Most of the laser speckle methods utilize subjective speckles in that speckles are recorded through a lens whose aperture determines the admitted spatial frequencies. Objective speckles are those intrinsically formed by the scattering wavelets. In this paper we propose the use of far-field objective speckles generated from a ground glass to measure surface displacement and strain. The object's surface is coated with a photosensitive material and placed inside the speckle field. Double exposure is made on the photosenstive coating before and after the application of load. Young's fringes are generated by probing the surface coating with a narrow laser beam. Surface strain can then be calculated from these fringes. The method can be applied to doubly-curved surfaces and opaque materials.
F. P Chiang,
C. C. Kin,
"Strain Determination On Curved Surfaces Using Far-Field Objective Laser Speckles," Optical Engineering 21(3), 213441 (1 June 1982). https://doi.org/10.1117/12.7972928