A technique for calculating a second aerial image associated with a photo-mask that can be used to determine whether or not the photo-mask (which may include defects) is acceptable for use in a photolithographic process is described. In particular, using a first aerial image produced by the photo-mask when illuminated using a source pattern and an inspection image of the photo-mask, a mask pattern corresponding to the photo-mask is determined. For example, the first aerial image may be obtained using an aerial image measurement system, and the inspection image may be a critical-dimension scanning-electron-microscope image of the photo-mask. This image, which has a higher resolution than the first aerial image, may indicate spatial-variations of a magnitude of the transmittance of the photo-mask. Then, the second aerial image may be calculated based on the determined mask pattern using a different source pattern than the source pattern.