Radiometry and Radiometric Testing Tutorial UDT Instruments

Th e G u i d e To

RADIOMETRY

Making light work of light MeasureMent


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TAble of ConTenTSAbout UDT Instruments

Hstory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Servce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Qualty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Publcatons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Professonal Socetes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Applcaton InformatonIntroducton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Important Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Calbraton of Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6How to Specfy a Radometer System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Additional References from UDT Instruments: Photomety & Radiometry:

Guide to Photometer & Radiometer System Configuration (PDF; including a catalog of UDTi components) Application Pages (on the UDTi website): LED Test & Measurement Laser Test Fiber-Optic Test General Radiometry

Table of Contents



1

The early begnnngs of UDT Instruments can be traced to 1967 when a small group of nventors at Unted Detector Technology (UDT) began manufacturng the frst commercally avalable transmpedance amplfers for planar-dffused and Schottky barrer slcon photosen-sors . Over the next several years, ths same group of people went on to poneer leadng-edge technologcal nnovatons for photometers, radometers, fber-optc power meters and optcal poston-sensng nstruments . By the early 1980s, ths hghly sklled and successful group grew nto an autonomous entty known as UDT Instruments .

Drawng on the momentum generated by UDTs precson photomet-rc nstruments,the company developed an nventve handheld color-meter for the growng televson and computer perpherals markets . The development of UDTs SLS9400 colormeter promses to strength-en our companys poston as a leader n precson electro-optcs nstrumentaton, whle meetng the strngent demands of a multtude of CRT calbraton requrements . UDT s posed and ready to excel to greater technologcal excellence wth only one goal n mnd: to meet and exceed the ever-changng needs of ts customers worldwde .

We at UDT Instruments stand behnd our products and the companes who use them . For ths reason, we contnue to servce those same lght-measurng nstruments that we bult twenty years ago . By offer-ng these servces to our customers, both new and establshed, we stay nvolved wth our products and extend a personal touch to our busness relatonshps . We know of no other company n our ndustry that hres more qualfed sales engneers, people who really under-stand lght measurement prncples and practces . By hrng such knowledgeable engneers, we ensure you that you wll get the best electro-optc nstruments to ft your applcaton and budget .

The nstrument you receve s certan to be relable and accurate . We mantan a Qualty program that affects every ndcator module, sen-sor head, and optcal accessory we sell . And when t comes tme for re-calbraton, upgrades, or repars, youll dscover that our servce and metrology departments reflect ths same commtment to qualty and personalzed servce .

HiSToryHiSTory

ServiCeServiCe

QuAliTyQuAliTy

About uDT instruments



2

UDT Instruments has always been and contnues to be at the forefront of lght measurement technology . We hold U .S . and worldwde pat-ents on our QED products, whch are absolute radometrc reference standards n the vsble and near IR spectrum . Our QED-200 product won a prestgous IR-100 award as one of the 100 most sgnfcant U .S . nventons n 1986 . These products were developed n conjunc-ton wth the Natonal Insttute of Standards & Technology (NIST) and the Natonal Physcal Laboratory (NPL) . UDT Instruments contnues to work wth the NIST under Cooperatve Research And Development Agreements (CRADA) n order to develop even more state-of-the-art products nto the 21st Century .

In addton to our comprehensve "Gude To" tutoral seres, UDT regu-larly publshes artcles n trade journals and other scentfc lterature whch we've made avalable as applcaton notes to explan subtle detals and applcatons of our technology .

UDT s commtted to supportng the ndustry through ts professonal socety afflates . We are proud to be sustanng members of:

Socety of Photo Optcal Instrumentaton Engneers (SPIE)Optcal Socety of Amerca (OSA)Natonal Assocaton of Broadcasters (NAB)Laser Insttute of Amerca (LIA)Illumnatng Engneerng Socety of Amerca (IES)Socety For Informaton Dsplay (SID)

UDT also actvely partcpates n the Councl for Optcal Radaton Measurement (CORM) and the Commsson Internatonale l'Eclarage (CIE) .

