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, wherein the estimated pattern is determined using an inverse optical calculation in which the inspection image is in an image plane of the optical imaging system and the estimated pattern is at the object plane of the optical imaging system; and wherein the estimated inspection image corresponds to the estimated pattern.