The effects of shadow blockage on the efficiency of a dielectric Fresnel lens is investigated. Since shadow blockage is a strong function of the F/D value, it should be factored into the design, especially for compact lenses designed with a small F/D. Equations are derived, based on ray tracing, to determine the decrease in aperture efficiency due to shadow blockage. Design curves have been generated to plot shadow efficiency against lens diameter for various values of F/D. Good agreement with measured radiation patterns is also demonstrated.
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Petosa et al. (2000) studied this question.