduled maintenance outage 

replica test at specified intervals 

limited service until repair 

immediate repair 


FIGURE 62. Creep damage curve showing the typical relationship of strain to time for a material 

under stress in a high temperature atmosphere; note that development of creep related voids in 

this alloy occurs early in service life; their eventual linkage is shown schematically on the curve 

[see reference 27): fa) isolated cavities, (b) oriented cavities, (c) microcracks and (d) 

macrocracks (see Table 14) 

http://www.scielo.br/scielo.php?pid=S1516-14392004000100021&script=sci_arttext 


Cellulose Acetate Replication 

http://corrosionhelp.com/failures.htm 


Replication Microscopy Techniques for NDE 

Fig. 2 Schematic of the plastic replica technique 


http://www.asminternational.org/documents/10192/1850228/06070G_Sample.pdf/f08974f0-1eca-4072-a2e1-cf60d5ae7e5d


Figure 3 Positive carbon extraction replication steps, (a) Placement of plastic after the first etch. 

(b) After the second etch. (c) After the deposition of carbon. (d) The positive replica offer the 

plastic is dissolved 


http://www.asminternational.org/documents/10192/1850228/06070G_Sample.pdf/f08974f0-1eca-4072-a2e1-cf60d5ae7e5d











5.2 Silicone Rubber Replicas 

Silicone impression materials have been used extensively in medicine, 

dentistry and in the science of anthropology. In nondestructive testing, 

silicone materials are used as tools for documenting macroscopic and 

microscopic material detail. Quantitative measurements can be obtained for 

depth of pitting, wear, surface finish and fracture surface evaluation. Silicone 

material is made with varying viscosities, setting times and resolution 

capabilities. Compared to an acetate replica, the resolution characteristics of 

a silicone replica is limited. With a medium viscosity compound, fine features 

visible at 50 x can be resolved but difficulties are encountered at higher 

magnifications. With a low viscosity compound, slightly better resolution is 

obtained but curing times are long and not suited to field applications. The 

lower viscosity medium is also known to creep with time and is not 

recommended for applications where very accurate dimensional studies are 

needed. 


5.2.1 Use of Silicone Replicating Materials 

Silicone replicating materials are supplied in two parts: a base material and 

an accelerator. Although it is best to follow the recommended mixing ratios, 

these can be altered slightly to change the working time of the material. The 

two parts are mixed thoroughly and spread over the subject area. Additional 

material can he added to thicken the replica. Molding clay can also be used to 

build a dam around a replicated area. The dam supports the replica as its 

sets and allows thicker replicas to be made. Measurements of pit depth and 

surface finish can be obtained easily because of the silicone's ability to flow 

into crevices on the test object. To evaluate pit depth and surface finish, the 

replica is cut and the cross section is examined with a microscope or a 

macroscopic measuring device (a micrometer or an optical comparator). 


Wear can be determined in a similar manner by replicating and comparing a 

worn surface to an unworn surface (see Fig. 63). Fracture surfaces with rough 

contours can be easily replicated with silicone (taking an acetate replica of 

such surfaces is difficult). However, the resolution characteristics of a silicone 

replica are not as good as acetate replicas and this limits the amount of 

interpretation that can be performed. Macroscopic details such as chevron 

markings can be easily located with the silicone technique to determine crack 

propagation direction or to trace a fracture path visually to its origin. 


FIGURE 63. Silicon replicas used to determine wear variance on a failed pinion gear 


Brittle Failure Chevron Marking 


Brittle Failure Chevron Marking 



Cellulose acetate tape and silicone impression materials are commonly used 

for nondestructive visual tests of surface phenomena such as corrosion, wear, 

cracking and microstructures. Both types of replicating material have 

advantages and limitations but when used in the correct application, can 

provide valuable information. In terms of resolution, the silicone replica 

typically does not have the capability to copy fine detail above 50 x . The 

acetate replica can reveal detail up to 50,000 x on a transmission electron 

microscope. The acetate replica is limited, however, by the roughness of the 

topography it can copy. On rough fracture surfaces, difficulty is encountered 

in both applying and removing an acetate replica. The silicone material is not 

as restrictive in terms of the surface features it can copy. The need for fine, 

resolvable detail versus macroscopic features normally indicates whether 

acetate or silicone replicas are best for the application. 



6.0 General 

A temperature indicating stick (chalk or crayon) is typically made of materials 

with calibrated melting points and temperature measuring accuracies to ± 1 

percent. Indicators are available in closely spaced increments over a range 

from 38 °C (100 °F) to 1,370 °C (2,500 °F). The workpiece to be tested is 

marked with the stick. When the workpiece attains the predetermined melting 

point of the indicator mark, the mark instantly liquefies, notifying the observer 

that the workpiece has reached that temperature. 


