An optical method to compare the shape and the contours of optically opaque test objects with those of a master is proposed, analyzed and demonstrated. A grid pattern is projected onto the test sample and the diffusely scattered light is imaged onto a mask, fabricated to yield zero transmission for ideal samples. Deviations in shape or depth from the master result in light transmitted through the mask. Resolution is in the range of a few micrometers.
H. P. Weber,
"Incoherent Optical Correlator Using Projected Fringes Patterns," Optical Engineering 18(4), 184387 (1 August 1979). https://doi.org/10.1117/12.7972391