UDT Instruments warrants that ts products are free from defects n materal and workmanshp under normal use and servce for a perod of one year from the date of shpment from our factory . UDT Instrumentss oblgaton under ths warranty s lmted to the replace-ment or repar of any product determned to be defectve durng the warranty perod, provded the product s returned to the factory pre-pad . Ths warranty does not apply to any equpment that has been repared or altered, except by UDT Instruments, or whch has been subject to msuse, neglgence, or accdents . It s expressly agreed that ths warranty wll be n leu of all warranty of merchantablty . No other warranty s expressed or mpled . UDT Instruments s not lable for consequental damages .

TeCHnologyTeCHnology

PubliCATionSPubliCATionS

ProfeSSionAl SoCieTieSProfeSSionAl SoCieTieS

WArrAnTyWArrAnTy

About uDT instruments



3

Application information

Radometry s the measurement of radaton n the electromagnetc spectrum . Ths ncludes ultravolet (UV), vsble and nfrared (IR) lght .

Electromagnetc radaton s characterzed by ts frequency of oscllaton . The frequency determnes the "color" of the radaton (see Fgure 1) . The speed of lght s a constant, and frequency s related to wavelength by the relatonshp:

C =

C = speed of lght

= wavelength

= frequency

The preferred unts of measure for wavelength are nanometers (nm) and mcrometers (m or "mcrons") .

The vsble regon of the electromagnetc spectrum can dvde nto the basc colors of the ranbow: red, orange, yellow, green, blue, ndgo and volet . Red lght has the longest wavelength n the vsble regon (780 nm) . Volet has the shortest (380 nm) .

Ultravolet lght s shorter n wavelength than vs-ble lght . It extends approximately from 10 nm to 400 nm . And lke other colors of the vsble regon, UV can be subdvded nto 3 smaller regons: UVA, VUV and UVC . The UVA regon ranges from 400 nm down to 320 nm and s the least harmful of UV radaton . The vacuum-ultravolet (VUV) and UVC regons are shorter and mportant to the study of cancer .

Infrared lght extends from 700 nm to 100 mcrons . Its regons are known as near-IR, md-IR and far-IR .

Measurements of optcal radaton requre specfc methods to obtan accurate measurements . UDT Instruments sup-ples calbrated detector/flter combnatons that cover from 200-1800 nm (0 .2 to 1 .8m) . To obtan accurate measurements, one must understand the lght source ( .e . laser, lamp, LED); the optcal medum ( .e . ar, water, optcs); and the partcular response characterstcs of the detector .

inTroDuCTion

Figure 1

780



4

imPorTAnT TermS


In order to accurately descrbe an optcal source, one must use the correct unts and know how these unts apply to detector-based radometry . In prac-tcal lght measurement applcatons, the recever of optcal radaton s a detec-ton devce that converts optcal radaton to electrcal current accordng to a known relatonshp .

The chart dsplayed on the l left s a short breakdown of

radometrc terms and ther correspondng unts of measure .

radiant energyRadant energy refers to the amount of power reachng a gven pont accu-mulated over tme . Ths s referred to as joules (watt-second).

radiant fluxRadant flux s the fundamental unt n detector-based radometry . It s defned as the total optcal power of a lght source, and s expressed n watts.

To measure radant flux, the detector must collect all emtted lght . Examples of typcal flux measurements are shown n Fgure 2 . Focused lasers and fber optc cables requre only the proper sensor head because the source and detector can be confgured so that all radaton s ncdent wthn the actve area of the sensor . Dvergng lght sources, such as LEDs and lamps, may requre an ntegratng sphere to capture lght radatng n several drectons .

irradianceIrradance s the amount of radant flux ncdent on a known surface area . Its nternatonal unt of measure s watt/m2 . However, because many sen-sor heads have a 1-cm2 detector area, t s smpler to use watt/cm2 .

There are two ways to control the sze of the detector area . The frst s to use a sensor head wth a known detector area . The second s to place an aperture wth a known area between the source and the detector . When source radaton does not completely fll the detector, an aperture s the only relable method of

controllng detector area .

