Low-background large-aperture infrared measurement facility:performance

Rudolf H. Meier and Alan B. Dauger

A novel ir measurement facility that generates a 43-cm diam collimated beam of radiant flux in a simulated

exoatmospheric environment has been built and evaluated. The broadband and spectral irradiance of thebeam was measured with a calibrated integral three-grating spectroradiometer. The irradiance values covera range of 108, with near Planckian distribution at higher flux densities. The beam source can also generate

monochromatic radiation tunable from 2 ,4m to 32 Aim. Diffraction losses at the source aperture have been

estimated.

Introduction

A low-background measurement facility has beenbuilt for the evaluation and calibration of large-apertureir sensors that operate from space platforms. It is dis-tinguished from other facilities by its portability, whichpermits it to be operated at the location of various largefacilities that are capable of simulating nuclear radiationenvironments that may exist in space. Furthermore,being the latest facility of its kind, it incorporatesstate-of-the-art devices for target simulation, beamscanning, and irradiance calibration.

Evaluation of the facility's performance presentedtechnical challenges. It was designed to simulatethermal radiant objects whose radiance may vary overa range of 108. Irradiance calibration of the test beamover this range was required to an absolute accuracy ofh25% (0.25). Also, because the test beam has an un-

avoidable deviation from a Planckian spectral distri-bution, it was necessary to be able to measure thebeam's spectral content during operation.

The facility's established performance parametersare summarized as follows:

(a) The source assembly is capable of producingbroadband irradiance over a range of 108. It can alsogenerate narrowband (monochromatic) radiation fromabout 3 gm to 32 im.

(b) The collimator, an off-center section of a Cas-segrainian system with a 43-cm unobscured diam ap-erture and a 348-cm focal length, has a maximum blurangle of 0.13 mrad.

The authors are with McDonnell Douglas Astronautics Company,

Huntington Beach, California 92647.Received 14 October 1977.0003-6935/78/1115-3547$0.50/0.© 1978 Optical Society of America.

(c) The scan capability provides variable angularvelocities from 0 to 0.6 rad/sec. The total scan field isa square of 0.1 rad on a side.

(d) The spectroradiometer permits, during opera-tion, both broadband and spectral calibration of theradiant flux density at the aperture of the sensor overwavelength intervals from about 3 Am to 32 ,um.

(e) The sensor parameters that can be determinedinclude responsivity as a function of irradiance and ofmodulation, noise equivalent flux density, sensor noisefrequency distribution, mapping of individual detectorsand of detector arrays, crosstalk, dynamic spatial re-solving power, and spectral responsivities of detec-tors.

Description

A 3-D optical schematic of the facility is presented inFig. 1 of the preceding paper.' A more detailed opto-mechanical layout is shown in Fig. 1 of this paper. Thefacility's major components are housed inside a vacuumchamber about 3 m long and 1.2 m in diameter and aresurrounded by a cryogenic shroud cooled to an operatingtemperature of 25 K. Components include a sourceassembly, an off-center collimator with two scan mir-rors, and a spectroradiometer. A rotatable beam de-flector can deflect a portion of the collimated beam intothe spectroradiometer, and a reference collimator canaccept radiant flux from a reference source assembly(not shown) and dir, t it into the spectroradiometeralong the same optical path as the beam deflector. Allcomponents are cooled either actively or by conductionto about 25 K to ensure a low-radiation background.Figure 2 is a photograph of the chamber with the tophalf removed. Note the black anodized finish, thecryogenic plumbing, the large chamber sealing surfaces,and the ready access to all major optical components.

15 November 1978 / Vol. 17, No. 22 / APPLIED OPTICS 3547

SPECTRORADIOMET B-OTA--jLE

HYPERBOLICI TO SENSOR SECONDARY

SCAN MIRROR SOURCEREFERENCE (ELERATIONI ASSEMBLY OFF AXIS PARABOLICOLLIMATOR (ELEVATION T AE LPRIMARY

SOURCE ASSEMBLY

Additional support equipment required for thechamber includes a vacuum pump, a He refrigerator, aHe pressure controller for regulating the temperatureof the calibration detector, and an electrical controlconsole with automatic data recording capability.Figure 3 shows the facility with some of the supportequipment.

