By illuminating a ground glass with a laser beam, a spatial field of speckles is created. These speckles are used as displacement gauging ele-ments of curved 3-D objects whose surfaces are coated with photosensitive material. Equations are derived to show that all three components of the surface displacement can be obtained from two specklegrams.
C. C. Kin,
F. P. Chiang,
"Objective Laser Speckle Method For Three-Dimensional Displacement Measurement On Curved Surfaces," Optical Engineering 22(1), 221153 (1 February 1983). https://doi.org/10.1117/12.7973065