'ILUSAARL Report No. 92-34

USAARL Guide for MakingLaboratory Light Measurements

ByDTIC_ELECTE fl Clarence E. RashDECO 2 1992 Ellen H. Snook

A Patricia M. SennA rEverette McGowin III

Sensory Research Division

Visual Sciences Research Branch

MIM

a) 1 September 1992

Approved for public release; distrlbution unlimited.

United States Army Aeromedical Research LaboratoryFort Rucker, Alabama 36362-5292

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RIetOD 'R. LEVI.kE -LTC,\MS/Director, Sensory Research

DivisionReleased for publication:

,. Ph.D. DAVID H. KARNEYCh irmanrScientific JColonel, MC, SFS

eview committee Commanding

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11. TITLE (WW~* Security Casalfketion)(U) USAARL Guide for Making Laboratory Light Measurements

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Rash, C.1., Snook, B.R., Senn, P.M., and McGowin, K.13,. TYE OF REPORT 13b. TIME COVERED 14. DATE OF REPORT (Year, 1140th,D00) 1S, PAGE COUNTFinal I FROM TO 92 September 7016. SUPPLEMENTARY NOTATION

I?. cosATCODES 1. SUBJECT TERMS (Continue on revrs if necessary and identiy by ocMk number)FIELD GROUP . SUB-ROUP

20 06 radiometry, photometry, contrast, transmittance

19. ABSTRACT (Cntnue On rvern if necessry and Wenti; by bOc number)This report provides the basic information required to perform routine

light measurements encountered in the laboratory environment. Tutorial discussionsare provided in identifying the type of measurement required, selecting the appropriateinstrument(s), performing the measurement, minimizing errors, and reporting the data.Appendices provide lists of the light measurement instrumentation and light sourcesavailable within USAARL, with a summary description of each. In addition, safetysteps for the preventiovi of personal injury and equipment damage are presented.

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Table. of contents

List of figures . . . ........... ... ..... ... .. iii

List of tables ............. ov- v

Introduction ................................................ I

Safety precautions .......................................... I

Identifying the measurement task ............................ 3Radiance and luminance ................................... 4Irradiance and illuminance ............................... 5Spectral radiance ........................................ 5Contrast ... .............................................. 6Transmittance ............................................ 8Color .................................................... 8

Correlated color temperature ............................. 8.Polarization .... 11Temporal characteristics. ................................ 14

Selecting the light measurement instrumentation ............. 14Light measurement instrumentation fundamentals ........... 17Special instrument considerations ........................ 19Common and standard light sources ....................... 19Optical accessories .................................. 25Filters ..... 25

Integrating spheres .................................. . 28

Making the measurement ...................................... 29Zeroing and calibration ............. o ................... 29Optical alignment ........................................ 29Focusing................................................. 29Aperture and filter selection ............................ 30Signal range selection. .................................. 31

Sources of error in light measurement ....................... 31Calibration ...................................... . ... 32Source-detector geometry ................................. 32Stray light............................................. 32Polarization ................. o........................... 32Time constant considerations ................. .99..*....... 33Spectral considerations ....... .......... ............... 33Instrument accuracy ................................... 33

Reporting the data ............... ....................... 33

Glossary. .. .. . .. .. ... .... . .. ...............9 o .......9.....9 99 9 *9 ** 36

References ........ .. ...... o......... 40

Bibliography . ............................................... 40

i

Appendices

Appendix A - USAARL light measurement instrumentation..,... 41

Appendix B - USAARL light sources ......................... 53

Accesion ForNTIS CRA&I

DTIC TABUnannoufice jJustification

-. .. ..... .... ........... 4..

.y .........................Distributio n/! ...........

II

ii

Dist•I II I I I I

List of figures

1. The electromagnetic spectrum ......................... 2

2. Energy absorption characteristics of the eye ......... 2

3. Relationship between radiance, luminance, irradiance,and illuminance ................................... 5

4. Spectral distribution of a tungsten filament source

over the range from 370 to 950 nanometers........ 7

5. The 1931 CIE tristimulus response curves ............. 9

6. The 1931 CIE chromaticity diagram ...... ........ 10

7. The 1976 UCS chromaticity diagram .................... 10

8. Planckian blackbody radiation curves ................. 12

9. Illustration of polarization ......................... 13

10. Block diagram of light measurement instrument ........ 17

.11. A detector with an aperture and filter ............... 18

12. Line emission spectra for sodium vapor gas ........... 20

13. Spectral distribution for fluorescent lamp ........... 21

14. Typical light emitting diode (LED) spectra (green)... 22

15. Spectral output of Helium-Neon laser................. 24

16. Transmittance curves for 1.0 and 2.0 neutral density(ND) filters ...................................... 26

17. Transmittance curve for high-pass filter having acut-on wavelength of approximately 475 nm ......... 27

18. Transmittance curve for a 200 nm bandpass filterwhich transmits energy from 475 to 675 nm ......... 27

19. Integrating sphere depicting source, filter, ports,and detector ...................................... 28

20. optical axis of an experimental setup ................ 30

iii

A- 1. ZG&G Gamma Scientific ANVIS spectroradiometer system. 42

A- 2. Minolta chroma meter ......... . . . . . . . . . . . . . . . . . . . . . . . . 4 3

A- 3. Minolta model T-1H illuminance meter ................. 44

A- 4. Minolta model Nt-10 luminance meter ...... . . . . . . . . . . . . 45

A- 5. Photodyne Inc. model 22XL optical multimeter ......... 46

A- 6. Photodyne Inc. model 88XLA radiometer/photometer ..... 47

A- 7. Photo Researcha PR-710 Spot SpectraScanTM fastspectral scanning system .......................... 48

A- 8. Photo Research 1980A photometer ...................... 49

A- 9. Photo Research LiteMate III photometer ............... 50

A-10. Photo Research Spectre Spotmeter'm modelUBD ............... ... . . ...... .... * .. ........ 51

A-li. Photo Research PR-1530AR NVISPOT meter ............... 52

B- 1. BHK Inc. Analamp ultraviolet lamp .................... 54

B- 2. EG&G Gamma RS-10 tungsten lamp ....................... 55

B- 3. EG&G Gamma RS-12 tungsten lamp ....................... 56

B- 4. Melles Griot 2 and 5 milliwatt HeNe lasers ........... 5 7

B- 5. Oriel 6325 20 watt quartz tungsten-halogen lamp ...... 58

B- 6. Oriel 6436 1000 watt quartz tungsten-halogen lamp.... 59

B- 7. Oriel C-13-96 spectral lamps ......................... 60

B- 8. Oriel 6340 point source, arc ................. *...... 61

B- 9. Oriel 6363 infrared source ........................... 62

B-10. Oriel 6611 HeNe research laser ....................... 63

B-11. Photo Research PR 2301 tungsten lamp ................. 64

iv

List of tables

1. Light measurement instrumentation ....................... 15

2. Light sources and accessories ........................... 16

3. Common photometric units ................................ 35

4. Conversion factors for photometric units ................ 35

V

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This page intentionally left blank.

vi

Introduction

The objective of this manual is to provide the basicinformation required to perform light measurements encountered inthe laboratory environment. Typically, laboratory measurementsare performed on ambient room lighting from incandescent orfluorescent bulbs, computer or television monitors, or lighteddisplays. The primary step in making light measurements isidentifying the type of measurement required. The section on"Identifying the measurement task" will list and discuss thetypical light parameters measured in the lab. After defining themeasurement of interest, the appropriate instrument(s) must beselected in order to collect reliable data. The section on"Selecting the light measurement instrumentation" will describethe most common laboratory light measurement instrumentation andtheir capabilities and limitations. Basic procedures for makingthe measurements will be presented in the section "Making themeasurement." The section on "Sources of error in lightmeasurement" will discuss some common measurement errors andprovide recommendations for avoiding them. Finally, the sectionon "Reporting the data" will give guidance on selecting thecorrect units of measure and provide techniques for presentingthe data. Appendices A and B provide a list of the lightmeasurement instrumentation and light sources available withinUSAARL, with a summary description of each. Most importantly,before beginning any measurement, safety must be considered inorder to prevent personal injury or equipment damage.

