Radiometric characterization of a large-aperture variableradiance calibration source for remote sensing applications R. D. Taubert, C. Baltruschat, S. Schiller, J. Hollandt Abstract The increasing demand for SI-traceable pre- and post-flight calibrations of airborne multi/hyper-spectral imagers has been met by the Physikalisch-Technische Bundesanstalt (PTB) with the development and characterization of a novel, integrating sphere based, large-aperture variable- radiance source (LAVRAS). Within the framework of the European Metrology Research Program (EMRP), as an objective of the EMRP Joint Research Project “European metrology for earth observation and climate – MetEOC”, LAVRAS was extensively characterized with respect to its temporal stability, lateral and angular radiance distribution, followed Objectives Radiometric characterization and calibration Optimization, radiometric characterization and calibration of the large aperture variable radiance source (LAVRAS) of PTB. Target aims: Stability by an absolute calibration in terms of its spectral radiance over its entire dynamic radiance range of two orders of magnitude. The results obtained confirm that LAVRAS is particularly suitable for linearity investigations as well as for flat-field calibrations when homogeneity below 1% is required. Angular radiance distribution detector: radiation thermometer LP3, λ = 650 nm dedicated kinematic stage with five degrees of freedom viewing angle: ± 30° horizontal and ± 10° vertical seven different lamp operation configurations, including deliberately chosen asymmetric illuminations detector instrumentation: LAVRAS monitor detector unit and a high stability radiation thermometer as an independent radiance detector (LP3, λ = 650 nm) Wavelength range: 400 nm to 1000 nm 0.00% UV monitor diode relative radiance change -0.20% Radiance range: (0.01 – 0.3) W·m-2·nm-1·sr-1 Stability: < 0.5 % over one hour Apparatus -0.40% VIS monitor diode -0.60% -0.80% NIR monitor diode -1.00% -1.20% LP3 @ 650nm -1.40% -1.60% 0:00 Φvertical / ° Uniformity: 1% or better over entire aperture 12:00 24:00 36:00 48:00 60:00 Φhorizontal / ° 72:00 Fig. 6: Relative angular distribution of the spectral radiance in the center of the radiating area as a function of the horizontal resp. vertical viewing angle with a 32×2-lamps in operation configuration. One colour change corresponds to a 0.2 % radiance change. time / hh:mm Fig. 3: Relative change of the spectral radiance as a function of time, monitored with three filter radiometers of the LAVRAS internal monitor detector unit and with the radiation thermometer LP3 Lateral radiance distribution Absolute spectral radiance calibration full aperture scanning with LP 3 at 650 nm and 950 nm 5 mm × 5 mm scanning grid focus setting: aperture plane of LAVRAS 400 mm Calibration performed at the Spectral Radiance Comparator Facility of the PTB Spectral Radiance Primary Standard: High Temperature Blackbody HTBB 3200pg Five different lamp / lamp aperture settings 0.70 0.60 Fig. 1: The integrating sphere based large-aperture variableradiance source (LAVRAS) of PTB 0.50 1.0E+09 0.40 0.3 W·m-2·nm-1·sr-1 spectral radiance / W·m-3·sr-1 0.30 0.20 LAVRAS – jointly developed by PTB and Czibula und Grundmann GmbH, Berlin. Main parameters: 0.10 0.00 -0.10 -0.20 integrating sphere with 1.2 m diameter, BaSO4 coating -0.30 -0.40 -0.50 400 mm aperture diameter -0.60 optical radiation sources: 64 (32×2) reflector type tungsten -0.70 -0.80 halogen lamps (50 W), actively air cooled and individually controllable stepper motor controlled, variable aperture system for continuous radiance adjustment 3 broadband-filtered Si-photodiode detectors as monitor Fig. 4: Relative spectral radiance distribution across the 400 mm aperture of the integrating sphere, measured at 650 nm and with a 32×2-lamps in operation configuration. One colour change corresponds to a 0.1% radiance change. measurement of the horizontal radiance distribution at five different wavelengths and for four different lamp configurations at the Spectral Radiance Comparator Facility of PTB reflector-type tungsten halogen lamp (50 W) relative spectral radiance 0.010 0.000 410 nm -0.005 -0.010 -0.015 -0.020 -250 -200 -150 -100 -50 0.010 relative spectral radiance adjustable apertures 0.005 4 lamps 12 lamps 24 lamps 31 lamps 0.005 0.000 0 50 100 150 200 250 horizontal position / mm 4 lamps 12 lamps 24 lamps 31 lamps 800 nm -0.005 -0.010 -0.015 -0.020 -250 -200 -150 -100 -50 0 50 100 150 200 250 horizontal position / mm monitor detector unit integrating sphere wall baffle Fig. 2: Schematic, cross-sectional detail view of LAVRAS 0.005 0.000 4 lamps 12 lamps 24 lamps 31 lamps 550 nm 0.000 0 50 100 150 200 250 horizontal position / mm 4 lamps 12 lamps 24 lamps 31 lamps 1000 nm Le(UK, 24 lamp pairs) Le(UK, 12 lamp pairs) Le(UK, 4 lamp pairs) Le(UK, 1 lamp pair) max. radiance min. radiance 300 400 500 600 700 800 wavelength / nm 900 1000 1100 1200 radiance stability: < 0.1 % / h in the UV, VIS and NIR spectral range angular radiance distribution: lambertian radiator (1 %, 650 nm) within ± 10° horizontal and vertical viewing angle incl. asymmetric illuminations radiance level range: variable within two orders of magnitude (350 nm – 1100 nm) maximum variation of the spectral shape for two orders of magnitude variation of the spectral radiance: 10 % in the UV resp. 5 % in the VIS/NIR spectral range -0.005 -0.010 -0.015 -0.020 -250 -200 -150 -100 -50 Le(UK, 32 lamp pairs) homogeneity: ± 0.7 % at 410 nm, ± 0.3 % in the VIS and NIR wavelength range -0.015 0.005 1.0E+06 Results / Summary -0.010 0.010 0.01 W·m-2·nm-1·sr-1 Fig. 7: Calibration results for the spectral radiance of LAVRAS for five different lamp configurations (fixed lamp aperture setting). The horizontal lines denote the targeted spectral radiance range -0.005 -0.020 -250 -200 -150 -100 -50 1.0E+07 1.0E+05 0 50 100 150 200 250 horizontal position / mm Fig. 5: Relative horizontal spectral radiance distribution across the 400 mm aperture of the integrating sphere for four different lamp configurations at 410 nm, 550 nm, 800 nm and 1000 nm Physikalisch-Technische Bundesanstalt Braunschweig und Berlin Dr. Richard Dieter Taubert Working Group 7.31 High-temperature Scale 0.010 relative spectral radiance actively cooled lamp housing relative spectral radiance detectors in the UV, VIS and NIR wavelength range 1.0E+08 Abbestr. 2 – 12 10587 Berlin phone: +49 30 3481-7480 fax: +49 30 3481-7490 e-mail: [email protected] www.ptb.de/cms/en/fachabteilungen/abt7/fb-73/ag-731.html
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