Inversing the image from the aim orbit with the known image sensor and auxiliary data is a feasible and important way to support the construction of aim satellites and sensors. The differences of images from the same piece of background on different orbits can be summarized into geometric distortion and radiation difference. Geometric distortion is caused by the observation geometrical position and the sensor parameters; Radiation difference is mainly decided by the radiation distribution in space. This paper researched on radiation difference and improved the model by Nilson and Kuusk, which is a bidirectional reflectance distribution function (BRDF) model.