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Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
Photometry guide contents
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Photometry guide contents

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  • 1. Making light work of light MeasureMentThe G uide ToP H OT O M E T RY
  • 2. Making light work of light MeasureMentPhotometry Guide ContentsPhotometry Guide Contents Photometry GuiDe ContentS About UDT Instruments Hstory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Servce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Qualty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Publcatons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Professonal Socetes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Applcaton Informaton Introducton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Basc Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Important Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 How To Specfy A Photometer System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 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): Display Measurement LED Test Measurement Laser Test Fiber-Optic Test General Photometry General Radiometry 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 3. Making light work of light MeasureMentAbout uDt instrumentshiStory 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 1980‘s, ths hghly sklled and successful group grew nto an autonomous entty known as UDT Instruments . Drawng on the momentum generated by UDT‘s precson photomet- rc nstruments,the company developed an nventve handheld color- meter for the growng televson and computer perpherals markets . The development of UDT‘s SLS9400 colormeter promses to strength- en our company‘s 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 .ServiCe 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 .QuAlity 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, you’ll dscover that our servce and metrology departments reflect ths same commtment to qualty and personalzed servce . 1 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 4. Making light work of light MeasureMentAbout uDt instrumentsteChnoloGy 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 .PubliCAtionS In addton to our comprehensve Gude To tutoral seres, UDT regu- larly publshes artcles n trade journals and other scentfc lterature whch weve made avalable as applcaton notes to explan subtle detals and applcatons of our technology .ProfeSSionAl UDT s commtted to supportng the ndustry through ts professonal socety afflates . We are proud to be sustanng members of:SoCietieS • 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 lEclarage (CIE) .WArrAnty 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 Instruments‘s 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 . 2 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 5. Making light work of light MeasureMentApplication informationintroDuCtion Photometry s the scence concerned wth measurng human vsual response to lght . Because the eye s a hghly complex Green organ, ths s by no means a smple task . It nvolves the meetng of many dscplnes: Relative Response UV IR psychology, physology, and physcs among them . Photometry can be sad to have become Blue Red a modern scence n 1924, when the Commsson Internatonale de l‘Eclarage (CIE) met to defne the response of the average human eye . The Commsson measured the lght-adapted eyes of a sz- Wavelength in Nanometers able sample group, and compled the data Cie phototopic response curve. nto the photopc curve . Smply stated, the curve reveals that people respond strongest to the color green, and are less senstve to the spectral extremes, red and volet . Scotopic Vision The eye has an altogether dfferent Relative Response response n the dark-adapted state, where- n t also has dffculty determnng color . Ths gave rse to a second set of measure- ments, and the scotopc curve . Yellow Havng defned the eye‘s spectral response, Orange CIE sought a standard lght source to serve Green as a yardstck for lumnous ntensty . The Violet Red Blue frst source was a specfc type of candle, gvng rse to the terms footcandle and Wavelength in Nanometers candlepower . In an effort to mprove repeatablty, the standard was redefned Cie scotopic response curve. n 1948 as the amount of lght emtted from a gven quantty of meltng platnum . 