Pre-marking with a stick is not practical under certain circumstances- when a 

heating period is prolonged a highly polished surface does not readily accept 

a mark or the marked material gradually absorbs the liquid phase of the 

indicator. In such instances, the operator frequently marks the workpiece with 

the stick. The desired temperature is noted when one ceases to make dry 

marks and begins to leave a liquid smear. A similar procedure can be 

employed to indicate temperature during a cooling cycle. But a melted mark, 

on cooling, will not solidify at the exact same temperature at which it melted, 

so solidification of a melted indicator mark cannot be relied on for temperature 

indication. Temperature ratings are in increments as small as 3.4 °C (6 °F) 

but increments ranging from 14 to 28 °C (25 to 50 °F) are typically used for 

welding applications. For most applications, a jump of 28 to 56 °C (50 to 100 

°F) and a range of sticks up to 650 °C (1,200 °F) are usually adequate. 


Temperature indicating sticks were developed in America by a metallurgist 

working on submarine hulls in the 1930s. At the time, preheat was measured 

with so-called melting point standards, granules of substances with known 

melting points used to calibrate heat sensing instruments. The engineer used 

the granules directly, spreading them on the preheated metal and using their 

melt as a signal to proceed with welding. 

The melting point granules were next formed into sticks held together with 

organic hinders. Different temperature ratings were added and some 

refinements have been made but the principle of indicators has remained 

unchanged. The sticks make physical contact with the heated test object, 

reach thermal equilibrium rapidly and do not conduct heat away from the test 

surface. For temperature ratings less than 340 °C (650 °F), indicator marks 

can usually be removed with water or alcohol. For ratings above 340 °C 

(650 °F), water is preferred. If the mark has been heated well above the 

rated temperature and has become charred, abrasion may be needed for 

complete removal. 



In addition to the stick, temperature indicating pellets and liquids are available. 

The liquid indicator is brushed on before welding starts and is useful on highly 

polished surfaces or for making large marks viewed at a distance. Heat 

indicating pellets, about the size and shape of an aspirin, have greater mass 

than stick or lacquer marks (see Fig. 64). Pellets are sometimes selected for 

use with large, heavy pieces requiring prolonged heating- applications where 

stick or lacquer marks could fade with time. 


Levels of 


FIGURE 64, Temperature indicating pellets 



Temperature indicating sticks are mixtures of organic and inorganic 

compounds. The purity of the source materials directly affects the accuracy of 

the predicted melting point. There is the possibility of contamination with trace 

quantities of other elements, which may be detrimental to the accuracy of the 

indicator. In some cases, low melting point materials (lead, tin, sulfur, 

halogenated compounds) may be undesirable for the welding procedure. 

Most manufacturers can provide certification supported by analyses of typical 

batches. Documentation indicates which temperature ratings may contain 

contaminants that can be avoided by the user. 

In some critical applications (nuclear fabrication, aircraft assembly), actual 

chemical analysis of the specific lot number of the temperature indicators may 

be required. If the customer supplies a written certification requirement listing 

the compounds to be tested for, most manufacturers will send lot numbered 

samples for laboratory analysis. The customer is usually expected to pay lab 

charges for such specialized requirements. 


Marking materials used on austenitic stainless steels typically have a certified 

analysis that meets the following specified maximum amounts of detrimental 

contaminants: 

1. inorganic halogen content less than 200 ppm by weight; 

2. halogen (inorganic and organic) content less than 1 percent by weight; 

3. sulfur content less than 1 percent by weight (measured in accordance with 

ASTM D129); and 

4. total content of low melting point metal (lead, bismuth, zinc, mercury, 

antimony and tin) less than 200 ppm by weight and no individual metal 

content greater than 50 ppm by weight. 

The certification typically indicates the methods and accuracy of analysis and 

the name of the testing laboratory. 



Temperature indicators can be used for preheat temperature tests and in 

annealing and stress relieving procedures, hardfacing, overlaying for 

corrosion resistance, flame cutting, flame conditioning, heat treating, pipe 

bending, shearing of bar steel, straightening hardened parts, shrink fitting, 

brazing, soldering and nonferrous fabrication. The indicators can help find hot 

spots in insulation and engines, help monitor temperatures in curing and 

bonding operations and help check pyrometric calibration. 


6.3.1 Tests of Railway Bearings 

Bearing breakdown can be detected by using fluorescent temperature 

indicating pellets as heat sensors for inboard journal boxes. The pellets are 

inserted in a specially fabricated stainless steel holder that contains two 

pellets. The holder is inserted into the hollow axle of each rail car with an 

insertion tool. The tool has a mechanical stop to ensure that the holder is 

located at a predetermined depth. This permits proper monitoring of journal 

box operating temperatures. Once a specified temperature is exceeded, in 

this case 100 °C (212 °F), the pellets melt and flow completely out of the 

holder. The fluorescent material is easy to detect and clearly indicates that 

excessive heat has been conducted from the bearing to the axle. 


