RadiometrySensing the World

STEVEN W. BROWN, B.CAROL JOHNSON, AND KEITH R. LYKKE

Figure 1. Photograph of a laser beam entering NIST’s high-accuracy–cryogenic radiometer.

RADIOMETRY

February 2001 ■ Optics & Photonics News 25

L ight is the supreme messenger. Itconveys information about thecosmos, about the make-up of our

bodies, and about nearly everything in be-tween. Light can carry structural informa-tion, chemical information, and informa-tion on position and speed, as well as ontemperature. This information is codedby pattern, color, and intensity.This article describes some ofthe methods developed tomake accurate measurement oflight a reality. We describe threeongoing research programsaimed at establishing measure-ment chains between funda-mental optical flux measure-ments and the more practicalquantities of commercial andscientific interest: spectral irra-diance, spectral radiance, andradiance temperature.

We need to measure opticalpower before we can measureirradiance, radiance, or radi-ance temperature.1 In the Unit-ed States, the national opticalpower standard is the high-accuracy–cry-ogenic radiometer, or HACR,2 housed atthe National Institute of Standards andTechnology (NIST). (see Fig. 1). Light en-ters HACR through a window and under-fills an absorbing cavity. The cavity iscooled by liquid helium, then maintainedat a temperature slightly higher than 4.2 Kby heating using electrical power. The lightis alternately absorbed (signal) in the cavi-ty and blocked (background) in front ofHACR. The difference in the electricalpower necessary to hold the temperatureconstant between the two configurationsis the optical power in the laser beam inwatts absorbed by the cavity. HACR there-fore functions as an electrical substitutiondevice, directly relating the optical watt tothe electrical watt.

The degree of uncertainty in measure-ment of the laser beam’s power is now onthe order of 1 part in 104. With an accuratemeasurement of the laser beam’s power,we can measure the responsivity of a de-tector to that power in Amps/Watt. Elec-trical substitution radiometry at cryogenictemperatures is the basis of detector cali-brations in which a detector’s response tooptical flux is measured as a function ofwavelength. Since HACR is large, expen-sive to operate, and slow, it is impracticalto use it to calibrate a large number of de-vices. To measure detectors’ optical re-

sponse on a routine basis, transfer devicesare required.

The typical transfer device is a trap de-tector. Trap detectors are comprised ofseveral separate detectors oriented in sucha way that light reflected from one detec-tor is incident on the next one. The devicetraps, or absorbs, almost all of the incident

light. To obtain the absolute responsivityin A/W of the transfer device, it is posi-tioned in the laser beam calibrated byHACR and the output current from thedetector is measured. If this is done at anumber of laser wavelengths, the transferdevice can be calibrated for spectral re-sponsivity over a certain wavelength rangewith typical uncertainties of less than 5 parts in 104. This gives us a transfer stan-dard detector that can be used to calibratethe response of other detectors. At NIST,special spectral comparator facilities(SCFs) use transfer standard instrumentsto measure the spectral power responsivityof customers’ detectors over a spectralrange from the UV (192.5 nm) to the farIR (20 �m). In place of lasers, a tunablemonochromatic light source is used. Thefacility measures the detector’s response tothe optical power incident upon it.

Although such measurements are un-deniably important, there is generallygreater interest in questions such as howmuch light falls on a surface (irradiance)or how much light emanates from a par-ticular source (radiance). Most radiomet-ric devices throughout the world do notmeasure power, but rather quantities likethe irradiance on a surface (in W/m2) or

radiance of a source (in W/m2/sr). Oftenwe are also interested in how much lightthe human eye registers; in this case wemodify the irradiance and radiance meas-urements by considering the response ofthe human eye.1

