Carbon arc solar simulator

Robert A. Olson and Jack H. Parker

Measurements of the spatial, spectral, and temporal characteristics of the beam irradiance of a carbon arcsolar simulator are reported. Pyroelectric radiometer measurements of total irradiance and spectroradio-meter measurements of spectral irradiance are presented. The solar simulator spectral irradiance is com-pared with the ASTM standard AM 1.5 global solar spectral irradiance over a wavelength region of 300-2500nm. The suitability of the solar simulator for laser receiver testing is discussed. Key words: Carbon arc,solar simulator, spectral irradiance, laser warning receiver.

I. Introduction

Laser warning and laser optical intelligence receiv-ers capable of detecting laser radiation over a spectralrange from the ultraviolet to 10.6 Am are being devel-oped by the Air Force as well as other Department ofDefense agencies. These receivers and their relatedtechnologies frequently undergo extensive laboratorytesting. Laboratory testing permits careful paramet-ric characterization of performance in a well-con-trolled and often benign environment. Issues of inter-est include system sensitivity, optical dynamic range,field of regard, and discrimination algorithm effective-ness. While these tests serve to identify weakness intechnology implementation, actual system perfor-mance is often dependent on the operating environ-ment in actual field conditions. For this reason, ef-forts are ongoing in the Air Force Electro-OpticsLaboratory at Wright Patterson Air Force Base tosimulate aspects of the environment to which receiversmight be exposed in field operations. In particular,emphasis is being placed on the solar radiation back-ground in which the receiver must operate.

Laser receivers can be broadly classified in terms ofpulsed and cw detection schemes. These broad cate-gories can be further divided into time domain, spec-tral, and coherence detection techniques. In time do-main detection, receivers depend on the observance ofultrahigh speed pulses to isolate lasers from natural orfrom other man-made sources. Spectral discrimina-tion schemes employ narrowband filters or high reso-lution spectrometry to sort narrow laser emission lines

Robert Olson is with Schwartz Electro-Optics, Inc., 3404 N. Or-ange Blossom Trail, Orlando, Florida 32804, and Jack Parker is withWright Research & Development Center, Air Force Avionics Lab-oratory, Wright-Patterson Air Force Base, Ohio 45433-6543.

Received 26 March 1990.

from the background. Finally, coherence detectiontechniques utilize forms of interferometry to distin-guish between coherent and noncoherent sources. So-lar background radiation can, in specific conditions,affect the performance of each of the three methods.Solar glint, originating from specular reflections offsurfaces under high rates of angular translation, canform light pulses tens of microseconds in length, whichborders on laser pulse duration times. Solar glints,therefore, can give artificial source indications, i.e.,false alarms, in receivers utilizing time domain dis-crimination. In addition, time domain, spectral, andcoherence detection can all suffer sensitivity lossesfrom dc background, detector bias, and elevated noiselevels. Because of the nature of receiver discrimina-tion techniques, selection of a light source for solarsimulation must be judicious. Care must be taken toavoid source characteristics which, during testing, in-duce unnatural responses in the receiver. Most con-ventional solar simulators incorporate xenon arc lampsin their design. These sources contain narrow emis-sion lines which some spectral detection schemes can-not tolerate. Guard-band receivers, for example, areimplemented with two narrowband spectral filterchannels in adjacent regions of the spectrum. Onebandpass filter is centered about a laser line of interest.The other (guard-band) filter samples background ra-diation near the laser bandpass. Scaling and subtrac-tion of the guard-band video signal from the laserchannel video yields the laser signal in a much reducednoise background. The resulting signal-to-noise en-hancement permits a lower electronic threshold fordetection than a simple narrow-laser-line-filter receiv-er scheme. The guard-band algorithm breaks down,however, if the energy balance in the two adjacentchannels is artificially upset by xenon line spectra.Two outcomes are possible. If strong emission linesappear in the guard-band channel, an artificial signal-to-noise condition can set the threshold out of reach for

1290 APPLIED OPTICS / Vol. 30, No. 10 / 1 April 1991

any laser-induced signal, effectively reducing the sen-sitivity of the receiver. If, on the other hand, theemission lines fall into the laser channel bandpass,they are interpreted as laser signals, and thereforeresult in false alarms.

