A technique for reconstructing a mask pattern corresponding to a photo-mask using a target mask pattern (which excludes defects) and an image of at least a portion of the photo-mask is described. This image may be an optical inspection image of the photo-mask that is determined using inspection optics which includes an optical path, and the reconstructed mask pattern may include additional spatial frequencies than the image. Furthermore, the reconstructed mask pattern may be reconstructed based on a characteristic of the optical path (such as an optical bandwidth of the optical path) using a constrained inverse optical calculation in which there are a finite number of discrete feature widths allowed in the reconstructed mask pattern, and where a given feature has a constant feature width. Consequently, the features in the reconstructed mask pattern may each have the constant feature width, such as an average critical dimension of the reconstructed mask pattern.