Abstract: Objective A physically-based model for calculating radiance reflected from neighboring slopes is devel-oped.The path radiance and atmospheric transmittance of reflected terrain radiance between two adja-cent slopes are considered.The model computes terrain irradiance illumination and its ratio to total ir-radiance as quantitatively supported by DEM data and atmospheric parameters.According to an exper-iment with spot images and the principle of terrain irradiance,terrain irradiance illumination dependson the surface reflectance of the adjacent objects,terrain,the total irradiance reaching the adjacentslopes and other factors.The radiance reflected from neighboring slopes usually is small,but it has tobe accounted in high surface reflectance areas.The ratio of terrain irradiance to total irradiance de-pends on direct irradiance,diffused irradiance,and terrain irradiance,which are received by a targetpixel.The ratio of terrain irradiance to total irradiance of low illumination pixels is larger than that ofits surrounding pixels.For a shadow pixel,the ratio of terrain irradiance to total irradiance of theweak scattering bands is larger than that of the strong scattering bands.