The performance of optical processors, particularly crosstalk and dynamic range, normally does not measure up to what is predicted by first order optical design, ray tracing, and diffraction calculations. Contributing factors such as detector array and light source characteristics, discussed elsewhere by other authors, combine with optical performance factors such as large-and small-angle scatter and multiple reflections to determine the actual system performance. A stray light computer code, ASAP/RABET, has been used to model several simple configurations found in optical processors. Scattering of samples of material was measured using a low angle scatterometer in transmission (BTDF), and measured longscan surface roughness profile data (WYKO TOPO). The results including the stray light effects are compared with normal calculations (CODEV) and also with some measured performance data.