Detection of movement with laser speckle patterns

Abstract

The dynamic behaviour of laser speckle patterns, caused by in-plane motion of a diffusing object under illumination with coherent light, has been used in the past in many applications to measure the displacement and the velocity of moving objects. In this paper, we present a speckle pattern based detector which measures the movement (displacement) rather than the velocity of the object. Two pairs of interlaced differential comb-detectors produce two periodic signals in quadrature, which permit to measure the movement and its direction without ambiguity and at arbitrarily low speed. With two orthogonally oriented detector arrays of this type, two-dimensional movements in the plane can be determined. The output signals of these detectors are of statistical nature. The comb-detectors act as periodic filters to the spatial frequency spectrum of the intensity of the speckle pattern. By appropriate choice of the speckle size and the comb-detector period, the output signals versus displacement are quasi-sinusoidal with statistical amplitude and phase. First and second order statistics of these signals have been investigated to determine the reliability and the accuracy of the measurement. All theoretical results have been verified by numerical simulations and by experimental measurements.