3 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 6. Making light work of light MeasureMentApplication informationbASiC ConCePtSThe basc unt of photometry s the lumen, whch Scannng can be accomplshed wth dscrete-s related to ts radometrc analog, the Watt, by: wavelength, scannng monochromators, orlm = 683 x W x Vλ mult-channel detectors . In ether case, the ntensty of a lght source s measured wave-Where Vλ s the relatve lumnosty, a coeffcent length-by-wavelength, and then the results arescaled to vsual response . Unty occurs at the mathematcally ftted to the photopc curve .eye‘s peak response wavelength, 555 nanome- For ths reason, such technques do not occur nters . real tme, and requre mcroprocessor control .Two useful laws n photometry recur: the nverse Scannng approaches offer hgh accuracy, butsquare law and the cosne law . The frst defnes tend to be costly, and complex to operate .the relatonshp between llumnaton from a Optcal flterng offers a smple and cost-effec-constant-ntensty lght source and ts dstance tve soluton . Wth only one photo-current sgnalfrom a surface . It states that the ntensty per to process, sngle-channel electroncs can beunt-area on the surface, vares n nverse propor- used . Also, recent advances n flter desgn, andton to the square of the dstance between the mprovements n sold-state detectors, allow thssource and surface, or: method to rval scannng systems for photomet-∆lm/M2 α 1/∆d2 rc accuracy .Accordngly, successve llumnance measure-ments are only as accurate as the control ofsource to surface dstance . Further, f llumnances known at one dstance, t can, barrng nterfer-ence, be calculated for any dstance .The cosne law ndcates the ntensty of lght ona surface of fxed area, vares wth ncdent angle .In fact, the ntensty falls off as the cosne of theangle . Ths results because the projected surfacearea, n the plane perpendcular to ncdence, sproportonally reduced .Thus n measurements of envronmental lghtng,sensors requre cosne correcton to account foroff-axs lght . Wthout t, consderable errors wlloccur, especally wth brght sources at low nc-dent angles (e .g ., wndows) . Ths often accountsfor the dfference n readngs between two pho-tometers .The cardnal challenge n photometry s to recre-ate the spectral response of the human eye . Butelectronc sensors have dstnct response char-acterstcs whch bear no resemblance to the CIEstandard observer . Therefore, these sensors mustbe spectrally corrected . Two technques are con-ventonally used to accomplsh ths: wavelengthscannng, and detector/flter matchng . 4 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 7. Making light work of light MeasureMentApplication informationbASiC ConCePtS Photometrc to radometrc converson factors .Wavelength Vλ CIEPhotopc Photopc Wavelength Vλ CIEPhotopc Photopc(nm) Lumnous Lumen/Watt (nm) Lumnous Lumen/Watt Effcency Converson Effcency Converson Coeffcent Factor Coeffcent Factor380 0 .0000 .05 570 0 .9520 649 .0390 0 .0001 0 .13 580 0 .8700 593 .0400 0 .0004 0 .27 590 0 .7570 516 .0410 0 .0012 0 .82 600 0 .6310 430 .0420 0 .0040 2 .73 610 0 .5030 343 .0430 0 .0116 7 .91 620 0 .3810 260 .0440 0 .0230 15 .7 630 0 .2650 181 .0450 0 .0380 25 .9 640 0 .1750 119 .0460 0 .0600 40 .9 650 0 .1070 73 .0470 0 .0910 62 .1 660 0 .0610 41 .4480 0 .1390 94 .8 670 0 .0320 21 .8490 0 .2080 142 .0 680 0 .0170 11 .6500 0 .3230 220 .0 690 0 .0082 5 .59510 0 .5030 343 .0 700 0 .0041 2 .78520 0 .7100 484 .0 710 0 .0021 1 .43530 0 .8620 588 .0 720 0 .0010 0 .716540 0 .9540 650 .0 730 0 .0005 0 .355550 0 .9950 679 .0 740 0 .0003 0 .170555 1 .0000 683 .0 750 0 .0001 0 .820560 0 .9950 679 .0 760 0 .0001 0 .041 5 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 8. Making light work of light MeasureMentApplication informationbASiC ConCePtS Ths flter-matchng technque nvolves the To Surface layerng of colored-glass flters over an optcal detector . Each element functons to attenu- θ ate selectve wavelengths untl the detector‘s Effective Area presented response smulates the CIE curve . Planar df- to Incident Flux is fused slcon photododes offer the best pho- Measurement Area x Cos φ tosensor characterstcs, snce they afford hgh senstvty and lnearty throughout the vsble spectrum . Usng slcon photodetectors, and Irradiation = Area x Cos φ x Incident Flux advanced flter desgns, UDT Instruments matches the CIE human eye response curve wthn 1% total area error . Ths s the best the intensity of off-axis light decreases match achevable, accordng to CIE . relative to the cosine of incident light. There s another more mportant specfcaton of the qualty of a photometrc detector and that s the f₁, value . Ths s defned by the CIE and s a numercal value assgned to the aver- age devaton of the photometrc detector‘s response from the CIE curve . An f₁, 1 .5% s the best possble laboratory grade detector whle an f₁, 3% s consdered sutable for most applcatons . However, the relatonshp between a gven detector and flter s delcate . Once the two have been matched, they should not be nterchanged wth other photometrc detec- tor/flter pars . Each detector exhbts unque response characterstcs that requre a specfc combnaton of flter layers and thcknesses . the typical spectral response of silicon photodetectors. 