This manual is intended as an introduction to general lightmeasurement techniques. Questions or problems relating tospecific or unusual experimental setups should be directed toinvestigators within the Visual Sciences Branch of the SensoryResearch Division.

An excellent user oriented reference on lighting and lightmeasurement is the IES Lighting Handbook, published by theIlluminating Engineering Society of horth America.

Safety precautions

Although the human eye can only "see" energy in the visiblepart of the spectrum, which is typically defined to be between380 and 730 nanometers (nm), it is capable of absorbing lightenergy over the entire electromagnetic spectrum (Figure 1) fromultraviolet (UV) to infrared (IR). Individual parts of the eye,e.g., cornea, lens, ocular media, and retina have differentabsorption characteristics which are depicted in Figure 2.Energy in the visible and near-IR wavelengths (between 380 nm and1.4 microns) passes through the cornea, lens, and ocular media ofthe eye to the back surface called the retina. On the retina,this light energy is converted into an electrical signal and

S. .. ' ' , , i i I I I I II1

RadioIWaves Microwaves Infrared Ultraviolet X - RaysWaves

- -I101 10-1 10-3 10-i 1O- 7 10- 9

Wavelength (m)

Figure 1. The electromagnetic spectrum.

PVitible nd NeoIR , NUar-UV390 um - 1.4 p~m "300 - 390nm,

<3Conm

:1r> 1.4 jiFar.UV

< 300 nm

Figure 2. Energy absorption characteristics of the eye.

2

transmitted to the brain. Due to focusing of light by the eye'slens, very high intensity visible light, e.g., sun, arc lamp,etc., may damage the retina, and light in the near-infraredwavelengths (750 nm to 1.4 microns) can cause severe retinalburns. This can result from gazing directly at high intensitysources without proper eye protection to reduce the energy whichfalls upon the retina. Light energy in the near-ultraviolet (UV)wavelengths (300 to 380 nm) does not reach the retina because itis absorbed by the lens of the eye. Unprotected exposure toenergy in these wavelengths can result in cataracts or otheropacities in the lens. Energy in the far-UV wavelengths (lessthan 300 nm) and far-infrared 1IR) wavelengths (greater than 1.4microns) can cause damage by thermal processes that overheat thetissue when absorbed by the cornea. Therefore, when working withhigh intensity light sources such as lasers, arc lamps, highwattage incandescent lamps, and any device which may reflect suchenergy (mirror, lenses, etc.), extreme caution should beexercised and proper eye protection should always be used.

In the same way that the human eye contains a sensitive energydetector (retina), so do light measurement instruments.Therefore, it is necessary to protect instrument detectors fromdamage as one would protect the eyes. Never exceed therecommended maximum power input to the instrument, and use properfilters to protect the detectors when sources are high inintensity. All measurements should begin with internal orexternal filters in the highest density configuration, thengradually decrease the filter density until optimum sensitivityis attained.

If accidently exposed to excess energy levels, some detectorsmay be permanently damaged. However, others only may betemporarily affected. If this occurs, it may "recover" over aperiod of several minutes to several days.

The operating manuals for light sources and light measuringinstrumentation should be consulted for specific safety concerns.

Identifying the measurement task

The first step in any light measurement is to identify themeasurement task. A measurement of total energy over the entireelectromagnetic spectrum (UV to IR) can be made of light emittedor reflected from a light source or of light falling upon thesurface of an object. In the same respect, a measurement oftotal energy over that part of the spectrum detectable by thehuman eye (visible energy) can be made. Each of thesemeasurements of total energy are a summed value over therespective ranges of the spectrum. Additionally, a measurementof energy present at individual wavelengths within the spectrumcan be made for light emitted from a source or falling upon the

3

surface of an object. For materials such as filters, which allowlight energy to pass through, measurements of total energy andenergy present at individual wavelengths can also be measured.Color is an additional characteristic of a light source or filterthat can be quantified by a measurement. The orientation(polarization) of light waves emitted by a source can also bemeasured.

The tasks cited above can be categorized as radiometric,photometric, spectroradiometric, transmittance, color, orpolarization measurements. Raot involves the measure oftotal energy typically covering the wavelengths from ultravioletthrough infrared. Paamxy involves the measure of totalenergy visible to the human eye. SRntroradiometrv is themeasurement of light energy at individual wavelengths within theelectromagnetic spectrum. It can be measured over the entirespectrum or within a specific band of wavelengths. TransMittanceof a material is a measure of the light energy passing throughand is expressed as a ratio of transmitted energy to incidentenergy. The golor of a light source or filter is defined by apoint in a standard color coordinate system or by a correlatedcolor temperature. Polarization is a measure of the orientationof light waves.

Each of these measurement tasks can be performed in thelaboratory. The following sections define specific terminologyassociated with each task and is intended for assisting the userin determining the appropriate measurement task(s).

Radiance and luminance

If the total energy output of a light source is of interest,then radiance or luminance measurements would be made (Figure 3).Radiance and luminance are two closely related measures; radianceis a measure of the total energy output of a source emitted overthe entire spectrum, while luminance is a measure of only thetotal energy output detectable by the human eye. Consider acommon light bulb which emits energy from the ultraviolet throughthe visible and infrared wavelengths. The radiance measure ofthe light bulb would include energy in the visible and non--visible (UV and IR) wavelengths of the emitted energy. However,the luminance of the light bulb would include only energy in thevisible wavelengths. Onc. common error made in discussing visibleenergy is the use of the term "brightness" to describe luminance.Luminance is a quantifiable measure; brightness describes howvisible energy ic perceived. The perception of brightness canvary from one observer to another.

4

IL

RadisAnc7 or

Figure 3. Relationship between radiance, luminance,

irradiance, and illuminance.

Irradiance and illuminance

When the amount of light energy falling upon an object is ofinterest, then irradiance or illuminance (Figure 3) measurementsshould be made. Irradiance and illuminance are related in muchthe same way as radiance and luminance. Irradiance is a measureof the total energy falling upon an object's surface; illuminanceis a measure of only the visible energy falling upon the samesurface. Using the common light bulb as mentioned above,consider the light as it falls on a surface such as a desk top.The irradiance measure of the light energy falling upon thesurface would include the visible and non-visible (UV and IR)energy. The illuminance measure would inclzde only the visibleenergy falling upon the surface.

Spectral radiance

The radiance or luminance of a light source is a single valuewhich is the sum of all energy measured over the respectivespectra. If the individual energy values at particularwavelengths are of interest, then a spectral radiance measurement

5

would be made. Spectroradiometry is the measurement of theenergy of a source as distributed over the wavelengths in theelectromagnetic spectrum. Figure 4 depicts the energy output ofa common light bulb (tungsten filament source) over the spectralrange from 390 to 950 na. Using this plot of the spectralradiance measure, an energy value at a particular wavelength canbe determined.

Contrast

When two or more adjacent areas have different luminancevalues, the human visual system will distinguish between them onthe basis of their relative luminance. This relative luminance isdefined as the contrast between the areas. By convention, thetwo areas are labeled as target and background, where the targetis the smaller area. These labels are very convenient when thedesired contrast measurement involves alphanumeric characters orsymbols on a chart or display.

Several methods of calculating the contrast value are incommon use. The simplest method is the contrast ratio, which bydefinition is the ratio of the target luminance (Lt) to thebackground luminance (Lb):

Cr = Lt/%

The contrast ratio can take on values from 1 (when the target andbackground luminances are equal) to infinity as the targetluminance increases with respect to the background.