A new laboratory, the facility for spec-tral irradiance and radiance responsivity

calibrations using uniformsources (SIRCUS), has been de-veloped at NIST to measureboth spectral irradiance and ra-diance responsivity of detectorsagainst our transfer standarddetectors3,4 (see Fig. 2). Thelaboratory is divided into threeseparate facilities (a veritablethree-ring SIRCUS!), with onefacility designed for calibrationsin the 200 nm to 1100 nmwavelength range, a second forthe IR region (1 µm to 20 µm),and a new-generation cryo-genic radiometer (HACR 2) sit-uated in the middle. SIRCUSconsists of an integratingsphere irradiated by powerful,continuous-wave, tunable laser

sources. The output of this sphere is ahighly uniform monochromatic light fieldthat can readily calibrate a number of dif-ferent instruments. The lasers are tunableover wide spectral regions, allowing us toperform spectral radiance and irradiancecalibrations.5

Below we discuss applications and cali-brations of irradiance meters for the pho-tolithography industry and for NASA’s re-mote sensing program. We will also touchon applications in the fields of spectral ra-diance and radiance temperature.

Spectral irradianceThere are many applications in which irra-diance is the measured quantity. It is inter-esting to look briefly at two examples.

LithographyOptical lithography is the key technologydriver of the semiconductor industry: theindustry’s growth has been the direct re-sult of improved lithographic resolutionand overlay across increasingly largerwafer areas. Lithography is also a signifi-cant economic factor, currently represent-ing over one-third of the cost of manufac-turing the chip. Two crucial variables arethe tightening of photolithography expo-sure control and the extension to shorterwavelengths. The tightening of exposurecontrol requires improved measurement

Figure 2. Diagram of the SIRCUS facility.

1047-6938/01/02/0024/6-$0015.00 © Optical Society of America

RADIOMETRY

26 Optics & Photonics News ■ February 2001

of absolute dose and uniformity across thewafer. Since manufacturers must be able todetermine how much light is falling on awafer, they need instrumentation calibrat-ed for irradiance responsivity which al-lows them to relate the signal from theirdetector to the irradiance on the wafer. Wehave developed a program on SIRCUS tocalibrate 365 nm irradiance meters withstandard uncertainties of 5 x 10-3 or less.Programs have also been developed to cal-ibrate irradiance meters further into theUV using synchrotron sources available atNIST.

Most photodiodes are subject to dam-age when exposed to UV radiation. Thischanges the detector’s responsivity andcan lead to large uncertainties when meas-uring the dose of radiation delivered to thewafer plane. In addition to calibrating de-tectors for UV irradiance responsivity,many national measurement institutionslike NIST have begun intense programs tocharacterize UV detectors.

NIST Advanced Radiometer (NISTAR)Staff in the Optical Technology Divisionled the development of a space-flight in-strument, the Scripps-NISTAR,6 sched-uled to fly as part of NASA’s Triana Mis-sion (see Fig. 3). Plans are for the Trianaspacecraft to be launched aboard the spaceshuttle and placed in an orbit about L1(the Lagrange libration, or neutral gravitypoint between Earth and the Sun) approx-imately 1.5 million kilometers from Earth.From there, an imaging camera (the EarthPolychromatic Imaging Camera) andNISTAR will have a continuous, nearlyfull-disk, sunlit view of Earth.

NIST worked with Ball Aerospace andTechnology Corp. in Boulder, Colorado,7

to develop NISTAR to requirements of theScripps Institution of Oceanography in LaJolla, California, and to integrate it aboardthe Triana spacecraft at NASA’s GoddardSpace Flight Center in Greenbelt, Mary-

land. NISTAR includes three active–cavityelectrical-substitution radiometers for ab-solute irradiance measurements and onesilicon photodiode. Besides broadbandmeasurements of total Earth irradiance,filtered channels on NISTAR allow separa-tion of reflected solar from Earth-emittedinfrared radiation. Techniques for im-provement of signal-to-noise in active-cavity radiometers recently developed atNIST were essential in making feasible the measurements to be performed byNISTAR.