A layout of the source assembly and spectroradi-ometer is shown in Fig. 4. The path of the beam fromthe source to the collimator and from the collimator tothe spectroradiometer is indicated but not detailed.The source assembly consists of a blackbody andtransfer optics incorporating variable apertures andvarious other means of attenuation. The 108 irradiancerange is achieved by the use of four source modes:specular, filter, dual filter, and integrating cavity. Themode selection is accomplished by changing the angularorientation of the trisurface reflector inside the inte-grating cavity and by inserting one of the two identical

SIc loE t

Fig. . Optornechanical layout.

Fig. 3. Facility with some support equipment.

Fig. 2. Optical assembly within the vacuum tank. The top halvesof the tank and the shroud have been removed. On the left side arethe scan mirrors with their cooling tube labyrinths. The collimator'ssecondary mirror is just to the right of the scan mirrors. On the rightis the table containing the shrouded source assembly and spectrora-diometer. The collimator's primary mirror is beneath the table.

Fig. 4. Schematic layout of the source assembly and spectroradi-ometer.

3548 APPLIED OPTICS / Vol. 17, No. 22 / 15 November 1978

SCAN MIRRORIEIEVA-ION)

1.

1.

0.

0 4 8 12 16 20 24 28 32 36 40

WAVELENGTH, (/LM)

neutral density filters that are part of the two aper-ture-filter wheels into the beam. In addition, the sourcemay be operated as a tunable monochromator bymaking use of the three ruled faces of the rotatablegrating cube. A detailed discussion of the variousmodes of source operation has been given previous-

l.2,3

The spectroradiometer consists of a telescope,transfer optics, a rotatable cube having three ruledgratings and a planoreflector, and a gallium-doped Gebolometer detector. A filter wheel contains the re-quired order-sorting filters. The spectroradiometermay be operated as a spectrometer over a 3-32-umwavelength range or, with the planoreflector in position,as a broadband radiometer.

Calibration of the Spectroradiometer

The reference standard for calibration of the spec-troradiometer was the blackbody source. Its exitancewas calculated, and its temperature transducers werecalibrated against a temperature standard that istraceable to the NBS.

When operated as a radiometer, the spectroradi-ometer's relative spectral responsivity is that of its de-tector, modified by the effect of the compound spectralreflectance of its eight optical surfaces. The spectralresponsivity of the detector only can be written

R(X) = Rr(X) (V W-1). (1)

The detector's peak normalized relative spectral re-sponsivity r(X) was measured at the Naval Ocean Sys-tems Center in San Diego. The wavelength-indepen-dent quantity R was obtained from a separate face-to-face calibration with the blackbody source.

The effect of r(X) on this face-to-face calibration wastaken into account by making use of the relation for thebolometer signal voltage

V=RG' S r()e'()Mx(T)dX (V), (2)

where G' is a term that incorporates amplifier gain andgeometry of the measurement arrangement, e'(X) is theeffective spectral emissivity of the blackbody assembly,and Mx(X,T)dX is the spectral radiant exitance of theblackbody cavity. The spectral responsivity of thebolometer R (), shown in Fig. 5, was obtained by arbi-trarily setting e'(X) = 1. The peak value is 1.49 X 105

VW-1 .

Fig. 5. Spectral responsivity ofthe bolometer.

Ideally, the specific responsivity of the three-gratingspectrometer should be determined with a calibratedreference blackbody. However, the facility, as presentlyconfigured, does not allow this direct measurement.Instead, the standard source assembly was used, which,of course, required taking into account the effects of thesource and collimator optics. To minimize these effectsand to obtain a sufficient SNR, it was operated in thespecular mode. Also, to minimize possible diffractionlosses at the source aperture, an aperture diameter ofabout 1 mm was used.