A second method of expressing contrast is:

C = (Lt- 1%)- ,

where the target luminance (Lt) is assumed to be greater than thebackground luminance (L.). This method of calculating contrastproduces a value of 0 when the target and background luminancesare equal. As the target luminance increases with respect to thebackground, the contrast values increase towards w. For targetshaving luminances less than the background, this formula forcalculating contrast is changed to:

C = (t - L,

and can take on values of 0 to 1. These two formulae are oftenused when defining the contrast of a real scene where therelationship between the background and target is known. Insituations where the background and target cannot bedistinguished, a third contrast expression, referred to asmodulation contrast, should be used. Modulation contrast is mostoften used for periodic patterns such as gratings.

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Contrast modulation is defined as:

Ca (tM - %i.d)/(L + Laid

which can take on values of 0 to 1.

Transmittance

It is often desirable to measure the amount of light energytransmitted through an optical material, e.g., glass, plastic,etc. The ratio of the radiant power transmitted through amaterial to the incident radiant power from the source is a valuecalled the transmittance. A measure of only the visible energytransmitted is referred to as luminous transmittance. Luminoustransmittance can be classified further as photopic or scotopictransmittance. These values refer to the luminous transmittanceas measured for the "day" and "night" responses of the eye,respectively. Transmittance of a filter may also be expressed asa function of wavelength, which would be referred to as spectraltransmittance.

Color

Color is a characteristic of light determined by its spectralcomposition and the properties of the human eye. Because coloris perceived differently by each individual, a system for colorstandards was developed. There are many standards by which toevaluate color (also referred to as chromaticity). Colorperception models are based on the theory that the retina of theeye consists of three different color receptors (tristimulusresponses). Each receptor responds to specific wavelengths"corresponding to blue, green, and red light (Figure 5). Variouschromaticity coordinate systems plot the color of a light sourceas a two-dimensional point on a chromaticity diagram. The twomost widely used coordinate systems are the 1931 CommissionInternationale 1'Eclairage (Commission on Illumination, CIE) andthe 1976 Uniform Color Scale (UCS) systems (Figures 6 and 7,respectively). The 1931 CIE coordinate system uses x,y and the1976 UCS coordinate system uses u',v' as the coordinatevariables. Equations exist for converting between these andother chromaticity coordinate systems.

Correlated color temperature

The spectral distribution of tungsten light sources varies asa function of filament current. Therefore, for reproducibilityin experiments using tungsten light sources, it is standardprocedure to specify the source's spectral energy distribution.One method of standardization is to specify the source'scorrelated color temperature.

i..

1.4

0!L 1.2

0.4-

0.

0D 1.02

-. 0.8

k 0.6 /

I0.4 1

/ I0.2 / /

- ~~0.0 " -,400 500 600 700

Wavelength (nm)

Figure 5. The 1931 CIE tristimulus response curves.

9

520 n

0.8 -931 CIE

0.6

0.4 -

0.2

0.0 3at

0.0 0.2 0.4 0.6 0.8

x

Figure 6. The 1931 CIE chromaticity diagram.

0.6 ,

0.4.~V•

0.2

0.0 1976 UCS

0.0 0.2 0.4 0.6

U I

Figure 7. The 1976 UCS chromaticity diagram.

10

I

The correlated color temperature of a source is determined bycomparing its spectral radiance curve to a set of theoreticalcurves known as the blackbody spectral emittance curves. Allobjects, when heated, emit a continuous spectrum of radiation.The emitted spectrum is dependent on temperature of the source aswell as the composition of the object. A blackbody radiator is atheoretically ideal emitter of uniform temperature whose radiantemittance in all parts of the spectrum is the maximum obtainablefrom any radiator at the same temperature. The physicist MaxPlanck proposed an empirical formula which described all featuresof the blackbody radiator as a function of wavelength andtemperature. When plotted for different temperatures, theblackbody curves appear as shown in Figure 8.

If the spectral distribution of a heated material canapproximate a blackbody spectral emittance curve, a correlatedcolor temperature for the source can be determined. Thistemperature will be that of the blackbody curve which mostclosely corresponds to the source curve. For example, thespectral distribution curve of a tungsten source is similar to ablackbody spectral emittance curve so that a correlated colortemperature can be determined by matching curves. On the otherhand, light emitting diode (LED) sources have narrow bandspectral emittance curves which cannot approximate any blackbodycurve and therefore have no correlated color temperature.Sources such as these are considered non-Planckian.

Polarization

One property of light sources which is often overlooked ispolarization. Light is an electromagnetic wave with associatedelectric and magnetic fields which oscillate as the wavepropagates through space. If the electric field alwaysoscillates in some fixed direction, the wave is said to belinearly polarized in that direction. The associated magneticfield is always perpendicular to both the electric field and thedirection of wave propagation. Therefore, the magnetic fieldalso oscillates in a single direction. By convention, thepolarization of a wave refers to the orientation of the electricfield vector. Unpolarized light consists of electromagneticwaves that have transverse vibrations of equal magnitude in aninfinite number of directions. Figure 9 depicts an unpolarizedlight beam as it strikes a vertical polarizer; only lightparallel to the polarization axis is transmitted. When theresulting vertically polarized beam then encounters a horizontalpolarizer, no light energy is transmitted.

Most common light sources such as the sun, incandescent lamps,and fluorescent lamps produce unpolarized light. However, thislight can be polarized by absorption, scattering, or reflictionand refraction of the transmitted waves.

11

.10

i6

-- ! • 107

5• 2000 KS10

210

S10oC. 1 0 --

V 0 10-1

10-1 100 101 102

Wavelength (am)

Figure 8. Plankian blackbody radiation curves.

12

wool

hpoizzore

ROM SOUrce

Figure 9. Illustration of polarization.

Absorption is achieved through the use of polarizing filterswhich absorb light waves propagating in all but the direction ofpolarization, e.g., horizontal or vertical. A high percentage ofpolarization can be obtained by this method, but at a loss ofluminous transmittance.

Scattering of sunlight within the atmosphere results inpolarization. On a clear day, light in the sky is partiallypolarized by dust particles in the air. This effect can bedemonstrated through the use of a polarizing analyzer (a pair ofpolarizing filters). As the sunlit sky is viewed through therotating end of the analyzer, the transmitted light intensitywill vary markedly.

Light may be polarized when reflected from or refractedthrough ordinary materials such as glass or water. Generally,light is only partially polarized upon reflection and refraction,but may be completely polarized if the conditions are just right.The degree of polarization depends upon the angle of incidence ofthe light and the indices of refraction of the media on eitherside of the reflecting or refracting surface.

13

Temporal characteristics

Light sources can be described by the temporal characteristicsof their luminous output. Most sources have a uniform output(luminance) over a time period. Other sources have outputs whichvary with time, but appear to the eye as constant, e.g., cathoderay tube displays (CRTs). And some sources, such as flashinglights, display visually apparent changes in luminance as afunction of time. Less common sources exist which produce only asingle pulse of light energy.

Any source which changes its output periodically over time canbe associated with a frequency. This frequency defines how manytimes within a certain time period the light undergoes itsvariation in luminance. In turn, this frequency is associatedwith a time period between each maximum (or minimum) outputvalue. This period, expressed in seconds or milliseconds, isreferred to as the time constant of the source.

All sources have a time constant. Uniform sources, which havea frequency of zero, have an infinite time constant value.Sources with short time constants (as compared to the human eye)are seen as steady sources, while those with relatively long timeconstants are seen as flashing sources. For the short timeconstant sources (e.g., CRTs and office fluorescent lamps), theeye responds only to their average "brightness." on the otherhand, the output of long time constant sources such as strobelights appears as distinct flashes. The time constant of a lightsource is an important factor when luminance measurements arebeing made.

Selectina the light measurement instrumentation

Having determined the type of measurement desired, the nextstep is the selection of the appropriate measurement instru-mentation. The simplest approach is to select the instrumentwhich allows the most direct determination for the desired typeof measurement. Luminance measurements are most easily performedwith a photometer. However, the lumiaance of a light source canbe calculated from spectroradiometric data. This spectraldistribution requires the use of a spectroradiometer. Color canbe measured directly with a specialized color meter (chromameter), or indirectly using a photometer (with color filters) ora spectroradiometer. Illumination measurements are generallyperformed using illumination meters. However, photometers can beused to measure the effective luminance of the illuminatedsurface, from which the illumination can be calculated.