Spectral radianceThe color of the ocean reveals informationon the presence and concentration of phy-toplankton, sediments, and dissolved or-ganic chemicals. By studying the color ofthe light scattered from the oceans, opticalsensors can quantify the amount ofchlorophyll and other constituents invarying regions of the ocean. The opticalsensors can be deployed in or on theocean, from aircraft, or from satellites. Theprime ocean-color satellite instrument forNASA is the Sea-viewing, Wide Field-of-view Sensor (SeaWiFS)8, launched in Au-gust 1997. Recently a second ocean color-measuring instrument, the moderate reso-lution imaging spectrometer (MODIS)was successfully launched. Correct inter-pretation of the data acquired with thesesatellite-based instruments involves ac-counting for light scattered or absorbed bythe atmosphere and scattered from theocean.

Optical instruments in space tend todegrade, and therefore require a coordi-nated sensor calibration program to en-sure the accuracy of ocean-color measure-ments over extended time frames. TheSeaWiFS program involves establishmentof traceability to national standards for ra-diometric calibrations prior to launch andon-orbit calibration strategies that ofteninclude measurements of ground-truthingreference points in conjunction withmeasurements by field calibration systemsand airborne sensors. For example, theNational Oceanic and Atmospheric Ad-ministration (NOAA) is acquiring dailymeasurements of ocean color using spec-troradiometers on a tethered buoy, theMarine Optical Buoy (MOBY),9 at a fixedlocation in the ocean. SeaWiFS andMODIS periodically overfly this site andmake simultaneous measurements ofocean color in the area. The measurementsare compared, and the MOBY measure-

Figure 3. Picture and diagram of NISTAR.

RADIOMETRY

February 2001 ■ Optics & Photonics News 27

ments are used to validate the data ac-quired with SeaWiFS and MODIS.

The radiometric response of instru-ments used to study ocean color are gener-ally traceable to national standards main-tained at NIST through standard spectralirradiance lamps calibrated on the Facilityfor Automated Spectroradiometric Cali-brations, or FASCAL.

However, the link may involve manyseparate sets of measurement compar-isons, resulting in larger uncertainties. Toensure the accuracy of the measurements,the SeaWiFS Project and NIST developeda program that allows for direct compari-son of spectral radiance, provides fortraining, and establishes a means to trackthe performance of commercial sensorsused for in-situ measurements. NIST builtand characterized the SeaWiFS transfer ra-diometer (SXR) (see Fig. 4). The SXR is asix-channel filter radiometer used tomeasure or validate the spectral radianceof radiometric sources. It has been used tomeasure the radiance of the sphere sourcesused to calibrate OCTS, SeaWiFS, MOBY,and MODIS. Later, NIST built a field-de-ployable stable light source, the SeaWiFSquality monitor (SQM), that has beenused by the SeaWiFS project on several At-lantic Meridional Transect cruises. TheSQM has quantified, for the first time, theradiometric stability of optical sensorsused for the in-water and at-surface radio-metric measurements. NIST has alsoplayed a key role in five SeaWiFS Intercali-

bration Round-Robin Experiments (SIR-REXs), that allow for a direct comparisonof sources and detectors, as well as trainingin proper measurement technique. For thesupport of NOAA’s MOBY program, NISTdesigned field-deployable filter radiome-ters with interchangeable optics for spec-tral radiance and spectral irradiance meas-urements. This allows the MOBY team tomonitor the stability of both types of stan-dard sources at the field site in Snug Har-bor, Honolulu.

A number of additional instrumentshave been developed for NASA satellite

sensor programs. These instruments, de-signed to operate from the ultraviolet tothe short-wave infrared, are calibratedagainst lamp-based integrating spheresources with uncertainties ranging from1% to 5%. Calibrating the radiometers onSIRCUS will reduce the uncertainty intheir radiometric response to the 0.l% to0.5% level. This in turn will reduce the un-certainty in the validation of NASA cali-bration sources and field calibration facil-ities as well as the uncertainty in ocean-color measurements by satellite-basedsensors.

Figure 4a. Photograph of the SXR calibrating an integrating sphere that, in turn, calibrates SeaWiFS.