Tungsten lamps are another traditional sourcechoice for solar simulation. These sources poorlymatch solar blackbody color temperature and flux lev-els necessary for proper evaluation of receiver sensitiv-ity to background levels in selected spectral regions.The results described in this paper suggest that solarsimulation using carbon arc sources provides excellentspectral matching with fewer unnatural attributesthan previously achieved by other techniques.

II. System Description

A schematic diagram of the optical train for thecarbon arc solar simulator is shown in Fig. 1. A recon-ditioned carbon arc lamp (Peerless Magnarc), previ-ously used for motion picture projection, is the sourcefor the solar simulator. The arc is formed in a 6-mmgap between a 7-mm diam negative carbon electrodeand an 8-mm diam positive carbon electrode. The arcis sustained by a 35-V power supply providing 70 A ofcurrent or -2.4 kW of electrical power. The incandes-cent crater in the positive electrode is imaged on aground Pyrex diffuser by an ellipsoidal mirror. Thearc is at one ellipsoid focus and the image of the arc is atthe other. In these conditions, there is coma but nospherical aberration.

A 3.8-cm diam arc image is formed on the diffuser,which is used to improve the uniformity of the beamirradiance. A perfect diffuser would scatter the trans-mitted light into 2vr sr, resulting in low on-axis irradi-ance. To avoid this, ground glass (Pyrex), which is arelatively poor diffuser, is used. This results in -75%of the incident radiation being transmitted throughthe diffuser, strongly peaked in the forward direction.'A steel aperture plate is used to limit the spot size to adiameter of 2 cm. The 2-cm diam spot taken with the22.86-cm focal length of the collimating objective re-sults in a nominal beam divergence of 0 = dif = 87mrad or 5.010.

The radiation transmitted through the diffuser iscollimated by an aspheric glass condenser lens. Thislow f No. lens is ideally suited to efficiently collect theradiation from the 2-cm diam spot. Also, it can with-stand temperatures up to 177°C, which is a desirablecharacteristic in view of the high thermal loading in thevicinity of the diffuser. Initially, consistent with thebroadband nature of the solar simulator, an achromatwas used to collimate the radiation from the diffuser.However, the adhesive bonding the elements togetherdiscolored and became more absorptive with time untilthe lens failed catastrophically.

The collimated region between the aspheric andachromatic lenses is useful for inserting filters in theoptical train. For example, interference filters couldbe used to simulate atmospheric absorption bands byattenuating radiation over the appropriate band-widths. Placing the filters in a collimated beam avoids

CARBONELECTRODES

DIFFUSER STEEL APERTUREAPE RT U RE

/- CNDENASPHERIC ACHROMATCONDENSER

ELLIPSOIDAL COLLIMATINGMIRROR OBJECTIVE

Fig. 1. Carbon arc solar simulator optical layout.

the shift of the filter passband to a shorter wavelengthwhich occurs for off-normal incidence.

The achromat reimages the (filtered) diffuser imageat an aperture which serves to block stray light. Theradiation is then recollimated by a two-element con-denser having a 22.86-cm focal length. This results ina 12.7-cm diam beam having a divergence of -5°.

Ill. Experimental Results

The solar simulator output was characterized bymeasuring the spatial, spectral, and temporal proper-ties of the beam irradiance. Total irradiance measure-ments were made with a Molectron model PR 200pyroelectric radiometer. Spectral irradiance mea-surements were made with a computer-controlledspectroradiometer, equipped with a 20.3-cm integrat-ing sphere input, manufactured by Geophysical Envi-ronmental Research Corp. This spectroradiometer isparticularly appropriate for solar radiation measure-ments because of its broad spectral coverage (300-3000nm) and its good spectral resolution (2 nm from 300 to1000 nm, 4 nm from 1000 to 3000 nm). The spectrora-diometer was calibrated for wavelength using mercuryand krypton lamps and for irradiance using an FEL1000-W spectral-irradiance standard lamp.

The temporal dependence of the solar simulatoroutput is shown in Fig. 2, which is a 5-min segment of astrip chart recording of the pyroelectric radiometerreading. The radiometer had a 1-s response time.This measurement was made with the radiometer 3.05m from the exit aperture of the solar simulator, whichwas operating at an arc current of 64 A. Over the 5-min interval, the irradiance varied by 44 mW/cm2

around a value of 84 mW/cm2. Apparent in the stripchart trace is a cyclic variation in irradiance with aperiod of -16 s. This is because of discontinuouschanges in the arc gap (and therefore arc current) andpositive crater position as the gear-driven electrodefeeds compensate for electrode consumption. Attimes, manual adjustment of the electrode positions(as indicated on the chart) was required.