Tables I and 2 provide a list of light measurementinstrumentation, light sources, and accessories currentlyavailable within USAARL. Table 1 also indicates which types ofmeasurements each instrument can be used to perform.

14

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Light measurement instrumentation fundamentals

An understanding of the basic instruments for performing lightmeasurements is essential in preventing mistakes in such tasks.Figure 10 shows a block diagram of a generic light measurementinstrument. The basic functional blocks are collection optics,detector (including aperture and filters), amplifier, display,and power supply.

In order to g&in a reliable measurement of a source, the lightenergy of interest must be directed to the detector. Thecollection optics of an instrument perform the function offocusing the light energy from a source unto the detector.

The detector takes the light energy provided by the collectionoptics and converts it into an electrical signal. Often thedetector is integrated with an aperture and filter (Figure 11).An aperture is an opening which controls the amount of lightenergy which reaches the detector. Apertures are usuallycircular, but can be horizontal or vertical slits. The mostcommon filter is the photopic filter. This filter is speciallydesigned such that its spectral transmittance when coupled withthe detector's spectral response simulates the response of thehuman eye. In such cases the detector/filter combination acts asa photometer, measuring luminance instead of radiance. Scotopicfilters, which simulate the night response of the eye, are alsofrequently used. Most photometers have multiple apertures andfilters located on wheels which allow convenient user selection.Combinations of apertures and filters allow control over thelevel of energy entering the detector. This allows measurementof high energy sources which could not otherwise be measureddirectly.

CollectionMq~y Ap~ferDue=to Monochioniator dF HI

Pow' Supply

Figure 10. Block diagram of light measurement instrument.

17

J4

........... detector- E/filter

gaperture

Figure 11. A detector with an aperture and filter.

In spectroradiometry, the amount of light energy as a functionof wavelength is measured. To accomplish this, the collectedlight energy is separated into individual wavelengths (or bandsof wavelengths) before being delivered to the detector (Figure10). This is performed by an instrument known as amonochromater. The resulting output of a monochromater/detectorcombination is a spectral distribution as depicted in Figure 4.

The amplifier in the system increases the small signal outputof the detector to a level which will drive the displayelectronics.

The display of the instruments can be as simple as a meterneedle with appropriate scale markings; or state-of-the-art suchas a liquid crystal display (LCD), a light emitting diode (LED)display, or a cathode-ray-tube monitor. Spectroradiometricsystems typically are connected to additional output devices,e.g., printers and plotters. With electronic displays, e.g., LCDand LED, the units of measure may be selected. For example, anillumination meter vay display its reading in lux or infootcandles. This choice is usually accomplished by means of a

18

switch located somewhere on the meter. The user must be carefulin recording the readings in the correct units.

The power supply's function is to provide the necessaryelectrical power to the other sections of the instrument. Theuser's concern for this section of the instrument is minimal. Anexception may be the use of hand-held instruments. The powersupply for most hand-held instruments is one or more batteries.Such instruments may, or may not, indicate a "low battery"condition. The user should always view as suspect unusually lowor high readings or readings which do not change with conditions.

Special instrument considerations

Special consideration must be given to the time constant of alight source when selecting the measurement instrument. If thetime variation in the source is of interest, the time constant ofthe detector must be several times smaller than the source's timeconstant. If the "average" output of the source is of interest,then the time constant of the detector should be several timeslarger than that of the source. Measurement of the output ofsources with a short time period, but which appear constant tothe human eye, can be measured with most common instruments.These instruments have a time constant on the same order ofmagnitude as that of the eye (approximately 20 milliseconds) andmeasure the "average" luminance of the source. However, forlonger time constant sources (those appearing flashing to thehuman eye), instrumentation with time constants much smaller thanthat of the source is required. Some instruments have a "pulsedlight" or similar measurement mode which allows accuratemeasurement of pulsed light sources. The Photo Research 1980A-PLphotometer is one such instrument which can accurately integratetotal light energy in a single pulse or series of pulseu forsources with a minimum pulse-duration time of 1.6 to 16microseconds.

Common and standard light sources

Many laboratory measurements require the use of light sources.These sources may be common in nature, such as the incandescentlight bulb, or very specialized, i.e., having required spectraland/or energy content. A class of sources, known as "standard"sources, are available which provide known values of luminance or

* spectral content. Table 2 lists light sources and accessoriesavailable within USAARL.

Common sources of energy emission are divided into two broadcategories: continuous emission and line emission. A continuousemission source has some energy present at all wavelengths. Atungsten source is an excellent example of a continuous source(Figure 4). On the other hand, some sources of light only emitenergy at discrete or well-defined wavelengths, known as line or

19

discrete emission. A sodium vapor lamp is an example of a lineemission source. There are 12 discrete lines between 16 and 819nm with the two most intense lines at 589.0 and 589.6 nm (Figure12). Some light sources such as a conventional officefluorescent lamp have a low level continuous spectrum but with aconsiderable amount of energy at certain discrete wavelengths(Figure 13).

Another type of source is a light emitting diode (LED). Theenergy emitted from an LED is produced by electrons moving acrossa junction of semiconductor materials when a voltage is appliedto the junction. LEDs produce light in a fairly narrow range ofwavelengths (Figure 14). Visible LEDs are often identified bytheir color, e.g., red, green, yellow, etc.

S589.0 nml 589.6 nm

Q)

400 500 600 700

Wavelength (nm)

Figure 12. Line emission spectra for sodium vapor gas.

20

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Arc lamps are also a common source used in light measurementtasks. An arc lamp is an electric discharge lamp that producesillumination when voltage is applied across electrodes separatedby a very short distance. The electrodes are surrounded by a gasmixture which produces a luminous plasma when an electric chargeis applied to the electrodes. Arc lamps produce the highestluminance available. Great care should be exercised inprotecting the observer and the instrumentation when arc lampsare used. Carbon arcs and high pressure mercury and xenon arcsapproach the luminance of the sun and in some cases exceed it.

Lasers (an acronym for "light amplification by stimulatedemission of radiation") are another high intensity light sourcethat are commonly used in light measurement. The energy outputof lasers is confined within a very narrow band of wavelengthsand are often considered to be monochromatic (single wavelength)sources. The most common laboratory laser, frequently used foroptical alignment, is a Helium-Neon laser which emits at 632.8 nm(Figure 15). Light from lasers consists of nearly parallel raysand exhibits very little angular divergence. For this reason,they are very intense sources with a large amount ofelectromagnetic energy concentrated within a small area, even outto moderate distances. As with other high intensity lightsources, great care should be exercised in protecting theobserver and instrumentation from damage.

A multitude of different sources, mostly incandescent, can beused in the laboratory, all with varying spectral characteristicswhich can produce different results in distinct circumstances.Due to these differences, a "standard source" was defined by theCIE and is used to facilitate the specification of the spectralcomposition of illumination. This standard, CIE Illuminant A, isthe source that produces a spectral distribution emitted from aincandescent tungsten filament lamp at a color temperature of28560 kelvin (K). Additional filters are used with Illuminant Ain order to obtain other standards of illumination, the mostcommon of which are Illuminants B and C. Illuminant B is used tosimulate direct sunlight and has a color temperature of 48700 K.Illuminant C is used to simulate an overcast sky and has a colortemperature of 67709 K.

Light sources can also be classified by their geometry: pointor extended. Point sources are those having physical dimensionsseveral orders of magnitude smaller than the distance at whichmeasurements are being performed. In such situations, thesephysical dimensions can be ignored. In comparison, extendedsources are those having physical dimensions which are of thesame order of magnitude as the measurement distance. Thegeometry of a source may preclude direct measurement of aparticular parameter. In such cases, special formulae may berequired to calculate the required data.

