The application of atmospheric models to AVIRIS and other spectral imaging data to derive surface reflectance requires that the sensor output be calibrated to absolute radiance (Green et al, 1996; Chrien et al, 1996; Gao et al, 1993). Uncertainties in absolute calibration are to be expected, and claims of ±2% accuracy have been published (Chrien et al, 1996). Measurements of accurate surface albedos and cloud absorption to be used in radiative balance calculations depend critically on knowing the absolute spectral-radiometric response of the sensor. The Earth Observing System project is implementing a rigorous program of absolute radiometric calibration for all optical sensors (Butler and Johnson, 1996a) Since a number of imaging instruments that provide output in terms of absolute radiance are calibrated at different sites, it is important to determine the errors that can be expected among calibration sites. Another question exists about the errors in the absolute knowledge of the exoatmospheric spectral solar irradiance (Nekel and Labs, 1984).