A portable transfer radiometer covering the spectral range 400 nm to 900 nm has been built and calibrated. This radiometer was designed to measure the output of spherical integrating sources with large apertures such as those used to calibrate spacecraft sensors for the Earth Observing System (EOS) of the National Aeronautics and Space Administration (NASA). The radiometer is a simple, robust, narrowband multifilter instrument using a silicon trap detector. The temperature of the filters, apertures, detectors and electronics is controlled slightly above ambient. There are no imaging optics and the radiometer throughput is controlled by Invar-spaced apertures. The radiometer has been calibrated by two methods. First, the radiometer was calibrated in an irradiance mode by use of a National Institute of Standards and Technology (NIST) FEL lamp. Second, a Solar-Radiation-Based Calibration (SRBC) was carried out, in which the radiometer viewed a panel, calibrated with a bidirectional reflectance distribution function (BRDF), illuminated by the Sun. Differences between the NIST calibration and the SRBC method for a recent solar spectrum are quite small: less than 2.1 % for the seven bands between 412 nm and 868 nm, and well within the estimated uncertainties for the two calibration methods.
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