23

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Optical accessories

Several accessories may be utilized in performing lightmeasurements. Two such frequently encountered accessories arefilters and integrating spheres (Table 2).

Many experimental setups require modification of the lightoriginating from a source. This modification may be to theradiance (and associated luminance) of the light source or to thespectral distribution. Materials used to achieve thesemodifications are referred to as filters. Filters are placedbetween an energy source of known value and the instrumentdetector. The ratio of the radiant power transmitted through thefilter to the unfiltered radiant power from the source is calledthe transmittance. Filters may be classified as neutral densityor spectrally selective.

Neutral density filters are used to attenuate the radiance(and associated luminance) of a source. They are designed to bespectrally neutral, i.e.. attenuating all wavelengths uniformlywithin a spectral range. The experimenter must be careful aboutassuming the spectral range over which a neutral density filteris actually useful. For example, the spectral transmittancecurves in Figure 16 depict 3.0 and 2.0 ND glass filters whichattenuate uniformly only between 480 and 700 rnm. Filterscomposed of different media can have slightly differenttransmittance characteristics. Neutral density filters aregenerally characterized by their logarithmic attenuation factor.A neutral density filter labeled Ias ND-I attenuates the sourceluminance by a factor of 0.1 (10"), and a filter of ND- .0attenuates the source luminance by a factor of 0.01 (10 ).

The spectrally selective filters can be categorized as cut-offfilters or bandpass filters. Cut-off filters can be furtherdivided into low-pass or high-pass, depending upon whether theyallow transmission of energy at wavelengths below or above thecut-off wavelength, respectively. Figure 17 shows a high-passfilter having a cut-on wavelength of approximately 475 rm. Notethat the transition from blocking to transmission does not occurinstantaneously. The cut-off wavelength is most often defined asthe 50 percent point.

Bandpass filters allow transmission of energy to pass througha certain band of wavelengths. Above and below this band, energyis attenuated to a much lower level, in effect obstructing the"passing" of this energy. Figure 18 displays the spectralcharacteristics of a bandpass filter which permits energy at a475 to 675 nm band to pass (or be transmitted).

25

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0.3

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0.1 "0 .0 1 - # . .

300 400 500 600 700 800Wavelength (nm)

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1.0 -

0.9

0.8 -

0 0.7 cut-on cut-off

§0.6 wavelength wavelength475 nm 675 nm

- 0.5 -C

§0.4-

0.38

0.2-0.1L

0.0 - ---L I300 400 500 800 700 800

Wavelength (nm)

Figure 18. Transmittance curve for a 200 nz bandpass filterwhich transmits energy from 475 to 675 nm.

27

"Integrating spheres are nearly perfect diffusers used formeasuring sources or object reflectance and transmittance. Theyare hollow spheres coated internally with a white diffusingmaterial and have openings for the source input (input port),filter materials, and the detector (output port).

The input light is distributed inside the sphere (Figure 19),uniformly illuminating the output port. Every point on the innersurface reflects to every other point in the sphere, and theilluminance at any point is made up of two components: lightenergy coming directly from the source, and that reflected fromother parts of the sphere wall. Therefore, the illuminance (andassociated luminance) of any part of the wall, due to reflectedlight only, is proportional to the total energy from the sourceregardless of its intensity distribution. The diffusing effectof the sphere guarantees that the output is insensitive tospatial, angular, and polarization characteristics of the inputsource, while maintaining its chromatic characteristics.

i inpput1port

Figure 19. Integrating sphere depicting source, filter, ports, I

and detector.28

APPII

Making the meaaurement

Following a well defined procedure in making lightmeasurements usually will produce more accurate results. Theseprocedural steps consist of: instrument zeroing and calibration,optical alignment, focusing, selecting appropriate aperture andfilter, and selecting optimum instrument signal range.

Zeroing and calibration

To obtain an absolute measurement of any physical quantity,the measurement instrument must be zeroed and calibrated.Zeroing the instrument ensures that the output of the detector iszero when there is no input. This is accomplished by blockingall input energy to the detector and adjusting the displayreading to a zero value. Most instruments have a shutter whichcan be closed to perform this operation.

Calibration is achieved by inputting a known amount of energyand adjusting the detector's output until the display readingagrees with the known value. Special energy sources, referred toas "standard lamps," are available for this task. They aremanufactured to provide exact values of luminance, radiance, etc.If possible, the instrument should be calibrated against a sourcewhich has an output value of the same order of magnitude as thesource to be measured. In all cases, operating manuals should beconsulted for special calibration procedures.

Zeroing the instrument and calibrating against a known value(other than zero) establishes two points on the output curve ofthe detector. If the detector is linear (which we will assume),then any additional readings are assumed to be proportional tothe input energy.

Optical alignment

Another important aspect to consider in the setup for lightmeasurement is the alignment of a source and intervening opticswith respect to the detector. A line called the optical axis isdefined as one that is perpendicular to the face of the detector,through the center (see Figure 20). Any optics and light sourceshould be placed in alignment with the optical axis since this isthe most direct path for light energy to reach the detector (see"Sources of error in light measurements").

Focusing

In situations where lenses are used (e.g., collection optics),an image of the source is formed. The focusing of this imageonto the detector provides the maximum reading. Focusing isachieved using a focus adjustment located on or adjacent to thecollecting optics.

29

oSF

detector

- ~80t, l

Figure 20. Optical axis of an experimental setup.

Aperture and filter selection

Most instruments allow for user control of the amount ofenergy entering the detector. This control is achieved throughintegrated apertures and filters. Apertures regulate the amountof light entering a detector. Filters offer a more selectivemethod of regulating light energy entering the detector byabsorbing energy at certain wavelengths and transmitting energyat others. Often, a selection of internal apertures auid filtersare provided on a given instrument.

The amount of energy allowed to pass through an aperture isdirectly proportional to the size of the aperture, i.e., a largeraperture will allow a greater amount of energy to pass through.When an aperture is integrated into the collection optics, italso defines the area of the (extended) source from which energyis measured. This area is expressed as the angle it subtends atthe detector. Therefore, instrument apertures are often markedIn angular measure, e.g., 1-degree, 20-minutes, etc. Whencharactarizing a source, care must be taken to ensure that theselected aperture (subtended angle) is filled by the source (see"Sources of error in light measurement").

30

The selection of aperture size must take into account themaximum capacity of the detector. The selection of an aperturethat is too large for a given high energy source will causesaturation of the detector. This will be indicated by anoverranging of the display. A general rule of thumb forselecting the internal aperture size is to start with thesmallest aperture available and gradually increase size to thelargest aperture possible without overranging.

With instruments that include integrated filters, the filtersare used to modify the response of the detector. The two mostcommon filters are the photopic and scotopic filters. These aredesigned to modify the detector's response to simulate the humaneye's day and night response, respectively. These filters areusually mounted in a filter wheel which allows selection amongthem and other specialized filters. A wheel with an "open"position allows radiance measurements.

Signal range selection

The range of an instrument is defined by the lowest andhighest values which can be measured (and displayed). The lowestdisplayed value is usually zero. On many instruments, only onerange is provided, and the highest value defines the overrangecondition. However, some instruments allow a selection ofchoices for the maximum value. These range choices usuallyincrease by decade units, e.g., 0 - 0.999, 0 - 9.99, 0 - 99.9 fL.Instruments with multiple ranges usually provide autorangecapability where range selection is left to the instrument. Theadvantage of one range over another lies in the degree ofprecision. If, as in the example above, a measured value isapproximately 0.5 fL, the most precise reading is obtained usingthe 0 - 0.999 range. Caution must be used to ensure that displayreadings are not incorrectly associated with instrumentsensitivity (the smallest level of input energy measurable). A"0" display reading cannot be accepted literally . For a displayrange selection of 0 - 99.9 fL, a "0" display reading canrepresent a signal value from the lowest detectable level up toapproximately 0.05 fL. In other words,"0" is not actually "0".

