Calibration of a visible and near-infrared portable transfer radiometer

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 these 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 tra p detector. The temperature of the filters, apertures, detectors and electr onics 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 wa s calibrated in an irradiance mode by use of a National Institute of Standa rds and Technology (NIST) FEL lamp. Second, a Solar-Radiation-Based Calibra tion (SRBC) was carried out, in which the radiometer viewed a panel, calibr ated with a bidirectional reflectance distribution function (BRDF), illumin ated by the Sun. Differences between the NIST calibration and the SRBC meth od for a recent solar spectrum are quite small: less than 2.1% for the seve n bands between 412 nm and 868 nm, and well within the estimated uncertaint ies for the two calibration methods.