To enhance the accuracy of downward light sensors used for measuring solar downward radiation, a radiation transmission model for cosine correctors is developed based on the principles of radiation transfer on material surfaces. An analytical equation is established for the structural parameters of the shadow ring in a plate-type cosine corrector, and a structural optimization design method is proposed. For designing of a cosine corrector intended for a small unmanned aerial vehicle downlink optical sensor, a genetic algorithm is employed to determine the optimal structural parameters based from the analytical expression, with the model verifies through optical simulation. Results indicate that the cosine corrector with optimized structural parameters exhibits the minimal cosine correction global weighting error across an incident angle range of 0°?90°. The cosine response characteristics of a prototype cosine corrector with a 10 mm radius are then test in the laboratory. Findings reveal that the global weighted error in cosine correction is 4.62% within the incident angle range of 0°?90°, consistent with the simulation results. These findings confirm the accuracy of the analytical equation for the shadow ring's structural parameters and provides a valuable reference for further structural optimization and error analysis of cosine correctors.