Real-time velocity measurement for a diffuse object using zero-crossings of laser speckle

This paper proposes a new method for measuring the in-plane velocity of a moving diffuse object by using the technique of zero-crossings for the intensity fluctuation of spatially integrated laser speckles. The scattered speckle pattern is detected in the diffraction field by a finite-aperture photodetector whose output current is analyzed, after removal of its dc component, by counting zero-crossings. The number of zero-crossings per second for the signal is investigated theoretically and found to depend linearly on the object's velocity. The theoretical results are confirmed experimentally for translational speckles detected by the photodetector having circular apertures of various sizes. The excellent agreement between theoretical and experimental results shows that the new method allows measurement of the velocity of a moving object in real time with good accuracy.