Quantity Symbol units Abbrev. Radant energy Q joule=watt-second J=Ws Radant energy densty U joule/m3 J/m3

Radant flux (Power) ,P watts=joules/second W=J/s Irradance E watts/m2 W/m2

Radant extance M watts/m2 W/m2

Radance L watts/m2steradan W/m2sr

Radant ntensty I watts/steradan W/sr

Radiometric Quantities and Units

figure 3

Focused Beam

Fiber Optic

Laser Beam

Aperture

Detector

figure 2



5

imPorTAnT TermS


radiant exitanceRadant Extance, a property of the lght source, s the total radant flux from the source dvded by the surface area of the source . Its unt of measure s watt/m2, smplfed as watt/cm2 . Ths type of measurement only apples to extended lght sources and s useful for makng effcency measurements of dfferent lght source materals .

To make radant exstence measurements, one must know the surface area of the source and then measure the total radant flux leavng the source .

radiant intensity Radant Intensty s the amount of flux emtted through a known sold angle . It s measured n watts/steradian .

To measure radant ntensty, start wth the angle subtended by the detector at a gven dstance from the source (see Fgure 4) . Then dvde the amount of flux by that sold angle .

Radant Intensty s a property of the lght source and may not be relevant f the spatal dstrbuton of radaton from the source s non-unform . It s approprate for pont sources (and for close approxmatons, such as an LED ntensty measurements), but not for

collmated sources .

radianceRadance s the radant ntensty emtted from a known unt area of a source . Unts of radance are used to descrbe extended lght sources, such as a CRT or an EL/O Panel unt for characterzng pont sources .

To measure radance, you need to defne the area of the source to be measured, and also the sold angle receved (see Fgure 5) . Ths s usually smulated usng an aperture and a postve lens n front of the detector . It s expressed as watts/cm2ster .

figure 4

DetectorAcceptanceAngle

Source Aperture Detector Aperture

figure 5



6


Radometrc Sensors may be comprsed of a detector ( .e . slcon or ger-manum) and a flter, or a combnaton of flters . Flters can be spec-trally matched to detectors to create a desred response curve . Ths s accomplshed by attenuatng certan wavelengths .

The relatonshp between detectors and flters s delcate . A small var-aton n the thckness of the flter materal s enough to cause a dffer-ence n the way two dentcal sensors perform . Therefore, sensors must be calbrated ndvdually, to measure ther unque responsvtes (the relatonshp between detector output sgnal and ncdent flux) .

One method of detector calbraton s by "transfer of standards ." Usng ths method, a detector s calbrated by comparng t wth another detector of known response . Ther responsvtes are measured by alternately placng the two detectors n a radant beam of known wavelength and ntensty (refer to explanaton n left nsert) .

UDT Instruments calbrates each sensor head aganst a NIST-traceable detector . The calbraton data s then matched to a readout devce .

Calibration by Transfer of Standards

Rt = Responsvty of the test detector at wavelength () .(A/W)Rr = Responsvty of the reference detector at wavelength () .(A/W)It = Measurement of the test detector at wavelength () . (A)Ir = Measurement of the reference detector at wavelength () .(A)

CAlibrATion of SenSorS

Rt(A)=Rr(A)(I lt(A)AW

AW

It(A)Ir(A)



7

Selectng a properly calbrated Radometrc head and the rght readout devce are mportant n obtanng accurate results .

The sensor head converts electromagnetc radaton nto an electrcal sgnal . The readout devce then receves ths sgnal and nterprets t . A properly calbrated measurement system wll measure the lght source and dsplay the measurement n the approprate optcal unts .

The readout unt should be selected accordng to ts features, and the detector head should be selected accordng to ts power measurement range, wavelength calbraton and sze . The two matched together

wll accurately measure the source n the correct optcal unts .

Consider The SourceAs descrbed prevously n the mportant terms secton, optcal sources are character-zed by certain units

Collmated lght sources, such as lasers, are typcally characterzed by radiant flux mea-surements (Fgure 6) . A beam that overflls a sensor head may be characterzed by ts power density (irradiance) n watts/cm2 . An ntegratng sphere can also be used n a radant flux measurement n order to attenu-ate the laser power to be wthn the lmts of the sensng devce .