Sources of error in light measurement

Light measurement tasks are subject to errors which candistort data and cause inaccurate conclusions. Most of theseerrors can be classified into the following areas: calibration,source-detector geometry, stray light, polarization, spectralconsiderations, and instrument accuracy (Application Note,"Reduction of errors," Oriel Corp., Stratford, CT).

31

Calibration

Calibration is critical when making absolute measurements.The calibration of the instrument relates the output of theinstrument to a known value. Using an uncalibrated or improperlycalibrated instrument wll not affect relative measurements, suchas luminous transmittance, but will result in incorrect absolutevalues.

Source-detector geometry

One goal of good measurement technique is to ensure that thelight energy to be measured is independent of any changes inorientation. All experimental setups have an optical axis(Figure 20). Proper alignment of source, detector, and inter-vening elements along this axis will provide accurate measure-ments. Alignment can be verified by making small movements ofeach component and noting the effect on the measured value.Optimum alignment is achieved when the measured energy attains apeak value.

Another aspect of source-detector geometry which can produceserious errors is inadequate filling of the detector aperturewith the source light. Many measurements are based oncalculations involving the total energy collected and the area ofthe detector. If the aperture is not completely filled, then theresulting measurement will be inaccurately low.

Stray light

As a basic rule of thumb, prior to performing measurements, a"light audit" of the laboratory should be performed to assess thepresence of stray light. Stray light is unwanted light energyfrom the test source or the surrounding area which enters thedetector. This extra energy produces an artificially highmeasurement value. Stray light may originate directly from thesource (e.g., reflections ftom sample holders, walls, tables,light colored clothing, etc.) or from ambient sources (e.g.,overhead lights, light leaks around doors or windows, indicatorlights on instrumentation, etc.). Some suggested procedures forminimizing stray light include: check for reflections of testsource, select optimal external aperture size, baffle alongoptical axis, perform measurements in darkest environmentpossible, keep all personnel away from measurement area, andcover all indicator lights on instrumentation.

Polarization

Polarization errors can result when: a) comparative measure-ments aro made between sources which differ in polarizationcharacteristics, e.g., a polarized vs. an unpolarized source or avertically polarized vs. a horizontally polarized source; b) t.he

32

polarization characteristics of the source and detector do notmatch; and c) changes occur in the source's polarizationcharacteristics along the optical path, especially when mirrorsand beamsplitters are present.

Time constant considerations

When measuring light sources which are uniform over time, thetime constant of the measurement instrument is not important.However, measurement error for time varying sources can occurwhen the time constants of the source and detector are notproperly matched. When the measurement task is to define theactual luminance profile, the time constant of the detector mustbe several times smaller than that of the source. Failure toselect an instrument which meets this requirement will result inan integrated output, rather than a true output profile.Conversely, when the average output of a time varying source isdesired, too small a detector time constant will not produce aconstant measurement value.

Spectral considerations

Measurements of narrow spectral characteristics may giveinaccurate readings due to unwanted energy outside of thespectral band of interest. Stray light "leaking" through themonochromater (narrow band filter) can produce an incorrect highreading. This error is more significant when the source isrelatively weak.

Instrument accuracy

While not an error in the sense of the items above, theaccuracy of a reading can be affected by the accuracy of thelight measuring instrument. Due to the required matching of adetector with a photopic or scotopic filter to perform luminanceor illuminance measurements, the accuracy of the reading is afunction of how well the detector/filter combination's responsematches that of the human eye. Additional factors affectinginstrument accuracy include temperature and humidity. As a ruleof thumb, photometric measurements can expect to have an error of2-5 percent.

Reporting the data

The most common formats for reporting light measurement dataare graphical curves for spectral data (radiance or trans-mittance) and tables for radiometric and photometric values.Spectral data are usually presented graphically such as thespectral radiance curve in Figure 4. Radiance values are plottedalong the y-axis as a function of spectral wavelength (x-axis).

33

There is a large selection of units from which one may choosefor reporting radiometric and photometric measurements. Table 3provides a list of the most commonly used units for radiance,irradiance, luminance, and illuminance. The choice of units fora given physical quantity is usually a matter of the preferredmeasurement system. While the currently agreed upon units ofmeasure are those of the International System of Weights andMeasures (SI), units of the English system of measure are oftenencountered. Table 4 provides conversion factors for commonluminance and illuminance units. From the table, it c.an be seenthat 1 nt is equivalent to 0.2919 fL and 0.0929 cd-ft'. Toconvert a value expressed in "nt" into "tL", multiply the valuein "nt" by 0.2919. For example: A value of 1.2 nt is equivalentto (1.2 x 0.2919) or 0.35 fL.

Color (chromaticity) is expressed in either 1931 CIE (x, y) or1976 UCS (u', v') coordinates. These are usually presented ingraphical form (Figures 6 and 7, respectively).

When reporting contrast measurements, it is important todocument the method, i.e., definitions and formulas, used in thecalculation of the contrast values.

34

TWOl 3Common photomettic units.

Radkno. wat per dadsmlen We equws mete wWa in 4

Lun~min nk ntfoodaiteit ILcandela per squmkiort d ft'4

footesnie fo

Ow urdt wm eepuedaas W m-1

Ta"l 4.Conversion faftmr for photmetri units.

Luminance lluminanc_ _ _ nt adr kf

fit -102919 o.029

-L 3A420 1 0.3183 --

=iftd 10.764 3.1410 1____

fc -- 10.764

35

absorption - partial loss of incident energy in an optical mediumdue to conversion of energy into other forms.

AWNVI radiance - the measure of total energy output of a sourceas detected through third generation image intensification nightvision imaging systems.

aperture - the opening through which light energy passes throughto the detector. The amount of light received by the detectormay be regulated through the changing of aperture size. Internalapertures are integrated into an instrument, and externalapertures are placed between a source and the detector of aninstrument.

arc lamp - an electric discharge lamp that produces light whenvoltage is applied across electrodes separated by a very shortdistance. The electrodes are surrounded by a gas mix-cure whichproduces a luminous plasma when an electric charge is applied tothe electrodes. Arc lamps produce very high luminance values.

ban4pass filter - a filter that allows energy within a certainband of wavelengths to only pass. Above and below this band,energy is attenuated to a very low level, in effect obstructingthe "passing" of this energy.

beaasplitter - An optical element which divides a light beam intotwo parts.

blackbody radiator - a temperature radiator of uniformtemperature whose radiant emittance in all parts of the spectrumis the maximum obtainable from any temperature radiator at thesame temperature. Such a radiator is called a blackbody becauseit will absorb all the radiant energy that falls upon it.

chromaticity - the classification of color.

collection optics - any lens, mirror, or combination that is usedto gather or focus light energy.

oolorimetry - methods employed to measure color and to interpretthe results of the measurements.

c4rrelated color temperature (CCT) - the temperature of theblackbody curve which is best approximated by a source's spectralenergy distribution. For tungsten filament sources, this variesas a function of filament current.

36

cut-off filter - filter that allows transmission of energy atwavelengths below a specified wavelength. This particularwavelength is referred to as the cut-off wavelength.

out-on filter - filter that allows transmission of energy atwavelengths above a specified wavelength. This particularwavelength is referred to as the cut-on wavelength.

detector - a device that converts light energy into another formof energy, usually an electrical current.

eleotromagnetic spectrum - the total range of wavelengthsextending from the shortest (zero) to the longest (infinity)wavelength that can be generated physically.

emitted spectra (line, continuous) - the spectrum formed byenergy emitted from a source. Emitted spectra are divided intotwo broad categories: continuous emission and line emission. Acontinuous emission source has some energy present at AlUwavelengths. Line or discrete emission sources have energyemitted only at discrete or well defined wavelengths.

filter - a material placed between a light energy source and adetector that attenuates the total amount of energy beingtransmitted; this attenuation can be uniform across allwavelengths as in neutral density filters or can be spectrallyselective.

footlambert - a unit of luminance in the English system ofmeasure equal to 1/w candela per square foot.

illuminance - the measure of visible energy falling upon asurface.

