<title>Real-time 3D video moire inspection system with enhanced depth resolution</title>

Abstract

A variable resolution video machine vision systems has been built which generates twice the number of depth contours for a given grating spacing as a conventional moire system. This variable resolution system uses a Mach-Zender interferometer to project interference fringes onto a reference surface and onto a target surface. Video images of the two structurally illuminated surfaces are mixed in a video mixer with the resulting output being moire contours which are the intersections of the two surfaces. If the reference surface is a flat plate, we get equal depth contours of the target surface, and if the reference surface is a perfect target, we get error map contours of the 3D shape differences between the two targets. Theoretical analysis has shown that if the reference surface upon which the gratings are projected is the inside of the actual surface to be inspected, then the moire depth contours are twice as dense as would be observed with a flat reference surface. This surprising result is experimentally demonstrated both for a perfect target and for a target with 3D shape errors. Real time error maps of damaged targets made using this technique have many moire contours outside the area of interest, but this 'non-information' can be greatly reduced by video or computer subtraction of the perfect target images.