Spatial rejection is critical when exo-atmospheric or in situ radiometric measurements of faint sources, such as air glow emissions, are made in the presence of relatively intense sources such as the sun, moon, or earth. Scattering from atmospheric molecules and aerosols during evaluation in an earth-bound laboratory makes the high-rejection baffle appear to be less effective than it actually is in the measurement situation. A Monte Carlo computer program SCAT was written to predict the effects of atmospheric scattering upon field-of-view calibration measurements of optical baffling systems. It was found that Mie scattering could be reduced in a clean room environment and Rayleigh scattering was determined to be the limiting mechanism in a special chamber designed for off-axis measurements. The background flux at 40° off axis was found to be 1 x 10-9 of the on-axis incident flux for a 5° full-angle baffle when illuminated by a 10.2 cm diameter collimated beam. A two-step process was used to measure a baffle response at 40° off axis down to 1 x 10-11 of the on-axis incident flux.