6.3.2 Verifying Oven Temperatures 

Technicians can determine if self cleaning ovens reach the proper cleaning 

temperature using pellets with pre-calibrated melting points at 450 °C (850 

°F). The pellets are placed on a flat piece of aluminum foil situated on the 

oven's center rack (see Fig. 65). The cleaning cycle is activated and as the 

temperature reaches 450 °C (850 °F), the pellets begin to melt. When the 

cleaning cycle is completed and the oven has cooled, the pellets are 

inspected—complete melting of the tablet verifies that the nominal cleaning 

temperature has been achieved. 


FIGURE 65. Pellets used to verify oven temperatures over 450 °C (850 °C) 


6.3.4 Process Control Applications 

A gas tight seal is needed to prevent leakage of combustion gases through 

the glass portion of a spark plug. To obtain optimum fusion properties, it is 

important to know and control the temperature inside the ceramic insulator 

and this can be done using a temperature indicating pellet. Sample insulators 

are loaded with pellets and processed with production parts. Information 

obtained from analyzing the samples is used to adjust furnace conveyor 

speed and temperature. 


6.3.5 Monitoring Fabric Seam Temperature 

In the making of specialized cloth (protective clothing, aerostat balloons), 

seam integrity is an important manufacturing function. A good radiofrequency 

seal can be achieved on a given fabric substrate only within a specific 

temperature range, determined by the minimum temperature needed to 

ensure a complete seal and the maximum temperature possible before 

material degradation. Constant temperature control and verification are 

required. This can be achieved using temperature sensitive strips (one for the 

upper limit, one for the lower limit) applied to the sealing tape used in 

production. A visual test of each seam after sealing indicates whether the 

seam temperature was within the required range, allowing visual verification 

of conditions for all dielectric seams. 


6.3.6 Precise Post forming Heat Control 

Temperature indicating materials are incorporated into 

many industrial applications where an indication is needed 

to show that a critical temperature has or has not been 

reached. A phase changing fusible liquid is used to indicate 

optimum postforming temperatures when bending decorative 

laminate for the contoured edges of countertops, desks, 

tables and other surfaces (see Fig. 66). 

Postforming is the process of bending a flat sheet of laminate 

around a radiused core material (particle board, plywood 

or fiber board). The process is typically done after controlled heating 

monitored with temperature sensitive liquids. Postforming can be a manual or 

mechanical operation. Hand postforming is used for unusual configurations or 

limited quantity production and mechanical postforming is used 

for high quantity production. Both methods have the need for a heat source, 

prepared cores, postforming grades of decorative laminate, pressuring guides 

and evenly applied pressure. 


A core is prepared by first shaping the edges to be laminated. 

The core and laminate are evenly coated with a 

contact adhesive, preferably a spray. The laminate is positioned 

and registered with the core, allowing the laminate to 

overhang the radius. Postforming grades of decorative laminate 

are formable between temperatures of 156 and 163 °C 

(313 and 325 °F). A popular example of hand postforming is the 180 degree 

edge wrap. In this example, radiant heat is applied to the 

decorative surface of the laminate with the work supported 

over the heat. To determine the proper postforming temperature, 

the temperature indicating liquid is painted in stripes 

onto the laminate. When the liquid changes from a dry 

(matte) to a wet (melted) appearance, the assembly is wiped 

into the cavity of a fixture to form the 180 degree radius. The 

fixture is a U channel made by two boards attached to a base. 

The dimension of the U channel is the thickness of the core 

plus the thickness of the laminate, allowing about 0.5 mm 

(0.02 in.) clearance. 


Another example of handforrning is known as a full wrap. 

In this application, the core is positioned over radiant heaters 

with temperature indicating stripes painted on the adhesive 

in the area of the radius. When the melt indicates forming 

temperature has been reached, the assembly is moved back 

onto a flat supporting surface. The wrapping action uses the 

flat surface as a pressure point. 

An example of mechanical postforming is the roll forming 

machine. Radiant heaters are located above an assembly 

supported by a moving carrier. When the forming temperature 

has been reached, slanted forming bars wipe the laminate 

over the radius. After the laminate has been formed, a 

succession of rollers maintains pressure until the assembly 

has cooled. In this application, temperature sensitive liquid 

is painted onto the laminate in order to verify that the dwell time under heat 

has been sufficient for reaching forming 

temperature. 


FIGURE 66. Laminate postforming around a radiused core 


6.3.7 Pipeline Coatings 

Epoxy powders are specially formulated to enhance corrosion 

proof resistance of utility pipe: that is, pipe usually buried 

underground, where it is subject to widely varying 

pipeline operating conditions. Intimately bonded to the 

pipe, the bonded epoxy is unaffected by widely varying soil 

compaction, moisture penetration, fungus attack, soil acids 

and chemical degradation. To achieve a long lasting bond of epoxy coating to 

metal pipe, the pipe must be preheated very carefully to the recommended 

preheat of 230 °C (450 °F). A spot on the pipe needs to be touched with the 

stick; its melting shows that the correct temperature for coating has been 

reached. 