6 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 9. Making light work of light MeasureMentApplication informationbASiC ConCePtS PHOTOMETRIC FILTER RESPONSE Once the detector‘s response s fxed, t s calbrated usng the transfer of standards CIE Response curve technque . Ths requres a detector of known response, whch can be obtaned from theRelative Response Sensor Design Shape Natonal Insttute of Scence and Technology (NIST) . A detector/flter par s postoned before an optcal source wth constant wave- length and ntensty characterstcs (usually a tungsten halogen lamp) . The electrcal output of the detector under test s then compared to the standard detector‘s output . Once the sensor‘s lumnous response s deter- Wavelength in Nanometers mned, t can be matched to a precson gan-uDt instruments photometric filters match controlled electronc amplfer and readoutthe Cie curve to within 1% total area error. system . Calibration by transfer of Standards Rt = Responsvty of the test detector (A/lm) Rr = Responsvty of the reference detector (A/lm) It = Measurement of the test detector (A) Ir = Measurement of the reference detector (A) ( ) A A Rt lm = Rr lm ( )( ) lt ( A) lr ( A) 7 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 10. Making light work of light MeasureMentApplication informationimPortAnt termS luminous flux Lumnous flux s expressed n lumens, the fun- Detector damental unt of photometry . It s a measure of the total optcal output of a vsble lght source . The measurement requres all of a source‘s power to be concentrated on a detector . Ths Detector can be a problem wth dvergent sources lke LEDs, and lamps . In these cases, ntegratng spheres are often used . illuminance Illumnance s a measure of the amount of vs- in illuminance measurements, area ble lght ncdent upon a prescrbed surface area . In Englsh unts, one lumen of flux fallng is determined by the detector unless on one square foot s termed a footcandle . there is an external aperture. The metrc equvalent, one lumen per square meter, s called a lux (10 .76 lux = 1 footcan- dle) .Of course, detectors don‘t have such large areas . So the area of the detector s multpled proporton-ally . Specal attenton s due when the detector s under-flled or used behnd correctve optcs, sncethe sensor‘s area no longer defnes the surface beng llumnated .For example, llumnance measurements are partcularly susceptble to errors ntroduced by off-axslght . So cosne-correctng dffusers are used wth the detector head . Snce the cosne dffuser sessentally maged onto the sensor, the dffuser‘s area, not the sensor‘s, represents the measurementsurface . Photometric Quantities and units Quantity Symbol Units Abbreviations Luminous energy Q lumen•second…talbot lm•s…talbot Luminous Density U lumen•second/m3 lm•s/m3 Luminous Flux F lumen lm Illuminance E lumen/m2…lux lm/m2…lx lumen/cm2…phot lm/cm2…ph lumen/ft2…footcandle lm/ft2…fc Luminous Exitance M same units as illuminance Luminance (brightness) L candela/m2…nit cd/m2…nt candela/cm2…stilb cd/cm2…sb candela/π ft2…footlambert cd/π ft2…fl candela/π m2…apostilb cd/π m2…asb candela/π cm2…lambert cd/π cm2…L Luminance intensity Iu lumen/steradian…candela lm/st…cd 8 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 11. Making light work of light MeasureMentApplication informationimPortAnt termS luminous exitance Lumnous extance s an ntrnsc property of a lght source . It s calculated by measurng lumnous flux (lumens), and dvdng by the surface area of the source . Ths measurement s also expressed n lumens per square meter, but s not to be confused wth llumnance measurements or lux . The area referred to n lumnous extance s that of the lght source, not the llumnated surface . Ths measure- ment s most applcable to emtters wth flat surfaces . luminous exitance is calculated by measuring luminous flux and divid- ing by the source‘s area. Detector luminous intensity Lumnous ntensty s also a source property, but one where the source‘sLED drecton and dvergence come nto Ω play . Defned as the quantty of lum- nous flux emtted unformly nto a sold angle, the basc unt of lumnous ntensty s the candela, equal to one lumen per steradan . Several thngs are suggested by ths defnton . One, ths measurement luminous intensity is a measure of the flux s not applcable to collmated lght emitted into a solid angle. sources . Two, t s naccurate for non- unform emtters . To calculate lumnous ntensty, the detector‘s area (or the area prescrbed by the aperture n front of t), and ts dstance from the lght source must be known . From these, the sold angle can be calculated, and then dvded nto the flux readng . 