ASNT Level III- Visual & Optical Testing

ASNT Level III- Visual & Optical Testing 

My Pre-exam Preparatory 

Self Study Notes Reading 4 Section 1 

2014-August 

Charlie Chong/ Fion Zhang

Reading 4 

ASNT Nondestructive Handbook Volume 8 

Visual & Optical testing- Section 1 

For my coming ASNT Level III VT Examination 

2014-August 


Charlie Chong/ Fion Zhang 

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.

ASNT Level III- Visual & Optical Testing

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