The divergence and the range dependence of theirradiance of the solar simulator output beam weredetermined by measuring the beam diameter and irra-diance as a function of distance from the simulator'sexit aperture. The data from these measurements,which were made at a constant arc current of 62 A, areshown in Fig. 3. The slope of the visible beam diame-ter vs range curve yields a divergence of 5.04°, which isin good agreement with the calculated value of 5.09°.

1 April 1991 / Vol. 30, No. 10 / APPLIED OPTICS 1291

100 -

MANUAL GAP ADJUSTE

i03z

ItQs~

2 3TIME (min)

1s

Fig. 2. Temporal variation of solar simulator beam irradiance.

4 8 12 15 20POSITION (cm)

Fig. 4. Solar simulator beam profiles.

160-1

i 120-

z

00Ut

40X 40-

:Z

0 I C- . .14s ...DlfeerYurm

1 2 3 4 5RANGE (in)

6 7 8

-80

-60

1

-40E

M

-20

Fig. 3. Dependence of solar simulator beam diameter and irradi-ance on range.

The on-axis pyroelectric radiometer measurements(solid circles) of irradiance vs range show that irradi-ance fell off at R-2 2.

The optical configuration of the solar simulator isessentially the same as that of a searchlight, i.e., a finitelight source situated at the focal point of a lens.Smith' has stated that for the searchlight the illumina-tion beyond a distance D will fall off as the square ofthe distance to the lens. The distance D is given by theratio of the lens diameter to the angle subtended by thesource from the lens. In the case of the solar simula-tor, D is -1.27 m. The fact that beyond 1.27 m theirradiance falls off faster than R-2 may be because ofthe inability to collimate the radiation over its entirebandwidth of -300-2500 nm.

Transverse profiles of the solar simulator outputbeam were measured by translating the pyroelectricradiometer, fitted with a 2-mm aperture, across thebeam. Data obtained at ranges of 2.44 and 3.05 m areshown in Fig. 4. At the 3.05-m range, where the on-axis irradiance is close to the AM 1.5 level, the beamirradiance is within 10% of the on-axis value over an 11-cm region. This should be satisfactory for most laserreceiver testing purposes.

The efficacy of a solar simulator may be evaluatedby a comparison of its spectral irradiance with that ofthe sun for the appropriate conditions. For terrestrialsolar energy applications, SERI has generated AM 1.5global (1800 FOV, surface tilted at 370 from horizontaltoward the sun) and direct beam (5.70 FOV) irradiance

- ola SimItor (on-axis)--- Glob. Sola, (AM 1.5)

I, I "l"- -300 500 700 900 1100 1300 1500 1700 I000 0100

WAVELENGTH (nm)2300 2500

Fig. 5. Comparison of carbon arc solar simulator and global solarirradiance spectra.

spectra,2 which have been accepted as ASTM stan-dards. A NASA-JPL study has shown that 1.5 is areasonable average air mass value for the annual solarenergy available in the United States.2 The 370 tiltangle corresponds to the average latitude in the U.S.A.

A comparison of the spectral irradiance at a range of3.05 m from the solar simulator, measured along thebeam's central axis, with a standard AM 1.5 globalsolar irradiance spectrum is shown in Fig. 5. Theglobal solar spectrum is more appropriate than thedirect beam spectrum in relation to laser warning re-ceivers, which generally have large fields of view. Ex-cept for the absorption regions, the matchup betweenthe global solar and simulator curves is quite good.The solar spectrum peaks at 480 nm; whereas, thesimulator spectrum peaks at 515 nm. Much of thereproducible structure in the simulator spectrum isprobably the result of emission lines from the rareearths contained in the core of the positive carbon.These rare earths form carbides which have highervolatizing temperature than carbon so that higher cra-ter temperatures are possible.3

In contrast to the uniformly continuumlike spec-trum of the carbon arc, the emission spectrum of thexenon arc contains numerous, prominent discrete linesin the near infrared. This is illustrated in Fig. 6 by thespectral irradiance of an Oriel 300-W solar simulatorequipped with an AM 1 filter. These data were ob-

1292 APPLIED OPTICS / Vol. 30, No. 10 / 1 April 1991

0

i

60

40-

X 20 -

. o . . . . . . . . . _

01 , , , , , , . . If

If 0

L 4

80

40

Tonu . .

