The absolute sky-scanning radiometer ~ASR! intro-duced in this paper is conceived as a self-calibratingreference standard instrument for measurements ofatmospheric long-wave radiation. The instrumentuses a highly sensitive cavity-designed pyroelectric de-tector that is calibrated with a reference blackbodyradiation source during measurements. Absolute cal-ibration of the ASR is related to an accurate knowledgeof the blackbody’s inside-wall emittance and tempera-ture, which is traced to absolute temperature stan-dards. The instrument uses no window, and the onlyoptical component in the beam path is a highly reflect-ing gold mirror that directs the narrow field-of-viewbeam into any direction in the sky and into the cali-bration blackbody radiation source. Sky-radiancemeasurements are compared with radiance measure-ments in the well-characterized reference blackbodysource, and the value of hemispherical downward long-wave irradiance is determined by Gaussian quadra-
R. Philipona [email protected]! is with the Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center,Dorfstrasse 33, CH-7260 Davos Dorf, Switzerland.
Received 5 July 2000; revised manuscript received 2 January2001.
2376 APPLIED OPTICS y Vol. 40, No. 15 y 20 May 2001
ture integration over 32 hemispherically distributedsky-radiance measurements.
Atmospheric long-wave downward radiation isdirectly related to the greenhouse effect. This de-pendence and its small year-to-year variation makelong-wave downward radiation one of the most prom-ising parameters for monitoring climate change.The uncertainty levels of measurements of atmo-spheric long-wave radiation have been considerablyreduced in recent years, which now makes this ele-ment even more attractive for climate research.1–3
The pyrgeometer, the most prominent instrument forthe measurement of longwave irradiance in climato-logical networks, has been improved.4,5 The newlydesigned instruments that are now on the marketshow promising advantages with respect to domeheating and spectral transmission, resulting in morestable and reliable radiometers.6,7 Long-wave in-struments are calibrated in blackbody radiationsources, and an international pyrgeometer calibra-tion round-robin experiment8 for intercomparison ofblackbody radiation sources has shown good agree-ment among many different types of blackbody cali-bration apparatus. The relative measurementuncertainty between state-of-the-art pyrgeometersthat are properly ventilated and shaded and haveuniform blackbody calibration is now down to 2–3 Wm22. However, pyrgeometer measurements andtheir present calibration standard provide no infor-mation about uncertainties on the absolute values of
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measurements of atmospheric long-wave radiation.In contrast, solar radiation measurements have beenmade with absolute pyrheliometers since the early1960’s, and a group of seven absolute cavity radiom-eters constitute the World Radiometric Reference formeasurements of short-wave irradiance. In suchdetector-based absolute instruments, solar irradi-ance is substituted for electrical power, and accuratemeasurements of the latter relates solar short-waveirradiance to the absolute standards of metrologicalunits of the international system.
The absolute sky-scanning radiometer relates mea-surements of long-wave radiation to absolute stan-dards of internationally accepted metrological units.Although high-intensity solar irradiance is substitutedby electrical power that is measured in a detector-based absolute instrument, the low intensity of atmo-spheric long-wave sky radiance is related to absolutetemperature, measured in a well-defined and well-characterized blackbody radiation source. The newASR instrument described in this paper can possiblyserve as a reference standard and its intended use is tocalibrate pyrgeometers by comparison during fieldcampaigns. In Section 2 deficiencies of pyrgeometermeasurements and possible error sources in connec-tion with blackbody calibration sources are described.Details about the ASR, in particular the detector headand the reference blackbody source, are in Section 3.In Section 4 the principle and uncertainties involvedwith ASR measurements are presented. The long-wave sky-radiance distribution of clear skies is dis-cussed in Section 5; the ASR test and fieldmeasurements are presented in Section 6.
