A quantitative model has been developed to characterize the formation of an electro-static x-ray image on an amorphous selenium photoreceptor. The model is based on measurements of selenium x-ray photogeneration efficiency in the mammographic and radiographic energy ranges for internal fields E ranging from 1 to 12 volts per micron. The x-ray energy absorbed by the selenium for each neutralized surface charge is found to be 510 eV divided by E in volts per micron. X-ray source spectral distribution, kVp and mAs as well as photoreceptor thickness and initial surface potential are included in the comput-erized model. The effects of Compton back scatter and foreward scatter are included through judicious use of object absorption and attenuation coefficients. Discharge curves calculated from the model illustrating two extremes of object scattering are shown to be in close agreement with experimentally measured discharge curves.