The specific responsivity of the spectrometer is de-fined as the signal voltage V(X,T) divided by the effec-tive power received by the spectrometer from the sourceassembly in the selected operational mode. This ef-fective power is the emitted radiant flux per unitwavelength Ase'(X)Mx(X,T) times the subtended solidangle, times the compound reflectance of the sourceassembly and collimator surfaces, times the half-in-tensity bandwidth of the spectrometer ZAXSR. Thus, thespecific responsivity is the product of spectral respon-sivity and instrument bandpass. It can be written

i7rF2 V(X,T)RSR(X)AsR = X

ABASR Mx(X\,Thc'(X)

X (VW'gm), (3)[PAU (X)13[POCLI(X)16

where F = collimator focal length = 348 cm,A, = source aperture area = 8.107 X 10-3 cm 2 ,

V(XT) = detector voltage (V),PAU(X) = spectral reflectivity of gold-coated sur-

faces,POCLI(AX) = spectral reflectivity of OCLI proprietary-

coated surfaces, andASR = effective entrance aperture area of spectro-

radiometer = 56.13 cm2.

The results of measurements and of the above calcula-tions are shown in Fig. 6. The effective spectral emis-sivity e'(X) of the blackbody assembly has again been setequal to unity.

Calibration of the Test Beam

The final step in calibrating the facility was themeasurement of the spectral irradiance Ei (XT) of thetest beam at the sensor aperture, for various modes (i)of source operation. This was done by directing aportion of the test beam into the spectroradiometer and

15 November 1978 / Vol. 17, No. 22 / APPLIED OPTICS 3549

1

AE14

16

_ 12

i- 10

a: 62

LU4

'I

WAVELENGTH [MICROMETER]

Table I. List of Irradlance Plots for Each Mode of the Source Assembly

BB ICaperture aperture Sourcediameter diameter temperature

Mode (mm) (mm) (K) Figure

Specular 0.35 5.5 300 7Filter 0.35 5.5 300 8Dual filter 0.35 5.5 300 9Integrating 8.0 0.35 400 10

cavity

measuring the detector voltages as a function of wave-length, yielding

E\i(X,T) = ifXT) -(WCM-2 (A),ASRRSR()ASR (mm (4)

Total irradiance in any specified spectral band was thenfound by integrating Exi (X,T) over the wavelength in-terval. Figures 7-10 are plots of Ex(X,T) for the modesand operational settings of the source assembly shownin Table I.

For the integrating cavity (IC) case, the data weretaken with a 0.0964-cm2 aperture. The curve in Fig. 10has been extrapolated from the measured curve for anaperture area of 9.93 X 10-4 cm2 (0.35-mm diam).

Note that the curves in Figs. 7 and 8 bear a strongresemblance to the Planckian curve for blackbodyspectral irradiance. The deviations near the ends of thecurve are due to SNR limitations of the measurements.These limitations severely affect the low irradiance levelcurves in Figs. 9 and 10. The true spectral irradianceis probably much more nearly Planckian than thesecurves would suggest.

The irradiance, in mode-aperture combination i, ina spectral band B between X1 and 2, is calculated bynumerically integrating

Fig. 6. Specific responsivity ofthe spectrometer.

E(B,T)i = J E,\i(X,T)dX (W cm- 2 ). (5)

The broadband irradiance for the four modes at 300 Kis given in Table II. The irradiance was derived bynumerically integrating spectral data from 4m to 32Am in the specular and single filter modes, and from 8Am to 24 m in the dual filter and IC modes. The datawere extrapolated beyond these limits in order to ap-proximate the total integral.

Controlled irradiance values, well below those shownin Table II, are obtained in the IC mode by using aper-tures smaller than 8.0-mm diam in the BB and 0.35-mmdiam at the IC. The total irradiance in the IC mode wascalculated by ratioing the aperture areas:

E(T) = E(T,0.35,8.0) ( AiC ) ( ABB ) (W CM 2 ). (6)

AIC is the IC aperture area, and ABB is the blackbodyaperture area (cm2). The smallest available aperturesare AIC = ABB = 1.267 X 10-4 cm2 (0.127-mm diam).Then for this case, E(T) is about 109 smaller than forthe specular case presented in Table II.