6.3.8 Preheating before Welding 

Heating to the proper temperature before welding lessens 

the danger of crack formation and shrinkage stresses in many 

metals. Hard zones near the weld are reduced and lessen the 

possibility of distortion. Preheating also helps diffuse hydrogen 

from steel and helps reduce the likelihood of subsequent 

hydrogen inclusions. 

The need for preheating increases with the mass of the 

material being welded. It is most useful for the thick, heavy 

weldments used in bridge construction, shipbuilding, pipelines 

and pressure vessels. Preheating is also recommended 

for (1) welding done at or below – 18 °C (0 °F); (2) when the 

electrode is a small diameter; (3) when the joined pieces are 

of different masses; (4) when the joined pieces are of complex 

cross section; and (5) for welding of high carbon or manganese 

steels. 


The most common use for temperature indicators is the 

measurement of preheat, postheat and interpass temperatures 

for welding. In a typical application, the welder marks 

the test surface with an indicating stick of a specific temperature 

rating (see Fig. 67). When the mark changes phase 

(melts), the material has reached the correct temperature 

and is ready for welding. It is important for the user to 

understand that change of color has no significance; only the 

actual melting of the mark should be considered. 

Oxyacetylene equipment cannot be used for welding or 

cutting of high strength steels used in automotive components 

because too much heat can reduce their structural 

strength. However, in some instances an oxyacetylene torch 

may be used if the critical temperature of 760 °C (1,400 °F) 

for high strength steel is not exceeded. 

When preheat temperatures are 370 °C (700 °F) or when 

heating is prolonged, an indicating mark could evaporate or 

could be absorbed by the test material, Under these conditions, 

marks should be added periodically during heating. 


When the rated temperature is reached, the stick leaves a liquid 

streak instead of a dry mark and welding can begin. 

To ensure accurate temperature indication with no override, 

two or more indicators can be used to alert the operator 

that the test object is approaching the correct temperature. 

When a range of recommended preheat temperatures is 

given, use of several indicators might be appropriate. For 

example, carbon-molybdenum steel should he preheated to 

between 95 and 205 °C (between 200 and 400 °F). A bundle 

of indicators with ratings at 95, 120, 150, 175 and 205 °C 

(200, 250, 300, 350 and 400 °F) might be useful for 

determining how much of the test object is within the preheat 

temperature range. 


FIGURE 67. Temperature indicating stick 


Preheating 



7.0 General 

The information contained in this text is simplified and 

provided only for general instruction. Local health (OSHA) 

and environmental (EPA) authorities should be consulted 

about the proper use and disposal of chemical agents. For 

reasons of safety, all chemicals must he handled with care, 

particularly the concentrated chemicals used as aids to visual 

and optical tests. 

In visual nondestructive testing, chemical techniques are 

used to clean and enhance test object surfaces. Cleaning 

processes remove dirt, grease, oil, rust and mill scale. Contrast 

is enhanced by chemical etching. 


Macroetching is the use of chemical solutions to attack 

material surfaces to improve the visibility of discontinuities 

for visual inspection at normal and low power magnifications. 

Caution is required in the use of these chemicals—the use 

of protective clothing and safety devices is imperative. Test 

object preparation and the choice of etchant must be appropriate 

for the inspection objectives. Once the desired etch is 

achieved, the metal surface must be flushed with water to 

avoid over etching. 



Figure 68 shows typical test objects removed from their 

service environment. Governing codes, standards or specifications 

may determine the number and location of visual 

tests. Specific areas may contain discontinuities from forming 

operations such as casting, rolling, forging or extruding. 

Weld tests may be full length or random spots and typically 

cover the weld metal, fusion line and heat-affected zone. 

The service of a component may also indicate problem areas 

requiring inspection. Location of the test site directly affects surface 

preparation. The test site may he prepared and nondestructively 

inspected in situ. Removal of a sample for laboratory examination 

is a destructive alternative test method that typically requires a repair weld. 


FIGURE 68. Components removed from service for visual testing 



Preparation of the test object before etching may require 

only cleaning or a process including cleaning, grinding and 

fine polishing (improper grinding is shown in Fig. 69). The 

extent of these operations depends on the etchant, the material 

and the type of discontinuity being sought. 

7.2.1 Solvent Cleaning 

Solvent cleaning can be useful at two stages in test object 

preparation. An initial cleaning with a suitable solvent 

removes dirt, grease and oil and may make rust and mill scale 

easier to remove. One of the most effective cleaning solvents is a solution of 

detergent and water. However, if water is detrimental to the 

test object, organic solvents such as ethyl alcohol, acetone or 

naphthas have been used. These materials generally have 

low flash points and their use may be prohibited by safety 

regulations. Safety solvents such as the chlorinated hydrocarbons and high 

flash point naphthas may be required to meet safety standards. 