2. Pyrgeometer Calibration and MeasurementDeficiencies
In atmospheric and climate research as well as formeteorological atmospheric long-wave measure-ments, the Eppley precision infrared radiometer~PIR! pyrgeometer9 is the most frequently used in-trument. Major improvements in the past 25ears, such as the change from a KRS-5 to a siliconome, the dome-temperature measurement and itsppropriate evaluation formula,10 as well as the im-
proved dome-temperature measurement and ex-tended pyrgeometer formula5 made the PIR, ifcorrectly deployed, a stable and reliable instrument.Thermal dome effects, which were the largest sourceof errors, have been taken care of with the new dome-temperature measurement. Encouraging resultshave been achieved in a pyrgeometer calibrationround-robin experiment8 that showed that differentlackbody calibration apparatuses would lead to theame results. Individual calibrations of five PIR’s atix different calibration laboratorie showed respon-ivity values C that matched to between 1% and 2%.ence, for worldwide homogeneity of calibration and
elative measurements between individual instru-ents, blackbody calibration sources seem to be ad-
quate.However, with respect to absolute measurements,
pectral and directional dome errors involved with
blackbody pyrgeometer calibration have to be ad-dressed carefully. Planck spectral curves fromblackbody radiation sources ~see Fig. 1! are quite dif-erent from spectral curves of atmospheric long-waveadiation. If the spectral sensitivity of a detectorhat measures atmospheric radiation were flat overhe entire spectrum, say from approximately 2 to 100m, there would be no harm in calibrating the detec-
or with Planck curves because all wavelengthsould be treated equally. But PIR dome transmis-
ion is low ~Fig. 1, right scale!, and transmissionurves are not at all flat and can be quite differentrom one instrument to another, which means thatyrgeometers do not treat all wavelengths with equalensitivity. Depending on the calibration tempera-ure and the atmospheric long-wave spectrum, whichan vary significantly for different temperatures andater vapor amounts, certain wavelengths can beverestimated or underestimated during measure-ents and hence create uncertainties. Inhomogene-
ties of the interference filter deposited on the insideall could cause directional errors. Also, in contrast
o atmospheric long-wave radiation, which has a min-mum emission at the zenith and maximum at theorizon, blackbody radiation sources emit uniformambertian long-wave radiance over all zenith an-les. Uncertainties are therefore to be expectedrom directional effects as well as from errors that areue to the cosine law.Hence, although the thermal effects of pyrgeometer
omes have been overcome by adequate instrumentharacterization and accurate dome-temperatureeasurements, additional uncertainties, connected
o spectral and directional dome transmission thatranslates to receiver sensitivity, have to be ad-ressed with respect to measurements of absoluteong-wave irradiance. The ASR is based on a com-letely different design and measurement principlend omits all errors related to the pyrgeometer mea-urements described above.
Fig. 1. Inhomogeneous PIR dome transmission ~right scale! andsignificant differences between pyrgeometer calibration Planckcurves and real atmospheric irradiance spectra, which cause con-siderable uncertainties in long-wave downward irradiance mea-surements.
20 May 2001 y Vol. 40, No. 15 y APPLIED OPTICS 2377
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3. Absolute Sky-Scanning Radiometer
A. Schematic of the Absolute Sky-Scanning Radiometer
The ASR consists of a detector head that sits on avertical standing reference blackbody source ~see Fig.!. A rotation stage between the two elements al-ows the detector head to rotate azimuthally. Theighly sensitive cavity-shaped pyroelectric detectortype RkP-575! and a suitable chopper system sit in aetallic box ~see Fig. 3!. To the right of this box aetallic cylinder holds a highly reflecting gold mirror
hat deviates the viewing beam by 90 deg, and anntrance diaphragm built into the cylinder wall lim-ts the field-of-view angle to 6 deg. The cylinder it-
Fig. 2. ASR with reference blackbody ~lower part! and rotatabledetector head ~upper part!.
Fig. 3. Schematic of the ASR with the pyroelectric detector headand the reference blackbody calibration source. Two PIR travel-ing standard pyrgeometers are logged on the same data acquisi-tion.