Point sources, such as LEDs, are characterized n unts of radiant intensity, provded the spatal dstrbuton s unform . Ths type of measurement s easly acheved wth an aperture and baffled tube to defne the sold angle of detector acceptance . Measurements must always be made wth a consstent sold angle . Ths constant sold angle may be defned by the detector's area and ts dstance from the pont source . A pont source may also be defned by unts of radiant flux, provded all the rada-ton s captured n an ntegratng sphere .

Unform extended sources such as lamps may be characterzed by radiant flux or irradiance measurements .

Unform extended sources such as flat-panel LCD dsplays, are best characterzed by unts of radiance .

energy measurementsMeasurements of pulsed sources requre specal consderatons . The standard unt of optcal energy s the joule (watt-second) . Integratng the sgnal over a known tme perod makes energy measurements . When makng energy measurements of pulsed sources usng slcon and germanum detectors, one must consder the effects of peak power on the detector . Saturaton of the detector wll cause the detector to behave non-lnearly and wll result n measurement error .


Attenuator orIntegrating Shere

Laser

Radiometer

Filter

Detector

figure 6

HoW To SPeCify A rADiomeTer SySTem



8


Wavelength and optical filters Optcal flters can be desgned to allow certan wavelengths to pass through, whle screenng out others . A flter can be selected to modfy a detector's response n order to lmt the bandpass to match some desred response curve or to attenuate the nput sgnal by a known amount . Many flter and detector c combnatons must be calbrated to ensure measurement accuracy .

A detector/flter combnaton that acheves a spectrally flat response (Fgure 7) s especally useful for measurng broadband sources or sources where the peak wavelength s uncertan or may vary . UDT Instruments newest flat flters are accurate between 450 and 950 nm to wthn 5% .

Detector/flter combnatons that allow a specfc broadband transmsson are well suted for measurng arc lamp dstrbuton peaks, visible light or UV spectral content (Fgure 8) .

Narrow bandpass flters are usually utlzed for laser power measurements . Ths type of detector/flter combnaton assures that only the monochromatc radaton from the laser reaches the detector's actve surface .

An mportant consderaton to make before specfyng a sensor head s how much power wll be measured . In addton to havng a well-defned wavelength range, they should also have a well-defned power handlng capacty .

Silicon ingaAs, and germanium as Light Measurement materials S Slcon, InGaAs and germanum are especally well suted to measure lght . T these materals are specally processed to convert ncdent radaton to an electrcal sgnal by the photoelectrc effect . The converson rato or detector r responsvty s lnear over the sensor's nput range . For a slcon sensor, ths range spans 12 decades . For a germanum sensor t spans 9 decades . The sensor's response s also unform over the actve surface, makng t deally suted for both power and power densty measurements .

The InGaAs sensors are used n the Telecommuncatons/Fber Industres when hgh senstvty, low dark current, and hgh dynamc mpedance are needed .

Once the applcatons and characterstcs of the source and recever have been fully defned, a radometer system can be selected .

UDT Instruments offers radometer systems rangng from an extremely por- table handheld meter that s durable enough for feld use to a sophstcated benchtop model that nterfaces wth a computerzed data acquston system .

Rela

tive

Resp

onse

Wavelength (nm)

1

.9

.8

.7

.6

.5

.4

.3

.2

.1

0350 450 550 650 750 850 950 1050

figure 7

350360370380390400

0.8

0.4

0.0

Rel

ativ

e Re

spon

se

Wavelength (nm)

HPBW10nm

figure 8

HoW To SPeCify A rADiomeTer SySTem

jlelandText BoxSEE ALSO:

The Guide to Photometer & Radiometer System Configuration, available as a free PDF download at:

www.udtinstruments.com

jlelandText Box

Table of ContentsAbout UDT InstrumentsHistoryServiceQualityTechnologyPublicationsProfessional SocietiesWarranty

Application InformationIntroductionImportant TermsCalibration of SensorsHow to Specify a Radiometer System

Product DatasheetsModel S350 OptometerModel S380 Dual-Channel OptometerTramp

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Radiometry and Radiometric Testing Tutorial UDT Instruments