Historically, software tools for the design of space-borne imaging sensors have been either typical geometric ray tracing codes or stray-light analysis programs. While these codes provide useful information about the system, they do not currently permit simulation of the image at the detector focal plane. This paper describes a novel software system designed to perform focal simulation, which we believe fulfills an important role in the design of certain systems. The Optical Simulation for Imaging Reconnaissance and Intelligence Sensors (OSIRIS) code will provide users with an environment in which to synthesize imagery to a very high level of fidelity as would be detected at a sensor focal plane. The method for the simulation utilizes a reverse ray tracing technology previously developed for radiometric analysis in the Los Alamos Radiometry Code (LARC). At the present time, the OSIRIS software is complete and undergoing preliminary testing. While this method is extremely computation intensive, it appears to provide the designer with some information which could not otherwise be practically obtained. The primary objective of OSIRIS is to simulate imagery at the focal plane of a detector in space, taking into account laser light scattering in the earth's atmosphere.