In this communication, we present a method to model the light absorption in a resonant photoconductor based on a photoconductive layer sandwiched between a metallic grating in front side and a metallic backside mirror. We started from a modal expansion formalism previously developed in order to model the transmission and the reflection of an infinite array of holes and we added the effect of a stack of homogeneous layers ended by a metallic mirror. It is finally shown that the electromagnetic response of the structure calculated with this numerical method is in good agreement with a more rigorous method such as the rigorous coupled wave analysis, whereas it requires a much lower computing power.