Figure 4b. Diagram of SXR.

RADIOMETRY

28 Optics & Photonics News ■ February 2001

Radiance temperatureIt is possible to determine an object's tem-perature from measurements of the emit-ted radiant flux, since all bodies that are

not at absolute zeroemit radiation. If theobject radiates like anideal blackbody, thenthe temperature is de-termined from the Ste-fan-Boltzmann equa-tion or Planck’s law, de-pending on the spec-tral coverage of the ra-diation thermometer.If the source is not ablackbody, then thetemperature can be as-signed as if it were, thedetermined quantity istermed the radiancetemperature. The spec-tral emittance of asample is the ratio ofthe actual spectral ra-diance to that from anideal blackbody at theobject temperature.Note that comparisonof the spectral radiancefor an ideal blackbodyat a known tempera-ture and the emittingobject at an unknowntemperature using a ra-diation thermometerdoes not yield the ob-ject's temperature un-less its spectral emit-tance is known fromancillary methods.

Thermodynamic temperatures are con-sistent with the laws of thermodynamics.To realize thermodynamic temperaturefrom measurements of the radiant fluxfrom a blackbody, the radiation ther-mometer must be a primary device, thatis, the equation of state cannot depend onunknown, temperature-dependent pa-rameters. An example is an absolute radia-tion thermometer traceable to electricaland dimensional standards: e.g., a filter ra-diometer calibrated for spectral irradianceresponsivity.

Implementation of primary thermom-etry over the range of required tempera-tures is often inconvenient or impractical.Instead, the International TemperatureScale of 1990 (ITS-90), which is based ondefined procedures, reference points, andstandard interpolating instruments, isused to realize the kelvin. On the ITS-90,the procedure for determining radiancetemperatures above the freezing point ofsilver (1234.93 K) is to use the ratio of thespectral radiances from blackbodysources. The Optical Technology Divisionat NIST (OTD) maintains the ITS-90above the silver freezing point using spec-tral radiance ratios; the standard black-body is a fixed-point blackbody sourcemade to operate at the unique tempera-ture of the equilibrium liquid to solidphase transition in pure gold, termed thegold-point (1337.33 K on ITS-90). The ra-tioing radiation thermometer, termed thephotoelectric pyrometer (PEP, data fromSIRCUS shown in Fig. 5), is a filter ra-diometer with a photomultiplier as the de-tector. The spectral coverage, which is cen-tered at 655.3 nm, is very narrow; recentmeasurements with SIRCUS gave a full-width half maximum of 1.1 nm and anout-of-band rejection between 10-6 and10-10. Establishment of the out-of-band re-jection for the PEP, an ideal application forSIRCUS, is critical for ITS-90 because therelative spectral shape of the blackbody ra-diance depends strongly on temperature.

For ITS-90, the component of uncer-tainty that accounts for the uncertainty inthe temperature of the fixed-point black-body is proportional to T2. At high tem-peratures, > 2000 K, thermodynamic tem-peratures are being realized using calibrat-ed filter radiometers to research alterna-tives to ITS-90 that result in reduced un-certainties. NIST has used six filter ra-diometers (see Fig. 6), with center wave-lengths between 350 nm and 950 nm, tocompare the temperature of a blackbody

Figure 5. Graphs of responsivity of PEP taken onSIRCUS showing narrow band nature of the facili-ty and the large dynamic range not achievable withtypical lamp/monochromator facilities.

Figure 6. Filter radiometers spectral responsivity (above) and agreementbetween filter radiometers and radiance temperature from pyrometry (below).

4. K. R. Lykke, P. -S Shaw, L.M. Hansen, G. P. Eppel-dauer,“Development of a monochromatic, uniformsource facility for calibration of radiance and irradi-ance detectors from 0.2 micron to 12 micron,”Metrologia 1998, 35, 479-84.

5. B.C. Johnson, S.W. Brown, G. P. Eppeldauer, K.R.Lykke,“System-level calibration of a transfer ra-diometer used to validate EOS radiance scales,” In-ternational Journal of Remote Sensing 2000, accept-ed.