X0 0

0.

- Ori.1 S.

--- Glob.1

300 00 700 oo 1000 1300 1500 1700

WAVELENGTH (nm)

Fig. 6. Comparison of Oriel xenon arc solar solar irradiance spectra.

14

Fig. 7. Normalized plots of carbon arc solarspectra at several ranges

tained by T. W. Cannon of the SolarInstitute using a GER Corp. spectrornto the one used for the carbon ar(measurements. Although the Orieltral irradiance is a good match tospectrum in the visible, xenon emissia matchup in the 700-1700-nm regiolar simulator. The curves in Figs. 5 athe carbon arc's superior solar simul

The solar simulator curve in FiEscaled (1.25X) version of data takenm. Scaling of the solar simulator d,tween those where data were takenthere is little change in the shape ofcan be seen in Fig. 7. Irradiance sp(1.22-m intervals over a 1.22-8.54-min Fig. 7. The spectra are normalizeach curve and, except for the deveminima with increasing distance, areFor receiver testing purposes, a recei

tioned at a distance from the solar simulator such thatit was illuminated by an irradiance spectrum closelymatching an AM 1.5 global solar spectrum.

,olar Simulator (o-xi)As a check on the consistency of the spectroradio-Sola (AM t.5) meter data with the pyroelectric radiometer data, the

absolute spectral irradiance curves measured at vari-ous ranges were integrated to obtain values of totalirradiance. These values are plotted in Fig. 3 (opencircles). There is a maximum 10% difference betweenthese values and the pyroelectric radiometer readings.

IV. Discussion and Conclusion

Measurements of the spatial, spectral, and temporal

- ----------- characteristics of the beam irradiance of a carbon arcsolar simulator have shown that it is quite suitable for

simulator and global laser warning receiver testing purposes. This conclu-sion is based on the following observations of the solarsimulator output beam:

(1) The variation in total irradiance can be held to±5% over a 5-min time interval (up to 20 min withmanual adjustment of the arc gap).

(2) At a range of 3.05 m, the irradiance is within 10%of the on-axis value over an 11-cm region of the beam.

(3) At a range between 2.44 and 3.05 m, the spectralirradiance is a good match to the standard AM 1.5

global solar irradiance spectrum.There are several improvements which could be

|glgd \ made to the solar simulator to enhance its perfor-mance. Spectral irradiance in the UV could be in-creased by substituting fused silica lenses for thecrown glass lenses now being used in the optical train.This would shift the cutoff in transmittance from 300to -200 nm, resulting in better agreement between thesolar simulator and global solar irradiance spectra inthe UV spectral region. The temporal stability of theoutput irradiance could be improved by monitoringthe instantaneous level of total irradiance with a pho-todiode whose output is used to control the feed rate of

simulator irradiance the carbon arc electrodes to maintain a constant gapand, therefore, constant output irradiance. Finerelectrode feed screws would be required to minimize

Energy Research the magnitude of discrete jumps in the arc gap. Final-idiometer similar ly, either filters or an absorbing gas cell could be insert-c solar simulator ed into the collimated beam region of the optical trainsimulator's spec- to induce absorption bands in the solar simulator irra-the global solar diance spectrum.on lines precludea for this particu-nd 6 demonstrateation capability.g. 5 is actually aat a range of 3.05ita for ranges be-t is valid because'the spectrum, as,ctra measured atrange are plotteded at the peak ofLoping absorptionalmost identical.

ver would be posi-

The authors wish to express their appreciation to J.Glidewell for assistance with data acquisition and pro-cessing and to T. W. Cannon of the Solar Energy Re-search Institute for providing the Oriel solar simulatordata. The authors would also like to acknowledge thelate Paul Ballard's contributions in the conceptualiza-tion and design of the carbon arc solar simulator.

References1. W. J. Smith, Modern Optical Engineering (McGraw-Hill, New

York, 1966).2. R. J. Matson, K. A. Emery, and R. E. Bird, "Terrestrial Solar

Spectra, Solar Simulation and Solar Cell Short-Circuit CurrentCalibration: A Review," Sol. Cells 11, 105 (1984).

3. J. D. Cobine, Gaseous Conductors (Dover, New York, 1958).

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