The technique of retroreflective grating analysis is presented to measure surface contour. This noncontacting optical method can detect a dent with maximum depth less than 10 mm. A comparison with some physical measuring systems indicates that it has advantage over such contacting measurements, especially for specular surfaces.
Walter P. T. North,
"Retroreflective grating analysis versus physical measurements of surface contour," Optical Engineering 37(5), (1 May 1998). https://doi.org/10.1117/1.601661