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Velocimetry of moving phase plates using laser speckle patterns.

On passage through a random phase plate a collimated laser beam breaks up into a speckle pattern which can be imaged on a detector placed some distance away from the phase plate. If the phase plate is moving transversely to the direction of the laser beam, this movement is followed exactly by the speckle pattern. The magnitude of the transverse velocity can be obtained by processing the random signal produced by the detector. Experimental and theoretical investigations show this to be a robust technique for measuring surface or averaged fluid-flow velocities, with possible applications to radar and sonar.