Optical preprocessing in a laser-speckle correlation measurement technique for the determination of engineering strain within a specimen

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Abstract
This paper is concerned with a method of non-contacting measurement of mechanical strain within specimen. It describes a new optical set-up to perform high speed digital laser- speckle correlation with the ultimate aim to deduce surface element displacements associated with the translation of laser-speckles emanating from those surface elements. The novel optical set-up combined with the application of line scan cameras attached to digital signal- or very fast general- purpose processors allows measurement rates that for most practical purposes are only limited by the integration time of the camera necessary to obtain properly exposed images. Instead of obtaining a two-dimensional vector by searching for the best space-lag for a digitally calculated cross- correlation estimate of the initial and translated speckle images a single component of that vector parallel to the straining direction is obtained by finding the space-lag of optically preprocessed almost one-dimensional speckle fields. The necessary optical preprocessing is performed in the Fourier-plane of the imaging optics. This way the numerical complexity of the algorithm is greatly reduced resulting in lower processing time per frame. System considerations for practical strain measurements are detailed and the measured sensitivities are presented.

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