This paper discusses ways in which laser speckles may be used to determine strain on the surface of an object. This may be done either by measuring variations in speckle displacement across a specklegram, or by measuring the slewing of the speckle field relative to the object surface. The latter method requires the use of tandemly recorded specklegrams, and, although data processing is time consuming, it yields a complete solution for the surface strain rotation matrix. The first method is effective when the surface is observed in focus, along the surface normal, but it suffers serious problems when a significantly contoured surface is considered. Equations are derived that relate the derivatives of observed speckle displacements to surface strains and rotations as a function of illumination and observation direction and variations in defocusing.
Karl A. Stetson,
"Speckle And Its Application To Strain Sensing", Proc. SPIE 0353, Industrial and Commercial Applications of Holography, (7 July 1983); doi: 10.1117/12.933943; https://doi.org/10.1117/12.933943