378 APPLIED OPTICS y Vol. 40, No. 15 y 20 May 2001
elf is carried by a vertically mounted rotation stagehat allows the mirror to rotate around its horizontalxis and direct the beam to any elevation into the skyr down into the blackbody for calibration. There iso window and the 45-deg reflecting gold mirror ishe only optical component in the beam path from theetector surface to the bottom of the reference black-ody or into the sky. Since the narrow field-of-vieweam is subject to diffraction at the entrance dia-hragm, a cylindrical collar fixed to the blackbodyeaches up close to the entrance diaphragm. Theollar has a gold plating with low emittance on itsnside wall and is held as close as possible to thelackbody temperature to prevent stray-light errorsuring calibration.The detector head containing the pyroelectric de-
ector, the chopper, and the gold mirror is well iso-ated but has no thermal control and simply floats atmbient temperature. The chopper temperature iseasured with two thermistors sitting inside the
ront and the back walls of the chopper housing closeo the beam aperture. The detector temperature isess important and not measured but is assumed toe approximately equal to the chopper temperature,hereas the gold mirror has its own thermistor.he blackbody temperature is controlled by a circu-
ator and measured by six thermistors. All ther-istors used in the ASR have been calibrated with an
nstrument that is traced to the absolute temperaturetandard of the Swiss Federal Office of Metrology. AC controls the two stepper motors of the azimuthnd elevation rotation stages and monitors the mea-urement during sky-scanning by a data logger thatecords scanner position, detector signal, all temper-tures, and the measurements of two traveling stan-ard PIR pyrgeometers.
B. Pyroelectric Detector
The ideal detector for measuring long-wave radiationshould have a flat response of from 2 to ;100 mm.Also, to measure long-wave radiance at narrow view-ing angles, the detector needs to be extremely sensi-tive. The detector used in the ASR is a commerciallyavailable high-sensitivity cavity-shaped pyroelectricdetector of type RkP-575 RF enhanced. The cavitydesign in Fig. 4 enhances the absorptance of the de-tector. Gold-black coatings often used on pyroelec-tric absorber surfaces have rather flat absorptance.11
The relative response of the RkP-575 RF is shown inFig. 5. For our purpose the most critical part is from4 to ;15 mm, which includes the atmospheric win-
ow, and, in this range, we have a steady decrease of
Fig. 4. Cavity-designed pyroelectric detector.
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;4% in sensitivity. With low water in the atmo-sphere a second window opens at around 20 mm.
bove 30 mm where the relative response drops from0% to ;70% the measured signal from the atmo-phere is similar to the calibration Planck curve.The pyroelectric detector has been reliable. Reit-
rative calibration accounts for minor temperaturend humidity dependence. A major concern, how-ver, is noise that interacts strongly with pyroelectricevices. Even though the path from the entranceiaphragm to the detector is ;400 mm long, the facthat no window is in front of the detector makesccurate measurements with wind speeds above 3ys impossible.
C. Gold Mirror
The only optical component used on the ASR is agold-coated, heavy copper mirror positioned at 45 degin the beam path. Gold has a high reflectance12 of;98% and is spectrally flat from 1 mm to at least 50
m. A 45-deg reflection changes reflectivity witholarization, but this is not a concern for our appli-ation because thermal emission is unpolarized. Al-hough the mirror has a high reflectance, itsemperature is continually monitored and staysithin ;2 deg of the chopper reference temperature.
D. Blackbody Radiation Source
The blackbody radiation source built especially forthe ASR is shown in Fig. 6 and consists of an alumi-num cylinder with a helical groove that serves as aconduit, a convex-shaped bottom plate, and a coverplate with an entrance hole. The cooling fluid thatenters the bottom plate rises in the helical conduitand exits at the top. Six calibrated thermistors arepositioned in small holes in the inside wall of thecavity, which is painted with a high-emittance blackpaint. The cavity is well isolated and is designed tocover the temperature range from 50 to 250 °C.
Blackbody sources can have different shapes. Be-sides an isothermal inside wall with high gray emit-tance, the most important characteristic of ablackbody radiation source is its length over diameteraspect ratio. The thermal radiation characteristicsof a cylindrical cavity13 can be determined by an anal-ysis of the radiant flux balance of infinitesimal ele-ments of the whole internal surface. The solution ofappropriate integral equations provides the distribu-
Fig. 5. Spectral sensitivity curve of the RkP-575 RF pyroelectricdetector.
tion of the apparent emittance εa along the insidesurface as well as the total emittance εt at the en-trance hole of the cavity.14 The radiative propertiesare determined by the aspect ratio LyD ~lengthydi-ameter!, which is 4 in our blackbody, and its openingratio ryR ~entrance radiusycavity radius!, which is0.5 ~see also Fig. 6!. Given this geometry and anemittance of ε 5 0.97 for the black paint, the calcu-lated apparent emittance along the walls of the cy-lindrical cavity are presented in Fig. 7 for an opencylinder and for a cylinder with cover plate and en-trance hole. The difference along the cylinder wallcan be clearly seen with a much higher apparentemittance at any position and a minimum value ofεa 5 0.9973 with the cover plate mounted. Also,apparent emittance values at the bottom are higherwith the cover plate ~εa 5 0.9995! compared with theopen cylinder ~εa 5 0.999!.
