For many scanning IR systems a constant flux of stray radiation can be treated as a part of the background and ignored in subsequent processing. The optical system can, therefore, be less complex, and often less than 100% cold sheild efficiency is permitted. On the other hand, variations in stray radiation detected through scan can be a major concern. The evaluation of these fluctuations is often tedious, especially when many combinations of parameters must be considered (such as scan mirror position and detector element location). As an aid in analyzing these variations, a computer program has been developed which combines both ray-tracing and radiometry to provide quick evaluation of various mechanisms of stray light change over scan. This type of evaluation is especially useful in the initial design stages, when the system configuration is being selected and optimized, so that stray light control becomes part of the lens design process. An example will be used to describe the capabilities of the program, to illustrate some of the mechanisms which cause this unwanted scan-dependent noise, and to suggest methods for reducing these fluctuations while maintaining a simple design.