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Time-varying laser speckle for measuring motion and flow

Laser speckle is a high-contrast, random interference pattern seen when coherent (laser) light is scattered from a diffuse object. If instead of being solid, the illuminated object consists of individual moving scatterers (such as particles in a fluid), the speckle pattern fluctuates randomly. These intensity fluctuations contain information about the velocity distribution of the scatterers. After a brief introduction to laser speckle and its statistics, this paper reviews the possibilities of analyzing speckle fluctuations to obtain information about the velocity of moving scatterers. The account stresses the close connection between time-varying speckle and the Doppler- based techniques of photon correlation spectroscopy and laser Doppler velocimetry. One particular speckle technique, laser speckle contrast analysis (LASCA), is treated in some detail. The final part of the paper is a review of some biomedical applications of time-varying speckle.