Fig. 7. Calculated local emittance εa at the inside wall of a cylin-rical cavity with ratio LyD 5 4 for ~a! the open cylinder and ~b! theartly closed cylinder with a ratio of ryR 5 0.5.
20 May 2001 y Vol. 40, No. 15 y APPLIED OPTICS 2379
r
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Table 1. Gaussian Quadrature Integration Angles and Weighting
2
During calibration the narrow field-of-view beammeasures radiance from the bottom plate of theblackbody source, and the apparent emittance of thissurface is therefore the most important part in ourconsideration. Taking into account that a smallpart of the narrow field-of-view beam is diffractedand hence measures other surfaces in the cavity, amean value of total emittance of εt 5 0.999 for thiseference cavity has been calculated.
4. Irradiance Measurement with the AbsoluteSky-Scanning Radiometer
A. Calibration and Measurement Principle
The ASR is designed to measure the absolute valuesof atmospheric long-wave radiation, and therefore nocompromises have been made with regard to errorsfrom optical components in the beam path. How-ever, no windows implies that, to exclude short-waveradiation, the measurements can be made only dur-ing nighttime. Also, since the instrument scans thesky, measurements should be made during homoge-neous clear skies. Before scanning, the ASR needsto reach ambient temperature. Inasmuch as thechopper temperature is used as a reference point be-tween calibration and measurement, the blackbodytemperature is first set equal to the chopper temper-ature to find the zero point of the pyroelectric detec-tor. Once the zero is determined, the mirror can berotated up to scan the sky. The sensitive pyroelec-tric detector emits thermal radiation into the coldsky, and its negative signal is proportional to thedifference between the chopper temperature and theapparent sky temperature. Between scans thebeam points into the blackbody, which is set to tem-peratures that are chosen to cover the range of theapparent sky temperature. With the zero point andtwo or three different blackbody temperatures, thepyroelectric detector can be calibrated.
The 6-deg field-of-view beam measures at the bot-tom of the blackbody the same amount of solid angleas in the sky. Hence the viewing angle is not rele-vant. During calibration the narrow field-of-viewbeam measures a certain radiance Lc in the referencelackbody. We assume that the blackbody entrances a Lambertian radiator, and we can calculate theadiant exitance Mc or the irradiance Ec at the black-
body entrance by applying the simple law
Mc 5 Ec 5 pLc. (1)
Also, the temperature and the emittance of the black-body are accurately known and allow us to calculatethe irradiance Ec by applying the Stefan–Boltzmannaw:
Ec 5 εtsTc4, (2)
where εt 5 0.999 is the calculated total emittance ofhe blackbody source, s 5 5.67051 3 1028 W m22 K24
is the Stefan–Boltzmann constant, and Tc is the ab-olute temperature of the blackbody in degreeselvin. Hence by use of the calibration procedure
380 APPLIED OPTICS y Vol. 40, No. 15 y 20 May 2001
e attribute a certain irradiance, measured by a ra-iance value, to a certain voltage measured at theyroelectric detector.The air temperature inside the blackbody is as-
umed to be close to blackbody temperature, but fromhe blackbody entrance to the chopper the viewingeam traverses air at ambient temperature. Calcu-ations of the radiative-transfer model show emissionf this air into the field-of-view beam, which altershe blackbody emission of the order of 1% for mid-atitude summer conditions and a temperature dif-erence of 20 °C. This emission depends onemperature and humidity and needs to be corrected.
