An integrating-sphere source developed by MTL Systems, Inc. was calibrated for them by a commercial standards laboratory traceable to the National Institute of Standards and Technology (NIST), resulting in values of spectral radiance in the 400 nm to 2500 nm spectral region. The spectral radiance of the MTL sphere source was then measured using three filter radiometers, also NIST-traceable, that were developed for the Earth Observing System (EOS). In the visible and near-infrared, the values from two Si-photodiode filter radiometers and the supplied calibration values agree within their mutual uncertainties, but in the short-wave infrared, the agreement between the filter-radiometer results and the calibration values is less satisfactory. The reason for this discrepancy is not understood, and additional measurements should be performed.
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