Abstract
The zero-crossing problem is studied for the purpose of investigating the dynamic properties of speckle produced in the diffraction field by a moving diffuse object under illumination of coherent light. The zero-crossing method is applied to the time-differentiated speckle intensity fluctuations and studied in some detail with respect to the influence of noise and the low-pass filter used for suppressing the noise. From the theoretical and experimental studies, the velocity of the moving object is found to be accurately determined by measuring the number of zero-crossings per second of the time-differentiated speckle intensity fluctuations when both the optical configuration used for producing speckles and the characteristic of the low-pass filter for suppressing the noise are known beforehand.
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