Problem of defocusing in speckle photography, its connection to hologram interferometry, and its solutions

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Abstract
The movements, in three-dimensional space, of laser speckles observed from diffusely reflecting objects are described. The description connects these motions to relationships between fringe vectors and fringe localization in hologram interferometry. The results indicate the conditions under which defocusing leads to false measurements with conventional techniques, and methods are suggested that could solve the problem encountered due to defocusing when small displacements are to be measured on highly three-dimensional objects.