A unitless measure of the ratio of incoming to outgoing Radiation created from converting a radiometrically calibrated image to an innate characteristic of the target being observed. After removing the atmospheric component of calibrated at-satellite spectral radiance, Bidirectional Reflectance distribution Functions (BRDFs), bidirectional reflectance, and bidirectional reflectance factors (BRF) attempt to take into account target-related differences in reflectance as a function of four sources of variability of non-Lambertian surfaces: solar zenith and azimuthal irradiance angles and sensor viewing zenith and azimuthal angles. Also see planetary albedo and reflectance. (Source: Dr. John Barker)