A transmission-line model was used to study the asymmetric illumination of photoconductive switches. We studied the evolution of the electric field within the first 2 ns of optical illumination. The electric fields were significantly compressed in the regions of low photocarrier density. Also, the field collapse was slower and the switching efficiency reduced in the assymetric illumination case. Both the symmetric and the asymmetric cases were investigated experimentally with our electro-optic sampling system. The experimental results confirmed the predictions of the transmission-line model.