FIGURE 69. Improper surface preparation; the grind marks mask indications and even a severe 

etchant does not give good test results 


7.2.2 Removing Rust and Scale 

Rust and mill scale are normally removed by mechanical 

methods such as wire brushing or grinding. If appropriate 

for a particular test, the use of a severe etchant requires only 

the removal of loose rust and mill scale. Rust may also be 

removed chemically. Commercially available rust removers 

are generally inhibited mineral acid solutions and are not 

often used for test object preparation. 

Most surface tests require complete removal of rust and 

mill scale but a coarsely ground surface is often adequate 

preparation before etching. 

Grinding may be done manually or by belt, disk or surface 

grinding tools. Surface grinders are usually found only in 

machine shops. Hand grinding requires a hard flat surface 

to support the abrasive sheet. Coolant is needed during 

grinding and water is the preferred coolant but kerosene may 

be used if the test material is not compatible with water. 


7.2.3 Grinding and Polishing 

Fine grinding and polishing are needed for visual tests of 

small structural details, welds and the effects of heat treatment. 

Finer grinding usually is done with 80 to 150 abrasive 

grit followed by 150 to 180 grit and finally 400 grit (an American 

indication of grit size, 400 being the finest). At each 

stage, marks from previous grinding must be completely removed. Changing 

the grinding direction between successive 

stages of the process aids the visibility of previous 

coarser grinding marks. Coolant is required for grinding and 

typical abrasives include emery, silicon carbide, aluminum 

oxide and diamond. 


If the required finish cannot be achieved by fine grinding 

with 400 grit abrasive, the test surface must be polished. Polishing 

is generally done with a cloth-covered disk and abrasive 

particles suspended in paste or water. Common 

polishing media include aluminum oxide, magnesium oxide, 

chromium oxide, iron oxide and diamond with particle sizes 

ranging from 0.5 to 15 μm. During polishing, it is critical that all marks from 

the previous 

step he completely removed. If coarser marks do not 

clear, it may be necessary to repeat a previous step using 

lighter pressure before continuing. Failure to do so can yield 

false indications. 


7.3 Etching 

7.3.1 Choice of Etchant 

The etchant, its strength, the material and the discontinuity 

all combine to determine surface finish requirements (see 

Table 15). Properly selected etchants chemically attack the 

test material and reveal welds (Fig. 70), pitting (Fig 71), 

grain boundaries, segregation, laps, seams, cracks aria heat 

affected zones. The indications are highlighted or contrasted 

with the surrounding base material. 


FIGURE 70. Example of contrast revealing a weld in stainless steel 


FIGURE 71. Effect of etching: (a) unetched component with shiny appearance at rolled area 

and (t)) pits are visible in the dulled area after etching with ammonium persulfate 


7.3.2 Safety Precautions 

Etchants are solutions of acids, bases or salts in water or 

alcohol. Etchants for macroetching are water based. Etching 

solutions need to be fresh and the primary concerns during 

mixing are safety concentration and purity 

Safety precautions are necessary during the mixing and 

use of chemical etchants. Chemical fumes are potentially 

toxic and corrosive. Mixing, handling or using etchants 

should be done only in well ventilated areas, preferably in an 

exhaust or fume hood. Use of an exhaust hood is mandatory 

when mixing large quantities of etchants. Etching large 

areas requires the use of ventilation fans in an open area or 

use of an exhaust hood. Contact of etchants with skin, eyes or clothes should 

beavoided. When pouring, mixing or handling such chemicals, protective 

equipment and clothing should be used, including but not limited to glasses, 

face shields, gloves, apron or laboratory jacket. A face-and-eye wash fountain 

is recommended where chemicals and etchants are sorted and handled. A 

safety shower is recommended when large quantities of 

chemicals or etchants are in use. 


Should contact occur, certain safety steps must be followed, 

depending on the kind of contact and the chemicals 

involved. Skin should be washed with soap and water. 

Chemical burns should have immediate medical attention. 

Eyes should be flushed at once with large amounts of water 

and immediate medical attention is mandatory. Hydrofluoric 

and fluorosilic acids cause painful burns and serious 

ulcers that are slow to heal. Immediately after exposure, the 

affected area must be flooded with water and emergency 

medical attention sought. Other materials that are especially harmful in 

contact with skin are concentrated nitric acid, sulfuric acid, chromic acid, 

30 and 50 percent hydrogen peroxide, sodium hydroxide, potassium 

hydroxide, bromine and anhydrous aluminum chloride, These materials also 

produce vapors that cause respiratory irritation and damage. 


Etchant Safety 








7.3.3 Containers 

Containers used with etchants must be rated for mixing, 

storing and handling of chemicals. Glass is resistant to most 

chemicals and is most often used for containment and stirring 

rods. Hydrofluoric acid, other fluorine based materials, 

strong alkali and strong phosphoric acids can attack glass, 

requiring the use of inert plastics. 

