A fairly accurate analytical expression of the measured reflectance is established for the general case of a non-Lambertian and nonuniform ground by separating the atmospheric and surface effects. The signal is nearly linear in the function of intrinsic atmospheric reflectance, the actual target reflectance, and two average ground reflectances, angular and spatial, to be defined. Contrast reduction by the atmosphere, defined in the cases of Lambertian and directional ground reflectances, has been evaluated using this formulation.