6. J.P. Rice, S.R. Lorentz,T.M. Jung,“The next generationof active cavity radiometers for space-based remotesensing,” 10th Conference on Atmospheric Radia-tion, Madison,Wisconsin, 1999;American Meteoro-logical Society.

7. Identification of commercial equipment to specifyadequately an experimental problem does not im-ply recommendation or endorsement by NIST, nordoes it imply that the equipment identified is neces-sarily the best available for the purpose.

8. R.A. Barnes,A.W Holmes,“Overview of the Sea-WiFS ocean sensor,” SPIE Proc. 1993, 224-32.

9. D.K. Clark, H.R. Gordon, K.K.Voss,Y. Ge,W. Bro-kenow, C.J.Trees, “Validation of atmospheric cor-rection over the oceans,” Geophys. Res. 1997, 102,17209-17.

10. H.W.Yoon, C. E. Gibson,“Determination of radiancetemperatures using detectors calibrated for ab-solute spectral power response,” TEMPMEKO Con-ference '99; 737-42.

Steven W. Brown, B. Carol Johnson and Keith R. Lykkework in the Optical Technology Divison of NIST. Theycan be reached by e-mail at swbrown@nist.gov, cjohn-son@nist.gov, lykke@nist.gov.

RADIOMETRY

February 2001 ■ Optics & Photonics News 29

determined from the radiance flux meas-ured by the filter radiometers to the ITS-90 temperature assigned using the PEP.The agreement was good, better than 0.5 K.10 To date, the filter radiometers werecalibrated for spectral flux responsivity us-ing the visible and ultraviolet SCF facili-ties, and the areas of their defining aper-tures were determined from the NIST ab-solute area facility. In the near future, SIR-CUS will be used to determine the spectralirradiance responsivity. The goal is to cali-brate filter radiometers centered at about400 nm for spectral irradiance responsivi-ty with a relative expanded uncertainty of0.05 % (k = 2). If this is achieved, the un-certainty component in radiance tempera-ture arising from the uncertainty in radi-ance would be 100mK (k = 2) at 3000 K.At the gold point, this uncertainty compo-nent would be about 40mK (k = 2), aboutfive times smaller than the combined ex-panded uncertainty of the previous NISTradiometric measurement. This work,which is being carried out independentlyat several national measurement institu-

tions, will have a direct impact on devel-opment of the next ITS scale.

ConclusionsWe have described various key aspects ofradiometry and how it is applied in prac-tice. As technical applications becomemore demanding, new ultra-precise meas-urement techniques will need to be devel-oped. SIRCUS was created in response tothe need for good signal-to-noise, accurate"filling" of the radiometers’ entrancepupil, narrow band, and accurate wave-length calibration. This type of endeavorconstitutes a major goal of national meas-urement institutions like NIST.

References1. R. McCluney, Introduction to Radiometry and Photome-

try; Artech House, Inc., Norwood, MA, 1994.2. T.R. Gentile, J.M. Houston, J.E. Hardis, C.L. Cromer,

A.C. Parr,“National Institute of Standards and Tech-nology high-accuracy cryogenic radiometer,” Ap-plied Optics, 35, 1056-68, 1996.

3. S.W. Brown, G.P. Eppeldauer, K.R. Lykke, “NIST Fa-cility for spectral irradiance and radiance responsiv-ity calibrations with uniform sources,” Metrologia,37, 579-82, 2000.

The Chameleon Tilt Mountdoes more for less

TM

www.paradigmoptics.com

Paradigm

Optics

Phone: 509-332-2221

Fax: 509-334-0816

The Chameleon Tilt Mount system is based on a

simple “key ” concept. The Chameleon Mount

can be changed from a lense or mirror mount

to a prism table in seconds. All of Paradigm’s

products are based on flexibility and versatility.

TM

TM

The Chameleon MountTM

Patent Pending