The Earth Radiation Budget Experiment (ERBE) and the Clouds and Earth's Radiant Energy System (CERES) are systems that measure the earth's radiation budget, or the difference between earth-emitted radiation and radiation absorbed from the sun. Both these systems use three scanning radiometers based on a Cassegrain telescope design. Each of the three radiometers is sensitive in one of three wavelength intervals: shortwave, longwave, and total. A numerical model has been developed to investigate the radiative and optical characteristics of these instruments. The Monte-Carlo ray-trace method has been applied to the ERBE and CERES scanning radiometer instruments to calculate radiation exchange. In a radiative analysis, the surfaces of both instruments are found to emit a significant amount of radiation to the detector. In an optics analysis, both the ERBE and CERES instrument are found to have excellent directional selectivity. The field-of-view limits in the design of both instruments are verified using results from the optical analysis. / Master of Science