Abstract: A technique for comparing master parts with replicas to determine differences in shapes or sizes, is disclosed, which uses interferometric techniques as disclosed. A television camera is used to scan the pattern established by illuminating the subject with a set of optical fringes. An objective lens images these on a transmission line grating. A television camera scans the transmission grating. The spatial frequencies of the subject fringes and of the grating are especially selected so that the camera can not resolve either of them but can resolve their difference. As a result, the output signals of the camera represent a contour map of the object placed on a low frequency spatial carrier, but yet, the measurement sensitivity is still determined by the spatial frequency of the subject fringes.