In the monochromator mode, the source assembly isoperated as a tunable narrowband source with a 3-32-,m spectral range. The calibration of the test beamoriginating from the monochromator was obtained withthe spectroradiometer operated as a radiometer(broadband mode). Since the wavelength setting of thenarrowband radiant flux was known, the measurementwas done by using the detector's spectral responsivityR(X), modified by the spectral reflectivity of the radi-ometer's eight surfaces. The narrowband irradianceat the sensor aperture then can be written

Em(XT) = VM(X,T) (W 2)ASR[PAU()13 [PoCL(X)15R(X)G ( m) 7

3550 APPLIED OPTICS / Vol. 17, No. 22 / 15 November 1978

BLK 81FIPER4 ISPECUI

-r -

SPECTRAL IRRAIDIANCE

DY TEMP.-..'. .0 DRY.-. .; ;v209DIR ...... 30 JM ~ ~ ~HOURJ.... .i

LRR M .ODE.. .. .... :MNTE.. , .71

.- .. . 4-.. -I

.~~~~~~~~~~~~~~~~~~~~~

.-.--. ~~t~IELENTI, (ICROM..RS

10 3u

Fig. 7. Spectral irradiance, specular mode.

A~~~U . ~~~~SPECTRAL IRRADIANCE -.-

ELK EBDY TEMP...300.0 DR .+w . .t-RPER DI R. 3Z M -HOUR .;T ....;. 20

FILTER MODE - . . MINUTE ........... 7

It-

I 4 ' 4 ~ ~ ~ ~ ~ _ 4 .

-4-

'U

Fig. 8. Spectral irradiance, filter mode.

15 November 1978 I Vol. 17, No. 22 I APPLIED OPTICS 3551

1000.

100

10

1004

10

0

0

iI

zu

I

JVzu

SPEClTRlAL IR RADIANCE .

BLK 8DY TEMP.. 30. 0 .I

RPER DIR..... 3W JJM . .... HOUR ~ DUAL FILTER MODE .M INUTE .

-. I I I~~~~~~~~~~~~C

-- ~ -,-.-...-.1' L

74

.'> ,J. MI ~~~~~~WVELENGTH <MICROMETtRS~ I ILi 'u 20 30

Fig. 9. Spectral irradiance, dual filter mode.

-SPECTRAL IRRADIANCE

f - t ~~~~~~~~~~- - I

DRY..- ..

HOUR. ..M INUTE.......3

100

WAVELENG3TH (MI CROMETERS)~

10

1

U 'U 20

Fig. 10. Spectral irradiance, integrating cavity mode.

3552 APPLIED OPTICS I Vol. 17, No. 22 1 15 November 1978

100

l0(

lc

1

1000

BLK SDY TEMP .. 00RIPER D IR. . . ' .. .0 JMINTEGRRTING CVITY MODE

30A --- - - - . . . .

I

,Rfp,i5ENW!:

to � . .

I.i- .-

. i- --. . .I I

I

�111

Table HI. Total Irradiance Blackbody Temperature: 300 K

BB ICaperture aperture Irradiance

Mode (mm) (mm) (W CM-2)

Specular 0.35 5.5 1.13 X 10-10Filter 0.35 5.5 6.72 X 10-12Dual filter 0.35 5.5 3.5-4.4 X iO-'1

Integrating cavity 8.0 0.35 4-5 X iO'15

I-

0.10

Assessment of Diffraction Losses

The bolometer detector was calibrated alone in anarrangement whose principal dimensions are given inFig. 12. Both the nomenclature and the equations usedto assess the magnitude of possible diffraction lossesduring these measurements are from Ref. 4. The sec-ond aperture, 5 cm from the detector, was installed to

SUM ..MONOCHROMRDTO CRL I ERT N'

3LK EDDY TEMP".. .400K DRY.,- 7HOUR.'.IG

- . . , - . .,. . . . .. ~~~~~~~~~~~~M I NUTES..,...

I I ~~~RVELENSTH <MICROMETERS>

0 10ju

Fig. 11. Irradiance of test beam with source in monochromator mode. The sharp discontinuities at 8.5 Atm and 16.5 Aim indicate the grating

BAFFLE change points.

APERTURE

dl

d,= 1.90 cm~j= 1.55 cm

a 48 cmD - 0.0127, 0.03566, 0.1016cm

APERTURE

Dii

d2= 008 cmk?2 = 5c-

D2- 0.6 -

Fig. 12. Schematic diagram of the bolometer face-to-face calibration

arrangement. The parameters for calculation of the diffraction lossare shown on the figure.

