This paper proposes a combined ray-tracing and physical-optics model to analyze parallel-plate-waveguide lens antennas with a flat aperture. A family of rays is traced from the source to a set of target points on the lens radiating aperture, giving a description of the aperture electric field. On the basis of the physical-optics approximation, an equivalent magnetic current is then assumed and used to evaluate the far-field radiation characteristics in every direction. This numerical approach is validated by applying it to a particular planar Mikaelian lens antenna and comparing the results with those obtained using a commercial full-wave simulator.