9 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 12. Making light work of light MeasureMentApplication informationimPortAnt termS Measurement Detector Plane one luminous measurement technique involvesfixing the detector‘s field-of-view through the use of a lens.luminance luminous energyAlso known as photometrc brghtness, lum- Lumnous energy s a measure of the rate of flownance s a measure of the flux reflected by, or of flux, and so s expressed n lumen-seconds .emtted from, a relatvely flat and unform sur- Generally, t s appled to flashed or pulsed sourc-face . The technque takes nto account the area of es .the surface measured, and the angle subtended It s also possble to measure any photometrcby an observer lookng at t . quantty on a tme-dependent bass . For nstance,Lumnance may be thought of as lumnous the llumnance of a rotatng beacon n one drec-ntensty per unt area, and so n metrc terms s ton could be ntegrated over tme to yeld foot-expressed as candle-seconds .candelas per square meter . But a host of otherterms are used for ths measurement, some todescrbe acrcular measurement area rather than a squareone (see Photometrc Quanttes and Unts chart) .To measure lumnance, the detector feld-of-vew must be restrcted, and ts angle calculated .Usually, a lens or baffle s used to acheve ths . Infact, the human eye, wth ts lens and aperture,functons as a lumnance meter .Note that so long as the detector‘s feld-of-vews flled, ths measurement s ndependent of thedstance between the detector and measurementplanes . That‘s because feld sze and source nten-sty vary n drect proporton to one another as afuncton of dstance . 10 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 13. Making light work of light MeasureMentApplication informationhoW to SPeCify A Photometer SyStem Specfyng a photometer system s best Photopic Filter approached n three steps . Frst, evaluate the Luminance LensDetector source to determne whch measurement technque best apples . Then, select a detec- tor and optcal system (detector head) that sut the measurement . And fnally, match the detector head to the partcular electroncs whch provde the most effectve user nter- Photometer face for the applcaton . Crts and other displays are typically Consider the Source measured in terms of luminance. Common sense goes a long way n determn- ng the rght measurement for an applcaton . After all, photometry s concerned wth the relaton of lght to the human eye . So, the frst queston s: how wll people be affected by the source to be measured? For nstance, measurements of ambent or envronmental lghtng are concerned wth Integrating Sphere people‘s ablty to read prnt or safely see objects n an area . It s not the power of a partcular source that s of concern, but rather how well the source lghts the area of nter- est . For ths reason, lghtng for the outdoors, offces, factores, and photography are mea- LED sured n terms of llumnance . Filter However, f n the same room or space one wshed to determne the brghtness of walls, Detector fabrc, or panted surfaces, the measurement changes altogether . Because now the amount Photometer of reflected lght receved by the eye s of con- cern . Snce all of these surfaces are dffuse and relatvely unform, a lumnance measurement integrating spheres are the most accu- would best apply . rate means of measuring small, divergent sources like leDs. SEE ALSO: The Guide to Photometer Radiometer System Configuration, available as a free PDF download at www.udtinstruments.com 11 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 14. Making light work of light MeasureMentApplication informationhoW to SPeCify A Photometer SyStem entfc applcatons . But when ther potental damage to the eye s of concern, they would probably be measured for lumnous flux . A lensed LED, however, s a dvergent, though drectonal, source . Lumnous ntensty would best characterze t . But wth surface or edge emttng LEDs, emsson as a functon of sur- face area s sgnfcant . Ths descrbes a lum- nous extance measurement . Lumnous energy measurements apply to any perodc source . Pulsed LEDs, photographc flash unts, strobe lghts, arc lamp systems, for luminance measurements requiring and rotatng or scannng lghts are sev- eral examples of sources whose flux s tme small fields-of-view, a lens system with dependent . view-through optics is essential. Selecting the right detector headElectronc dsplays such as CRTs, avoncs, and The measurement type dctates your choceautomotve panels are ncdent drectly upon the of detector head assembles .eye too . But alpha-numerc characters and lne UDT Instruments offers a modular photometrcdetal are generally small . So the measurement sensor-head desgn approach . In all cases, a sl-system‘s feld-of-vew must be lmted or focused con