1. A method for inspecting a photo-mask, comprising: receiving an inspection image corresponding to the photo-mask, wherein the inspection image is determined using an optical imaging system; and prior to inspecting the photo-mask, iteratively modifying an estimated pattern corresponding to the photo-mask at an object plane of the optical imaging system, wherein the iterative modifying is based on merit function corresponding to the inspection image and an estimated inspection image; and wherein the estimated inspection image corresponds to the estimated pattern.