Abstract: An optical comparator for comparing a master printed circuit board or other complex assembly with a manufactured board to determine whether the board being tested matches the master in all respects. The master board is placed in the rear chamber of the comparator and the test board is inserted in the front chamber thereof. The optical arrangement is such that when these chambers are alternately illuminated at a periodic rate, an observer sees the master and test boards alternately and at coincident positions so that if the two boards match in all respects, the observer receives the visual impression that he is seeing a continuously-illuminated single board. But if a component is missing from the test board or there is any other disparity between the two boards, then the alternate illumination of the boards gives rise to light pulsations indicative of this disparity.