Keywords: 

Strong phosphoric acid attack glass 


7.3.4 Generation of Heat 

Heat may be generated when chemicals are mixed 

together or added to water. Mixing chemicals must be done 

using accepted laboratory procedures and caution. Strong 

acids, alkalis or their concentrated solutions incorrectly 

add to water, alcohols or other solutions, cause violent 

chemical reactions. To be safe, never add water to concentrated 

acids or alkalis. In general, the addition of acidic materials to alkaline 

materials will generate heat. Sulfuric acid, sodium hydroxide 

or potassium hydroxide in any concentration generate large 

amounts of heat when mixed or diluted and an ice bath may 

be necessary to provide cooling. Three precautions in mixing 

can reduce or prevent a violent reaction: 

Keywords: 

To be safe, never add water to concentrated acids or alkalis. 


1. add the acid or alkali to the water or a weaker solution; 

2. slowly introduce acids, alkali or salts to water or solution; and 

3. stir the solution continuously to prevent layering and a delayed violent 

reaction. 


7.3.5 Chemical Purity 

Chemicals are available in various grades of purity ranging 

from technical to very pure reagent grades. For etchants, the 

technical grade is used unless a purer grade is specified. For 

macroetchants, the technical grade is generally adequate. 

Water is the solvent used for most macroetching solutions 

and water purity can affect the etchant. Potable tap water 

may contain some impurities that could affect the etchant. 

Distilled water has a significantly higher purity than tap 

water. For macroetchants using technical grade chemicals, 

potable tap water is usually acceptable. For etchants in 

which high purity is required, distilled water is 

recommended. 


7.3.6 Disposal 

Before disposing of chemical solutions, check environmental 

regulations (federal, state and local) and safety 

department procedures. The steps listed here are used only 

if there are no other regulations for disposal. Spent etchants 

are discarded and must be discarded separately—mixing of 

etchant materials can produce violent chemical reactions. 

Using a chemical resistant drain under an exhaust hood, 

slowly pour the spent etchant while running a heavy flow of 

tap water down the drain. The drain is flushed with a large 

volume of water. 



After proper surface preparation and safe mixing of 

etchants, the application of etchants to the test object may be 

done with immersion or swabbing. The technique is determined 

by the characteristics of the etchant being used. 

7.4.1 immersion 

During immersion, a test object is completely covered by 

an etchant contained in a safe and suitable material- glass can be used for 

most etchants except hydrofluoric acid, fluorine 

materials, strong alkali and strong phosphoric acid. 

A glass heat resistant dish on a hot plate may be used for 

heated solutions. The solution should be brought to temperature 

before the test object is immersed. Tongs or other handling 

tools are used and the test object is positioned so that 

the test surface is face up or vertical to allow gas to escape. 

The solution is gently agitated to keep fresh etchant in contact 

with the test object. 


7.4.2 Swabbing 

Etching may also be done by swabbing the test surface 

with a cotton ball, cotton tipped wooden swab, bristled acid 

brush, medicine dropper or a glass rod. The cotton ball and 

the cotton tipped wooden swab generally are saturated with 

etchant and then rubbed over the test surface. 

Tongs and gloves should be used for protection and the 

etchant applicator must be inert to the etchant. For example, 

strong nitric acid and alkali solutions attack cotton and 

these etchants must be applied using a fine bristle acid 

brush. A glass or plastic medicine dropper may be used to 

place etchants on the test object surface and a suitable stirring 

rod can be used to rub the surface. The test object may 

be immersed in etchant and swabbed while in the solution. 


7.4.3 Etching Time 

Etching time is determined by: 

1. the concentration of the etchant, 

2. the surface condition and temperature of the test object and 

3. the type of test material 

(see Tables 16 and 17). During etching, the material surface loses its bright 

appearance and the degree of dullness is used to determine 

when to stop etching. Approximate dwell times are given in 

the table procedures but experience is important as well. 


7.4.4 Test Object Preservation 

Rinsing, drying, de-smutting and coating may be required 

for preservation of the test object. Rinsing removes the 

etchant by flushing the surface thoroughly under running 

water. Cold water rinsing usually produces better surface 

appearance than hot water rinsing. Hot water rinsing does 

aid in drying. 

If smutting is a problem, the test object can be scrubbed 

with a stiff bristled brush or dipped in a suitable de-smutting 

solution. The test object should be dried with warm dry air. 

Shop air may be used if it is filtered and dried. After visual 

inspection, the test surface may be coated with a clear acrylic 

or lacquer but such coatings must be removed before subsequent 

tests. If the component is returned to service, a photographic 

record of the macro-etched area should be made. 


TABLE 16. Etchant characteristics and uses 

TABLE 16. Etchant characteristics and uses* (continued) 

TABLE 17. Etchants for welds 

See Text. 



Visual testing is performed in accordance with applicable 

codes, standards, specifications and procedures. Chemical 

aids enhance the contrast of discontinuities making them 

easier to interpret and evaluate. This enhancement is 

attained by macroetching- a controlled chemical processing 

of the surface. Macroetching gives the optimum 

results on a properly cleaned and prepared surface. Chemicals 

for etching must be mixed, stored, handled and applied 

in strict accordance with safety regulations.'''' 


