A technique for providing information about defects in a mask pattern is described. In this technique, defects in the mask pattern may be determined based on differences between a calculated pattern produced at an image plane in the photolithographic process, when the mask pattern, illuminated by an associated source pattern, is at an object plane in the photolithographic process, and a target pattern that excludes the defects. Then the defect information may be provided to the user, such as a spatial map of the determined defects, where the spatial map is associated with at least the portion of the mask pattern.