photodetector, detector housng, and photo-n order to measure only the lghted portons metrc flter assembly are provded . And for thoseof the dsplay . Ths s, by defnton, a lumnance lumnous flux measurements where all ncdentmeasurement . So dsplay brghtness s usually lght s collmated or focused onto the detector,specfed n footlamberts . ths smple head wll suffce .Lamps are used n so many applcatons that t However, f flux levels exceed 70 lumens pers mpossble to defne just one way to measure square centmeter, the detector may becomethem . As prevously mentoned, lamps and lamp saturated, and ts output nonlnear . In suchsystems for area lghtng (rooms, streets, stad- nstances, attenuaton s recommended . Neutral-ums) call for llumnance measurements . But n densty flters, apertures, or ntegratng spheresautomotve exteror lghtng, headlghts are usu- acheve the desred effect . The correct selec-ally measured for llumnance, tallghts for lum- ton depends upon the amount of attenuatonnance . There are a number of mnature, lensed desred: t should be enough to avod detectorlamps on the market, and snce ther dvergence saturaton, but not so much as to lose senstvtys of concern, they would be measured for lum- and dynamc range .nous ntensty . Incandescent and fluorescentlamp manufacturers specfy products n termsof lumnous flux (or the radometrc equvalent,watts) snce these wll be placed n fxtures SEE ALSO: The Guide to Photometermeant to dffuse and measure ther total output . Radiometer System Configuration, available as a free PDF download atLasers and LEDs also requre a careful approach .They are measured n radometrc terms for sc- www.udtinstruments.com 12 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 15. Making light work of light MeasureMentApplication informationhoW to SPeCify A Photometer SyStemThe smple detector/flter arrangement s also mcroscopc or telescopc .effectve for ambent measurements f all lght s UDT Instruments offers a wde range of optcalat normal ncdence . But when off-axs lght, such accessores for out-of-the-ordnary measure-as from wndows and perpheral sources, contrb- ments . These nclude: fber optc probes, forutes to the total flux, a cosne dffuser s needed . convenence n measurng sources hdden nIn addton to beng wdely appled by lamp hard-to-reach places; LED measurement systemsmanufacturers, ntegratng spheres are useful for specfc to ether segmented or dscrete LEDs;measurements of small dvergent sources lke low-profle sensors forlensed LEDs or mnature lamps . These can be slppng nto tght spaces, such as n photolthog-nserted rght nto the sphere‘s entrance port to raphy exposure systems; and a varety of sensorensure that all lght s collected . heads customzed for Display lumnance measure-Lumnance measurements requre a prescrbed ments .sensor-head feld-of-vew . The sze of the sourcen the measurement-feld plane, and the sensor- SEE ALSO: The Guide to Photometerto-subject dstance determne the angle . Wth Radiometer System Configuration,large, but close felds, a smple baffle (steradanshade or aperture) wll do . But small mages, such available as a free PDF download atare those on CRTs or avoncs, call for a lens sys- www.udtinstruments.comtem, as do measurements at a dstance . A varetyof lens assembles and optcal accessores areavalable from UDT Instruments, to accommo-date most any lumnance measurement, whether uDt model 1120 telephotometer 13 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com
  • 16. Making light work of light MeasureMentApplication informationhoW to SPeCify A Photometer SyStemChoosing electronics matched to the Your choce of electroncs depends upon the answers to a few basc questons:application 1 . Is feld portablty needed?The lght sensor n each UDT Instruments photo-metrc head s a slcon photodode . Though sen- 2 . Wll the nstrument be nterfaced wth asor sze may vary, the output wll n all cases be a computer?low ampltude current sgnal . Ths sgnal wll be 3 . Is a vsual dsplay desred, or wll an analogconverted nto a output suffce?voltage by a transmpedance amplfer crcut, 4 . Wll more than one measurement be con-and then used accordng to the requrements of ducted concurrently?the partcular applcaton . UDT Instruments offers photometer controllersSEE ALSO: The Guide to Photometer and electronc amplfers that satsfy any comb- Radiometer System Configuration, naton of answers to these questons . The nstru- available as a free PDF download at ments range from smple analog amplfers and www.udtinstruments.com hand held photometers, to multchannel com- puter-controllable laboratory nstru- ments . Versons are avalable whch sut most any budget . 14 8581 Aero Drive, San Diego, CA 92123 • (858) 279-8034 • www.udtinstruments.com

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