Here - PMOD/WRC

Evaluation and improvement of the performance of a commercially available
detector stabilized radiance sphere source
E. F. Zalewski and S. F. Biggar
University of Arizona, College of Optical Sciences, Tucson, Arizona, USA
Abstract. Many national standards laboratories have
recently introduced new and more accurate radiometric
calibrations (White et al. 1996; Sperfeld et al. 1996; Yoon
et al. 2002). In order to improve measurements in the
field these new calibrations must be reliably transferred to
the user’s laboratory. Incandescent lamps are typically
used as calibration transfer devices but are of questionable
stability. Improvements in calibration transfer devices
have been made using silicon photodiodes to monitor and
correct for lamp instabilities (Fox et al. 1998; Windsor et
al. 2000). Recently sphere sources of spectral radiance
have become available commercially that are based on an
incandescent lamp and that incorporate a silicon
photodiode to control radiance within the sphere. The
incandescent lamp is located outside the sphere and its
output is attenuated by a variable slit at the entrance to the
sphere. The signal from the silicon photodiode is used in a
feedback circuit to control the slit width in order to correct
for lamp fluctuations. In addition, by changing the slit
width the radiance level can be varied continuously over a
very large dynamic range; presumably without changing
the spectrum of the incandescent light source.
Biggar, S. F., Calibration of a visible and near infrared portable
transfer radiometer, Metrologia, 35, 701-706, 1998.
Fox, N. P., C. J. Chunnilall, M. G. White, Detector based transfer
standards for improved accuracy in spectral irradiance and
radiance measurements Metrologia, 35, 555-561, 1998.
Sperfeld, P., K.-H. Raatz, B. Nawo, W. Moeller, J. Metzdorf,
Spectral irradiance scale based on radiometric black-body
temperature measurements, Remote sensing aids in sea-ice
analysis, Metrologia, 32, 435-439, 1996.
Spyak, P. R., D. S. Smith, J Thiry, C. Burkhart, Short-wave
infrared transfer radiometer for the calibration of the
Moderate-Resolution Imaging Spectrometer and the Advanced
Spaceborne Thermal Emission and Reflection Radiometer,
Applied Optics, 39, 5694-5706, 2000.
White, M. G., N. P. Fox, V. E. Ralph, The characterization of a
high temperature blackbody as the basis for the NPL spectral
irradiance scale, Metrologia, 32, 431-434, 1996.
Windsor, S. A., N. J. Harrison, N. P. Fox, The NPL detector
stabilized irradiance source, Metrologia, 37, 473-476, 2000.
Yoon, H. W., C. E. Gibson, P. Y. Barnes, The realization of the
National Institute of Standards and Technology detector-based
spectral irradiance scale, Applied Optics, 41, 5879-5890, 2002.
We have purchased and examined one of these commercial
sphere sources and found it to be repeatable within the
manufacturer’s specifications at one radiance level (within
+/-0.5% after the recommended warm-up interval).
However, we have found that upon changing the slit width
the spectral output of the sphere source varied outside the
limits of the specifications. In order to correct for these
spectral changes we have introduced additional spectrally
filtered detectors to measure the sphere spectral radiance.
Other changes we have incorporated include thermal
stabilization of the manufacturer’s control detector and
flushing the sphere with dry nitrogen to improve
repeatability in the short-wave ir where absorption by
water vapor would cause non-repeatability.
These modifications were tested as a prototype before final
incorporation into the sphere source system.
Repeatability of the absolute spectral radiance was
measured by comparisons with our highly stable VNIR
(Biggar, 1998) and SWIR (Spyak et al., 2002) transfer
radiometers.
Our prototype modifications have improved the
repeatability of the sphere source by a factor of five not
only at a single radiance level but also over a wide
dynamic range of radiance output. We will present details
of the design of our improvements to the sphere source and
the results of our repeatability tests.
References
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 293