ASNT Level III- Visual & Optical Testing

ASNT Level III- Visual & Optical Testing 

My Pre-exam Preparatory 

Self Study Notes Reading 4 Section 2 

2014-August 

Charlie Chong/ Fion Zhang

For my coming ASNT Level III VT Examination 

2014-August 


Reading 4 

ASNT Nondestructive Handbook Volume 8 

Visual & Optical testing- Section 2 

For my coming ASNT Level III VT Examination 

2014-August 


Charlie Chong/ Fion Zhang 

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

ASNT Level III- Visual & Optical Testing

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