IllumLant A, B, and C - illuminant A is the standard for asource that produces a spectral distribution which approximatesan incandescent tungsten filament lamp at a color temperature of28540 kelvin (K). Illuminant B is used to simulate directsunlight and has a color temperature of 48700 K. Illuminant C isused to simulate an overcast sky and has a color temperature of67700 K.

image intensification - an electro-optical device which convertscollected photons into electrons which are then "intensified" byphotomultiplication. After intensification, the electrons areconverted back into photons (light image) at a phosphor screen.

inoandescent lamp - a lamp that emits light when an electricalcurrent passes through a resistant metallic wire positioned in avacuum tube.

37

~i

index of refraction - the ratio of the velocity of light in airto the velocity of light in a refractive material for a givenwavelength; a measure of the bending properties of an opticalmaterial.

infrared spectrum - energy that lies between the wavelengths of750 nu to 1.4 microns; heat producing energy which is invisiblebut can be damaging to the retina of the human eye.

irradiance - the measure of total energy falling upon an object'ssurface; this measurement includes visible and nonvisible (UV andIR) energy.

laser - acronym for "Light Amplification by Stimulated Emissionof Radiation." A narrow band of wavelengths, often considered tobe a monochromatic (single wavelength) source, which consists ofnearly parallel rays and exhibits very little angular divergence.When coupled with the focusing capability of the human eye,lasers become an extreme optical hazard.

light emitting diode (LID) - emits light energy which is producedby electrons moving across a junction of semiconductor materialswhen a voltage is applied to the junction.

liumely polarimed - when the electric field (light energy)oscillates in some fixed direction, the wave is said to belinearly polarized in that direction.

luminanoe - a measure of energy output (weighted by the humaneye's response) leaving or arriving at a surface in a givenI direction.

luminous transmittance - the ratio of the luminance of a knownsource as measured through a filter material, to the luminance ofthe source itself.

monoohromater - an instrument which isolates narrow bands of thespectrum allowing the measurement of energy within the band (inpractice, the band is very narrow and often referenced by itscenter wavelength).

neutral density filter - a filter which attenuates the radiance(and associated luminance) of a source. They are designed to bespectrally neutral, i.e., attenuating all spectral wavelengthsuniformly.

objective lenses - the optical elements that collect light energyfrom a source and form the first image to be measured.

38

photometer - an instrument for measuring photometric parameterssuch as luminance and illuminance (indirectly). The response ofa photometer's detector is tailored to approximate that of thehuman eye.

photometry - the measurement of light energy as perceived by thehuman eye (visible light).

photomultiplier tube (PMT) - a tube consisting of a photocathodethat emits electrons in proportion to the incident photons.

photopia - values that refer to the light output of a source asmodified by the eye's cone photoreceptors' response.

polarization - If the electric field of an energy wave alwaysoscillates in some fixed direction, the wave is said to belinearly polarized in that direction; the polarization of thatwave refers to the orientation of the electric field vector.

radiance - the measure of total energy output of a.source; thismeasurement includes both visible and nonvisible (UV and IR)energy.

radiomtry - involves the measurement of total electromagneticenergy, typically covering the wavelengths from ultravioletthrough infrared.

scotopic - values that refer to the light output of a source asmodified by the eye's rod photoreceptors' response.

spectral radiance curve - a graphical representation of theenergy emitted per unit wavelength of a particular source over aspecified spectral range.

spectral reflectance - the ratio of the reflected energy to theincident energy as a function of wavelength.

spectral transmittance - transmittance of an optical material asa function of wavelength.

spectroradiometry - the measurement of the amount of energy at aparticular wavelength or within a band of wavelengths.

ultraviolet spectrum - energy that lies between the wavelengths100 to 380 nm. This energy is invisible and damaging to thehuman eye.

visible spectrum - the region of the electromagnetic spectrumwhich the human eye is sensitive to. It is typically defined tobe between 380 to 730 nm.

39

wavelength - the measure of the physical distance covered by onecycle of a sinusoidal wave of light energy; a variable used torelate to the spectral content of a light source.

-- " OrielrCorp.

1. Application Note, "Reduction of errors," Oriel Corp.,Stratford, CT.

2. IRS Lighting Handbook (Vol. 1 and 2), 1987. Edited byKaufman, J. K., and Christensen, J. F. New York, NY:Illuminating Engineering Society of North America.

Bliblioarap=7

1. Design Handbook for Imagery Interpretation Equipment, 1984.Farrell, R.J., and Booth, J.M.. Boeing Aerospace Company.Seattle, WA

2. Principles of Display Illumination Techniques for AerospaceVehicle Crew Stations, 1982. Godfrey, George W. Tampa,FL: Aerospace Lighting Institute.

3. Optical Radiation Measurements (Vol. 1 Radiometry), 1979.Edited by Grua, Franc, and Becherer, Richard J. New York,NY: Academic Press.

4. Optical Radiation Measurements (Vol. 2 Color Measurement),1980. Edited by Grum, Franc, and Bartleson, C. James.New York, NY: Academic Press.

40

USAARL light measurement instrumentation

41

EG&G Gamma scientificANVIS spectroradiometer system

Model C-12ASR

This spectroradiometer system is designed to measure ANVISradiance, which is a measure of the compatibility of lighting(i.e., instrumentation panels and auxiliary instrument lighting)with image intensification night vision imaging system equipment.The system is comprised of a GS-4100 intelligent radiometer, aDC-49C thermoelectric water-cooled photomultiplier tube, a NM-3DHholographic monochromater, a GS-2110A scanning telemicroscope,and an IBM-PC interface console. This system is used primarilyfor spectroradiometry. The response range of this system is 380to 950 nm.

Figure A-1. EG&G Gamma Scientific ANVIS spectroradiometer

system.

42

KinoltaChroma meter

The Minolta chroma meter is a battery operated, hand-heldtristimulus color analyzer utilizing three silicon photo cellswhich are filtered to match CIE standard observer response. Thecells are capable of making readings of light source or reflectedcolor. Chromaticity coordinates (1931 CIE), illuminance (lux),and color temperature (kelvin) are calculated by the meter.Measurements are indicated digitally on a liquid-crystal display.Color temperature range is 1600 to 40,000 K. The illuminationrange is 10 to 200,000 lux.

Figure A-2. Minolta chroma meter.

43

-inoltaModel T-IH

Illuminance motor

The Minolta illuminance meter is a battery operated, hand-held unit that contains a silicon photocell for its lightdetector and has a liquid crystal display. The unit can displaymeasurements in footcandles or lux. Measuring range is 0.1 to99,990 fc (0.1 to 3,000,000 lux). The acceptance angle is 30degrees.

The meter also has integrating and deviation functions. Theintegrated illuminance mode allows measurements in fc-hr or lux-hr for integration periods from 0.01 hours (36 seconds). In theilluminance deviation mode, the difference between two lightlevels can be determined.

Figure A-3. Minolta model T-1H illuminance meter.

44

MinoltaModel Nt-i*

Luminance meter

TIhe Minolta luminance meter is a battery operated, hand-4heldluminance meter which focuses using a single-lens-reflpxviewfinder. The measuring range is 0.1 to 99,900 cd/rn (0.:O1-to9199004.fL). -The acceptance angle is 1 degree. The instrument -canfocus between 1 meter and infinity.

Figure A-4. Minolta model Nt-i0 luminance meter.

45

Photodyne Inc.Model 22XL

Optical multimeter

The- Photodyne Model 22XL Optical multimeter is the equivalentof a digital multimeter developed specifically for fiber opticsapplications. The optical multimeter provides for absolutemeasurements of all aspects of fiber optics systems includinglight sources and emitters, photoreceivers, fiber cabletransmission, and connector and splice loss. Features of theoptical multimeter include selectable 0.1 decibel (dB) or 0.01 dBresolution, mode selection for absolute or ratio measurements,current regulated IR source for calibration, and optical plug-insensor heads for specific range spectral and power applications.

Figure A-5. Photodyne Inc. model 22XL optical multimeter.