B. Gaussian Quadrature Integration
By rotating the mirror to the sky we can measure theradiance in the sky at a given zenith angle. But thesky is not a Lambertian emitter, and we thereforehave to scan the sky. The distribution of the skyradiance is rather continuous, varying from a mini-mum at the zenith to a maximum at the horizon.We therefore do not need to measure a large numberof points but can use a Gaussian quadrature integra-tion. In the usual process of a scan we measureradiance at four elevations at predetermined Gauss-ian quadrature angles and at eight azimuthal direc-tions. With a variation from n 5 3 to n 5 4 angles,
aussian quadrature integration improves the resulty only 0.025%; but four angles have been chosen foretter detection of possible inhomogeneities of theky. Gaussian quadrature integration is describedy
*0
1
mf ~m!dm 5 (i51
n
wif ~mi!, (3)
where m 5 cos q and f ~m! is the radiance measured atgiven zenith angle. For n 5 4 the angles mi, the
enith angles qi, and the weighting coefficients wi areiven in Table 1.At the four given zenith angles qi the radiance Li is
determined and multiplied with the respectiveweighting coefficient wi. With the sum of theweighting coefficients ¥ wi 5 0.5, the irradiance Eadeduced from measurements of one azimuthal direc-tion is determined by
Table 2. Uncertainties due to Errors on ASR Absolute Long-Wave Irradiance Measurements
and one can calculate the final long-wave downwardsky irradiance E determined with a full scan by av-eraging the eight azimuthal directions.
C. Sky Scan
To allow the detector to adjust to the small radiancedifferences, each measuring point takes 30 s. Be-cause of the 32 points for the integration, the zenith,and the horizon as well as calibration measurementsincluded in the scan, the procedure lasted for ;24
in. Long-wave radiation does not vary much dur-ng nighttime clear-sky situations, and the final irra-iance measurement is considered as an integratedalue over the scan time.
D. Uncertainty on Absolute Measurements of Long-WaveRadiance
The basic principle of the ASR measurement is a com-parison of sky radiance with the radiance measured ina well-determined radiation source. Thus the uncer-tainty of the absolute value of the measurement pri-marily depends on the error of the temperaturemeasurement in the blackbody. Uncertainties arefurther involved with the temperature reading of thechopper, which serves as a reference between calibra-tion and measurement. Blackbody emittance alsohas an uncertainty. Although the sensitivity of thepyroelectric detector is almost flat, a minor mismatchthat is due to spectral dependence could still producean error. Stray light caused by beam diffraction iswell taken care of with the collar on top of the black-body and is compensated in the sky with higher andlower intensities on one side versus the other side ofthe field-of-view beam. Yet a small uncertainty isstill accounted for. Ambient air between the black-body entrance and chopper is corrected but a smallerror is still accounted for. Uncertainty values deter-mined for the ASR are shown in Table 2, and a com-bined standard uncertainty of 0.98 W m22 ~root sum ofsquares! is calculated on an absolute value of 260 Wm22 of long-wave irradiance measured. Summing alluncertainties, as a worst case, results in 2.15 W m22.
5. Long-Wave Sky-Radiance Distribution
The long-wave sky-radiance distribution is easilymeasured with the ASR as shown by measurementsmade on the roof platform of the Observatory at Da-
Uncertainties
TypeUncertainty of temperature of blackbody ~at 260 K!Uncertainty of chopper temperature during calibrationUncertainty of blackbody emittanceMismatch due to pyroelectric detector sensitivityField-of-view beam stray lightAmbient air passage
Combined standard uncertainty ~rss!Maximum uncertainty ~worst case!
vos in July 1998 ~see Fig. 8!. For the standard four-aussian-angle scan, a second 5° step scan waserformed going from southwest to northeast. Forresentation on the graphs, we used long-wave netrradiance assuming Lambertian emittance into allirections. The net irradiance over the hemisphereould be equal to two times the sum of the irradi-nces at the four Gaussian angles multiplied by theespective weighting coefficients. The largest netutgoing irradiance is measured at the zenith,hereas at the horizon the temperature of the atmo-
phere is equal to ambient temperature and no radi-tion exchange occurs. Over the eight azimuthalirections, signals are somewhat uniform, althoughhey were measured over a period of ;20 min ~see
Fig. 9!. At the largest zenith angle of 82°, the north-east and southwest directions show a larger net irra-diance, because at Davos only these two directionspoint toward the sky at this low elevation. In allother directions the horizon is above 8°, and becausemountains are warmer than the clear sky, the netoutgoing irradiance is strongly reduced in these di-rections. For accurate field calibration measure-ments a clear horizon is mandatory.
