Optical comparator for testing printed circuit boards

Claim 1

An optical comparator for testing a printed circuit board or other complex assembly to determine whether it matches a master printed circuit board, said comparator comprising: A. a casing divided by a partition into a normally-dark front chamber and a normally-dark rear chamber, the front wall of the casing having an observation port therein, the partition having an inclined see-through mirror therein in alignment with said port, said see-through mirror being effectively transparent when only the normally-dark rear chamber is illuminated and being effectively reflective when only the normally-dark front chamber is illuminated; B. means alternately to illuminate said normally-dark front and rear chambers at a periodic rate whereby in a first state in which the rear chamber is illuminated while the front chamber is dark, one may from said port look in a direct optical path through said see-through mirror which is then transparent into said rear chamber to see said master board which is disposed therein; C. means to support said master board in said rear chamber at a predetermined position therein where it is fully visible from said port in said first state; D. a reflecting mirror disposed in said front chamber at an angle to said see-through mirror, said see-through mirror being so disposed with respect to said reflecting mirror to provide an optical path in which the line from the eye of the observer to the see-through mirror is normal to the line from the reflecting mirror to the test board, whereby in a second state in which the front chamber is illuminated while the rear chamber is dark, the see-through mirror is then reflective and cooperates with said reflecting mirror so that one may from said port look by way of an indirect optical path into said front chamber to see said test board which is disposed therein; and E. means to support said test board in said front chamber at a position therein where it can be seen from said port in said second state at a position which appears to be coincident with the predetermined position at which the master board is seen in said first state, whereby when the test board matches the master board, the visual impression is that of a continuously-illuminated image, but if a disparity exists therebetween, light pulses are produced at said periodic rate indicating a disparity therebetween.