46

Photodyne Inc.Model 88XLA

Radiometer/photometer

The PhotodYne Model 88XLA Radiometer/photometer featuresdirect linear readout for radiant power in units of ianowatts.,micrematts, and milliwatts, from 1 picowatt lninum-rewo1mution -2 wattsmaximum reading. Direct linear photopic measurements weavAilable in footcandles, lux, and candelas. Plug-in sensorheads ave available for specific ranges of spectral and powerapplioctions.

Figure A-6. Photodyne Inc. model 88XLA radiometer/photometer.

47

:U

Photo ResearchPR-710 spot SpectraScan"T

Fast Spectral Scanning System

The SpectraScan PR-710 Fast Spectral Scanning System is adesigned for rapid measurement of radiance, luminance, spectraltransmittance, spectral reflectance, CIE color coordinates, andcorrelated color temperature. Typical time required formeasurements is less than 15 seconds. It uses a high-resolution,low-flare lens which can be focused from 2 inches to infinity;thus enabling the instrument to be used for either macro-photometry or telephotometry with no accessory lenses or changesin calibration.

Figure A-7. Photo Research PR-710 Spot SpectraScanT fastspectral scanning system.

48

Photo Research1980A and 19S01-PL

Photometer

The Spectraw Pritchardm Photometer consists of two separateunits: the photometer unit or "optical head," and the controlconsole. The two units are connected by an 8-foot cable. Theoptical system has a filter wheel containing neutral densityfilters (ND = 1, 2, and 3), a photopic correction filter,colorimetry filters, and a polarizing filter. An aperture wheelallows angular measuring fields from 2 minutes (2') to 3 degrees(30). The standard objective lens is a high-resolution, low-flare, seven inch, f/3.5 lens which may be focused from 4 feet toinfinity. A series of interchangeable and supplementary lensesenable the focusing distance to be reduced to as little as 0.625inches. The PL modification of the 1980A can measure theintegrated (average) value of pulsed sources from 10E-5 to 1OE+7foot-lambert-seconds (full-scale) for pulses as short as 1.6microseconds.

• -, ... .' .. ...."

Figure A-8. Photo Research 1980A photometer.

49

Photo RemearchLite~ato IIIPhotometer

The LiteMate Photometer, Model 501, is a compact, hand-helddigital photometer. It is capable of making cosine-correctedilluminance measurements over the range from 0.01 fcotcandles to19,990 footcandles, with ±5% accuracy and ±1% repeatability. Ithas a 1 degree acceptance angle.

Figure A-9. Photo Research Lite50ate III Photometer.

Photo ResearchSpectra! spotaeter,

Model UDO

The Photo Research Spectra" SpotmeterT' is a photomultipliertube photometer. Alone it can measure luminance and withaccessories or calibrations is capable of measuring illuminantd,luminous intensity, color temperature, light polarization,relative color coordinates, radiance, and irradiance.

The optical system consists of a photomultiplier tube, a IOOXattenuator, filter turret, a 1 degree aperture mirror, anobjective lens, and a focusing eye piece. This high resolution,low-flare lens can be focused from 2 inches to infinity, enablingthe instrument to be used for microphotometry or telephotometry.A six-position filter turret is located between the aperturemirror and the photo tube. This filter turret contains aphotopic, scotopic, red and blue filters for varying lightmeasurement conditions. The filter also contains an "open"position for radiometric measurements and calibrate position forinternal calibration verification. The attenuator reduces theinstrument's sensitivity by a factor of 10OX (ND = 2).

Figure A-10. Photo Research Spectra" Spotmeter~" model UBD.

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Photo ResearchPR-I253 fl

NVISPOT Rotor

The PR-1530AR NVISPOT is designed to perform AI4VIS radiancemeasurements, but has the capability to perform conventionalluminance and colorimetric light measurements.

The PR-1530AR uses an extended-red gallium arsenidephotomultiplier tube and is packaged as a portable unit. Themeasuring field aperture is 3 degrees.

Figure A-11. Photo Research PR-1530AR NVISPOT meter.

52

usAARL light sources

53

SHRO Inc.Ana lamp

Ultraviolet lamp

Wavelength reference source for 313.2, 404.7, 435.8, 546.1,730.8, and 809.4 nm.

Figure B-1. BH1( Inc. Analamp ultraviolet lamp.

54

EGSG GaMmA&R3-10

Tungsten lamp

Luminance referen~ce source for EG&G Gamma spectroradiometricsystem has a luminance of 179.0 footlamberts and correlated colortemperature of 26030K.

Figure B-2. EG&G Gamma RS-10 tungsten lamp.

55

EG&G Gamma

r•ungsten lamp

Luminance reference source for EG&G Gamma spectroradiometricsystem has a luminance of 358.2 footlamberts and correlated colortemperature of 28526K.

I-I

... .. . . . . ------....

-

Figure B-3. EG&G Gamma RS-12 tungsten lamp.

56

Melles Griat2 and 5 milliwatt

• HoNe lasers

z The helium-neon laser delivers relatively intense coherenti• electromagnetic radiation at 632.8 nm. The 2 milliwatt laser!• head has a beam diameter of 0.65 mm and typical beam divergence• of 1.3 milliradians with random polarization. The 5 milliwatt

laser head has a beam diameter of 0.8 mm and typical beam• divergence of 1.0 milliradians with random polarization.

Figure B-4. Melles Griot 2 and 5 milliwatt He~e lasers.

57

Oriel 632520 Vatt

Quartz tungsten-halogen lamp

This source has a 2.3 x 0.8 am dense filament in a smallhousing that can be mounted and positioned either horizontally orvertically on an accessory rod mount. Spring clips on thehousing can hold 2 x 2 inch or 50 x 50 am filters. The lamp ispowered by a constant voltage transformer.

Figure B-5. Oriel 6325 20 watt quartz tungsten-halogen lamp.

58

Oriel 64361000 watt

Quartz tungsten-llaloge~l lamp

This iiurce has a 5 x 18 am coiled filament lamp. Thehousing is blower cooled and allows for vertical and horizontaladjustment of the lamp during operation. condensing lenses and arear reflector increase the beam power by imaging the filamentback onto or adjacent to itself.

Figure B-6. Oriel 6436 1000 watt quartz tungsten-halogen lamp.

59

C-13-96spectral lamps

The Oriel spectral calibration set contains six differentline emission gas lamps -- Argon,. Helium, Krypton, Neon, Xenon,and Mercury (Argon). operated with low voltage power supplies,the'ne sourzes provide strong emission lines at specificwavelengths between 380 to 840 nui. These lamps are typicallyused for wavelength calibration of monocbromaters or detectors.

Figure B-7. Oriel C-13-96 spectral lamps.

60

Oriel 6340Point source

A.rc

This point source is a 0.18 mm diameter tungsten "point"heated by electron bombardment from an inert gas arc. Theresulting incandescent point is round and highly uniform with anaverage brightness of 25 cd/mm2 and an approximate temperature of30000K.

. '•. . . .. . .• . .- . .- . ". .......

Figure B-8. Oriel 6340 point source, arc.

61

Oriel 6363Infrared source

This infrared element continuously radiates in the 1.0 to 25micron range. The 6.2 mm diameter rod is 100 =m long with acenter of silicon carbide (carborundu~m) providing a 6.2 mumdiameter by 20 ma active area. It is heated by electric currentwith the capability of operating up to 10000K.

Figure B-9. Oriel 6363 infrared source.

62

Oriel 6611Rome

Rosoaroh laser

This 3 milliwatt helium neon laser delivers a highly stable0.8 mm diameter beam at 632.8 nm that is polarized with singlespatial mode and is concentric with the tube housing.

Figure B-10. Oriel 6611 HeNe research laser.

63

Photo ResearchPR 2301

Tungsten lamp

Luminance reference source for PR-1980A system has aluminance of 9.0 footlamberts.

Figure B-11. Photo Research PR 2301 tungsten lamp.

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