A laser-based strain sensor with optical preprocessing

This paper is concerned with a method of noncontacting measurement of mechanical strain within specimen, It describes a new optical setup 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 setup combined with the application of line-scan cameras attached to a digital signal processor 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 of a digitally calculated cross-correlation estimate of the initial and translated speckle images a single displacement value (associated only with the sensitive 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 running on the digital signal processor is greatly reduced resulting in lower processing time per frame, System considerations for practical strain measurements are detailed.