6. Test and Field Measurements of the AbsoluteSky-Scanning Radiometer
Test measurements with the ASR were taken in thelaboratory by measuring the irradiance of a well-characterized blackbody that was specifically madefor pyrgeometer calibrations. These tests actually
ErrorMeasured Unit
~W m22!
6 0.15 K 6 0.66 0.05° K 6 0.20.999 6 0.0005 6 0.14~100 W m22! 6 0.5% 6 0.5~100 W m22! 6 0.5% 6 0.5~100 W m22! 6 0.5% 6 0.2
6 0.986 2.15
Fig. 8. Long-wave net irradiance distribution at Davos measuredby a four-Gaussian-angle scan ~thick arrows! and a 5° step-anglescan ~thin arrows! in the southwest to northeast direction.
20 May 2001 y Vol. 40, No. 15 y APPLIED OPTICS 2381
CpPKAdwtoam
2
revealed the stray-light problem and led to the im-provement with the gold collar. Field measure-ments were first made on the roof of the Observatoryat Davos by comparing measured irradiances withmeasurements of the Physikalisch-MeteorologischesObservatorium Davos, World Radiation Center~PMOD WRC! modified pyrgeometers.5 Furthermeasurements were then taken at three stations ofthe Alpine Surface Radiation Budget network,2 andmeasurements at the Payerne station allowed ASRabsolute measurements to be compared with calcula-tions of the MODTRAN radiative-transfer model for thefirst time to our knowledge.15
In fall 1999 the Baseline Surface Radiation Net-work community in collaboration with scientists ofthe Atmospheric Radiation Measurements programconducted the first International Pyrgeometer andAbsolute Sky-scanning Radiometer Comparison~IPASRC-I! at the U.S. Southern Great Plains
loud and Radiation Testbed ~CART! site. Fifteenyrgeometers including eight standard EppleyIR’s, six PMOD WRC modified PIR’s, and oneipp&Zonen CG4 pyrgeometer were compared withSR measurements, with Atmospheric Emitted Ra-iance Interferometer ~AERI! measurements, andith radiative-transfer model calculations. Night-
ime intercomparisons have shown average pyrge-meter long-wave irradiance measurements, AERI,nd model calculations to agree to within 1–2 W22 with ASR absolute measurements.16
7. Conclusions
The world radiometric reference for short-wave radi-ation is inadequate as a reference for long-wave ra-diation owing to large intensity differences betweenshort-wave irradiance and long-wave radiance. Wehave introduced an absolute instrument for long-wave irradiance measurements that is traced to ab-solute temperature standards. The absolute sky-scanning radiometer uses a highly sensitive spectrallyflat pyroelectric detector to compare sky radiancemeasured at a narrow viewing angle with radiancemeasured from the bottom of a blackbody radiationsource. Hence an absolute temperature measure-
Fig. 9. Clear-sky hemispherical long-wave net irradiance distri-bution at Davos measured at four Gaussian angles and eight azi-muthal directions.
382 APPLIED OPTICS y Vol. 40, No. 15 y 20 May 2001
ment in the blackbody is transferred to an absoluteradiance measurement in the sky. The ASR scansthe sky and uses a Gaussian quadrature integrationto calculate atmospheric long-wave irradiance. Anabsolute uncertainty of 0.98 W m22 is determined forASR measurements of long-wave irradiance. Re-sults at the IPASRC-I pyrgeometer intercomparisonare satisfactory. The ASR has been used for fieldcalibration of pyrgeometers and has been suggestedto become part of a world standard group for mea-surements of long-wave radiation. The low level ofuncertainty now reached with pyrgeometer measure-ments make long-wave irradiance one of the mostpromising parameters for climate-change researchthat is greenhouse related.
The author thanks colleagues of the observatory fortechnical support and many encouraging and helpfuldiscussions. This research received great supportfrom A. Ohmura of the Institute of Climate Researchof ETH-Zurich. Financial support was granted bythe Swiss Federal Institute of Technology in Zurich,contract 3208.
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