Figure 11 shows the monochromator irradiance as afunction of wavelength. The blackbody temperaturewas 400 K, the blackbody aperture was one of the au-tomatically selectable 1 X 3-mm slit filters, and the ICaperture was 1.0 mm.

eliminate radiation from directions other than thesource. It had only a slight effect on the over-allloss.

The resulting diffraction loss, as a function ofblackbody temperature for D1 = 0.03556 cm, is plottedin Fig. 13(a). The diffraction loss, as a function of ap-erture diameter DI for T = 300 K, is plotted in Fig.13(b).

Assessing diffraction effects on the spectrometercalibration requires determining diffraction losses inthe source assembly only. Diffraction losses in thespectrometer itself need not be considered, inasmuchas they are the same when the test beam is being mea-sured. The nature of diffraction losses in the sourceassembly is illustrated in Fig. 6 of the preceding paper. 1

The solid angle of the intercepted blackbody radiationis determined by the source relay optics. The stillsmaller solid angle of the beam that was actually usedis determined by the relative aperture of the collimator.Computed diffraction los'ses, again obtained by applying

15 November 1978 Vol. 17, No. 22 APPLIED OPTICS 3553

10 I

JU2u

a

0

4

0

3.0W

2.0 - \____ ____

SOURCE APERTURE DIA 0.314

1.0 - _

0 200 300 400BLACKBODY TEMPERATURE, 'K

Table Ill. Percent Source Diffraction Loss, Specular Mode

Source aperture diameterX (mm)

(Mm) 1.016 0.3556 0.127

5 0.17 0.47 1.3010 0.33 0.93 2.6020 0.65 1.86 5.19

mm

3b ,0 _ __ X !

2 .0- - - - -- _

0

1.0r-

00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

SOURCE APERTURE DIAMETER. mm

Fig. 13. Diffraction loss for the bolometer calibration arrangementin Fig. 12.

expressions given in Ref. 4, are listed in Table III forthree wavelengths.

Conclusions

The performance of the low-background large-ap-erture ir measurement facility was shown to be up toexpectations. A test beam dynamic range in excess of108 was achieved, and the capability of self-calibrationwith its self-contained calibration standard was proven.The calibration may be repeated at any time, even whilethe facility is at a field location.

Calibration accuracy was determined to be wellwithin ±25% (0.25) for the higher level modes (specular,single filter, and monochromator). However, SNRlimitations caused the calibration uncertainty to in-crease to about ±40% in the lower level modes (dualfilter and integrating cavity). The calibration errors

may be reduced in several ways, e.g., taking repeatedmeasurements over a long period of time, improvingSNR by installing a more sensitive detector in thespectroradiometer, and calibrating precisely the elec-tronic measurement instrumentation just before eachtest. In addition, in order to reduce the uncertainty inthe amount of radiation reaching the detector, thespectroradiometer telescope and some of the relay op-tics may be eliminated in a simplified spectrometer.Finally, the installation of a separate reference sourcethat bypasses the source and collimator optics willgreatly enhance the quality of the spectroradiometercalibration.

This work was performed under U.S. Army contractDAHC60-73-C-0096. The project was monitored by B.L. Walker of BMD-ATC, Huntsville, Alabama. Theviews and conclusions contained in this document arethose of the authors and should not be interpreted asnecessarily representing the official policies, either ex-pressed or implied, of the U.S. Army.

Thanks are due to many individuals who in variousways have contributed to this program, in particular,J. R. Averett, A. M. Cech, G. W. Galleher, R. M. Hart-man, C. R. Horgen, J. J. Kacin, and B. L. Walker.

References1. R. H. Meier and A. B. Dauger, Appl. Opt. 17, 3541 (1978).2. R. H. Meier, Appl. Opt. 14, 1021 (1975).3. R. H. Meier and A. B. Dauger, Opt. Eng. 14, SR-144 (1975); 14,

SR-182 (1975).4. W. B. Fussell, Tables of Diffraction Losses, NBS Technical Note

594-8 (U.S. Government Printing Office, Washington, D.C.,1974).

3554 APPLIED OPTICS / Vol. 17, No. 22 / 15 November 1978

(a)