INPLANE 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 gener ated in situ using a converging beam illumination and two-beam couplin g recording geometry in a photorefractive barium titanate crystal. Int ermediate digital interfacing through an electrically addressed spatia l light modulator enables one to introduce digital processing techniqu es (viz., thresholding) for improving the fringe contrast resulting in a better quality of correlation output required for measurement accur acy and reliability. This also increases the range of displacement mea surement. In-plane object displacement being proportional to the separ ation between the correlation peaks, a measure of it gives the magnitu de of object displacement. Both simulation and experimental studies ha ve been performed on double as well as multiple exposure speckle inter ference patterns. The effect of threshold binarization on speckle corr elation has also been studied. The method promises high degree of accu racy and increased range for displacement measurement in real-time. (C ) 1998 Elsevier Science B.V.