In-plane displacement measurement using a photorefractive speckle correlator

The present work describes the use of a photorefractive correlator in the measurement of in-plane displacement. A correlation technique has been proposed and demonstrated for precise displacement measurement by recording objective speckles. Speckle interference patterns are generated in situ using a converging beam illumination and two-beam coupling recording geometry in a photorefractive barium titanate crystal. Intermediate digital interfacing through an electrically addressed spatial light modulator enables one to introduce digital processing techniques (viz., thresholding) for improving the fringe contrast resulting in a better quality of correlation output required for measurement accuracy and reliability. This also increases the range of displacement measurement. In-plane object displacement being proportional to the separation between the correlation peaks, a measure of it gives the magnitude of object displacement. Both simulation and experimental studies have been performed on double as well as multiple exposure speckle interference patterns. The effect of threshold binarization on speckle correlation has also been studied. The method promises high degree of accuracy and increased range for displacement measurement in real-time. (C) 1998 Elsevier Science B.V.