Techniques that use the design databases used in each of the expose/etch steps during construction of phase shift masks are described. A model or reference image is rendered, accounting for systematic variations, from the design databases to represent what a layer of the PSM should look like after processing. The reference image is compared to an optically acquired image of a specimen phase shift mask to find defects. The technique of the present invention can be used to inspect EAPSM, APSM and tritone masks. The technique inspects all layers in one pass and is therefore more efficient.