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Experts at Work 



Casting Definitions and Terminology 

Manufacturing Menu | Casting Manufacturing Services 

http://www.engineersedge.com/casting_definition.htm 

My ASNT Level III VI Self Study Notes 


A 

As-Cast Condition: Casting without subsequent heat treatment. 


B 

Back Draft: Reverse taper which would prevent removal of a pattern from a 

mold or a core from a corebox 

Bar, Flask: A rib in the cope of a tight flask to help support the sand. 

Backing Sand: The bulk of the sand in the flask. The sand compacted on top of 

the facing sand that covers the pattern. 

Binder: The bonding agent used as an additive to mold or core sand to impart 

strength of plasticity in a dry state. 

Blasting or Blast Cleaning: A process for cleaning or finishing metal objects by 

use of an air blast or centrifugal wheel that throws abrasive particles against the 

surfaces of the work pieces. Small irregular particles of metal are used as the 

abrasive in grit blasting; sand in sand blasting; and steel balls in shot blasting. 

Bleeder: A defect wherein a casting lacks completeness due to molten metal 

draining or leaking out of some part of the mold cavity after pouring has stopped. 

Burn-On Sand: Sand adhering to the surface of the casting that is extremely 

difficult to remove. 

Burn-Out: Firing a mold at a high temperature to remove pattern material 

residue. 


C 

Casting Yield: The weight of casting or castings divided by the total weight of 

metal poured into the mold, expressed as a percent. 

Centrifugal Casting: A process of filling molds by 1) pouring metal into a sand or 

permanent mold that is revolving about either its horizontal or its vertical axis; or 

2) pouring metal into a mold that is subsequently revolved before solidification of 

the metal is complete. See also Centrifuge Casting. 

Centrifuge Casting - A casting technique. 

Cavity: The portion of a cast which forms the external shape 

Chaplet: A small metal insert or spacer used in molds to provide core support 

during the casting process. 

Charge: A given weight of metal introduced into the casting furnace. 

Chill: A metal insert in the sand mold used to produce local chilling and equalize 

rate of solidification throughout the casting. 

Cleaning: Removal of runners, risers, flash, surplus metal and sand from a 

casting. 

CO2 Process: Molds and cores made with sand containing sodium silicate are 

instantly hardened by permeating the sand with carbon dioxide gas. 


Coining: A press metal-working operation which establishes accurate 

dimensions of flat surfaces or depressions under predominantly compressive 

loading. 

Cold Shot: Small globule of metal embedded in, but not entirely fused with the 

casting. 

Cold Shut: A casting defect caused by imperfect fusing of molten metal coming 

together from opposite directions in a mold or due to folding of the surface. 

Collapsible Core: A metal insert made in two or more pieces to permit withdrawal 

from an undercut mold surface. 

Cope: The top half of a horizontally parted mold. 

Core: A sand or metal insert in a mold to shape the interior of the casting or that 

part of the casting that cannot be shaped by the mold pattern. The portion of the 

cast which forms the internal shape 

Core Assembly: An assembly made from a number of cores. 

Corebox: The wooden, metal or plastic tool used to produce cores. 


Coreprint: A projection on a pattern that leaves an impression in the mold for 

supporting the core. 

Core Wash: A liquid suspension of a refractory material applied to cores and 

dried (intended to improve surface of casting). 

Crush: The displacement of sand at mold joints. 

Cuploa: A cylindrical, straight shaft furnace (usually lined with refractories) for 

melting metal in direct contact with coke by forcing air under pressure through 

openings near its base. 

Cure: To harden. 


D 

Die: A metal form used as a permanent mold for die casting or for wax pattern in 

investment casting. 

Die Casting: A casting process in which the molten metal is forced under 

pressure into a metal mold cavity. 

Die Cavity: The impression in a die into which pattern material is forced. 

Directional Solidification: The solidification of molten metal in a casting in such a 

manner that liquid feed metal is always available for that portion that is just 

solidifying. 

Draft: Taper on the vertical sides of a pattern or corebox that permits the core or 

sand mold to be removed without distortion or tearing of the sand. Angle of draft 

varies and is dependant on surface length as well as process employed during 

cast. 

Draft (Pattern) - The taper on the sides of pattern which are perpendicular to the 

parting plane that allows the pattern to be withdrawn from the mold without 

breaking the edges of the mold. 


E 

Ejector Pins: Movable pins in the pattern die that "push" to remove cast pattern 

from the dies. 

External Undercut: Any recess or projection on the outside of the die block which 

prevents its removal from the cavity. 

F 

Facing Sand: The sand used to surround the pattern that produces the surface in 

contact with the molten metal. 

Feeder: Also called "riser", it is part of the gating system that forms the reservoir 

of molten metal necessary to compensate for losses due to shrinkage as the 

metal solidifies. 

