Described herein is a method for simulating a three-dimensional spatial intensity distribution of radiation formed within a resist layer on a substrate resulting from an incident radiation, the method comprising: calculating an incoherent sum of forward propagating radiation in the resist layer and backward propagating radiation in the resist layer; calculating an interference of the forward propagating radiation in the resist layer and the backward propagating radiation in the resist layer; and calculating the three-dimensional spatial intensity distribution of radiation from the incoherent sum and the interference.