Abstract: Two substantially identically patterned transparencies, i.e. a master photomask and a copy thereof, are compared with each other by transluminating the master with an object beam, producing a hologram of that master by letting the object beam interfere with a reference beam from a common source of coherent light such as a laser, placing the copy in the path of the object beam formerly occupied by the master, and positioning the developed hologram at the intersection of the two beams to generate a compound beam of zero intensity if the two transparencies are identical. The luminous energy of the compound beam thus varies inversely with the degree of equivalency of the two transparencies. The compound beam may be imaged onto a receiving surface, with certain areas thereof blocked out to eliminate error indications from insignificant (e.g. marginal) zones.