Flask: A rigid metal or wood frame used to hold the sand of which a mold is 

formed and usually consisting of two parts, cope and drag. 

Foundry Returns: Metal (of unknown composition) in the form of gates, sprues, 

runners, risers and scraped castings returned to the furnace for remelting. 


G 

Gas Porosity: A condition existing in a casting caused by the trapping of gas in 

the molten metal or by mold gases evolved during the pouring of the casting. 

Green Sand: A molding sand that has been tempered with water and is 

employed for casting when still in the damp condition. 

Green Sand Mold: A mold composed of moist molding sand and not dried before 

being filled with molten metal. 

H 

Hotbox Process: A resin-based process that uses heated metal coreboxes to 

produce cores. 

Hot tear: Irregularly shaped fracture in a casting resulting from stresses set up by 

steep thermal gradients within the casting during solidification. 


I 

Inclusions: Particles of slag, refractory materials, sand or deoxidation products 

trapped in the casting during pouring solidifications. 

Internal Shrinkage: A void or network of voids within a casting caused by 

inadequate feeding of that section during solidification. 

Inverse Chill: The condition in a casting section where the interior is mottled 

or white, while the other sections are gray iron. Also known as Reverse Chill, 

Internal Chill and Inverted Chill. 

Investment Casting Process: A pattern casting process in which a wax or 

thermoplastic pattern is used. The pattern is invested (surrounded) by a 

refractory slurry. After the mold is dry, the pattern is melted or burned out of 

the mold cavity, and molted metal poured into the resulting cavity 


L 

Loose Piece: 

1) Core box; part of a core box which remains embedded in the core, and is 

removed after lifting off the core box. 2) Pattern; laterally-projecting part of a 

pattern so attached that it remains in the mold until the body of the pattern is 

drawn. Back-draft is avoided by this means. 3) Part of a permanent mold 

which remains on the casting, and is removed after casting is ejected from the 

mold. 

Lost Wax Process: A casting process in which an expendable pattern made of 

wax or similar material is melted or burned out of the mold rather than being 

drawn out. 

Lost Foam: 

A casting process in which a foam pattern is replaced by molten in a flask 

filled with loose sand to form a casting. 

Master Pattern: The object from which a die can be made; generally a metal 

model of the part to be cast with process shrinkage. 


M 

Mold: The form, made of sand, metal or refractory material, which contains 

the cavity into which molten metal is poured to produce a casting of desire 

shape. 

Mold Cavity: The impression in a mold produced by removal of the pattern. It 

is filled with molten metal to form the casting. Gates and risers are not 

considered part of the mold cavity. 

Mold Shift: A casting defect which results when the parts of the mold do not 

match at the parting line. 

Mold Wash: A slurry of refractory material, such as graphite and silica flour, 

used in coating the surface of the mold cavity to provide an improved casting 

surface. 

Mold Weight: A weight that is applied to the top of a mold to keep the mold 

from separating. 

Molding Machine: A machine for making molds 


N 

Nitriding: A process of shallow case hardening in which a ferrous alloy, 

usually of a special composition, is heated in an atmosphere of ammonia, or in 

contact with nitrogenous material, to produce surface hardening by formation 

of nitrites, without quenching. 

Nobake Process: Molds/cores produced with a resin bonded air-setting sand. 

also known a the airset process because molds are left to harden under ambient 

conditions. 


P 

Parting Line: A mark or line produced on the cast, formed at the junction of 

the parting dies. 

Patternmakers Shrinkage: Contraction allowance made on patterns to 

compensate for the decrease in dimensions as the solidified casting cools in 

the mold from freezing temperature of the metal to room temperature. Pattern 

is made larger by the amount of contraction that is characteristic of the 

particular metal to be used. 

Porosity: Holes in the produced casting due to : Gasses trapped in the mold, 

the reaction of molten metal with moisture in the molten sand, or the imperfect 

fusion of chaplets with molten metal. 

R 

Rod: A heavy wire or bar in a sand core used for reinforcing. 

Runner: The portion of the gate assembly that connects the down gate (sprue) 

with the casting ingate or riser. The term also applies to that part of the pattern 

which forms the runner. 

Runout: Unintentional escape of molten metal from a mold. 

Riser: See feeder. 


S 

Shrink Hole: A hole or cavity in a casting resulting from shrinkage and 

insufficient feed metal, and formed during solidification. 

Shrinkage: Decrease in volume of the metal as it solidifies. 

Shrinkage Cavity: Void left in cast metals as a result of solidification 

shrinkage. 

Shrinkage Defect: Jagged hole or spongy area of a casting lined with 

dendrites: generally due to insufficient feeding of molten metal during 

solidification. Not to be confused with Patternmakers shrinkage. 

Sprues: Channels cut into a mold to allow for the entry of metal. Also the 

name given to the metal rods that assume this shape in the final casting. 

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ASNT Level III Visual Testing, VT

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