Spectral analysis of dynamic laser speckle patterns by using a full-field temporal modulation method

We introduce an approach for analyzing the power spectrum of the time-varying intensity in the dynamic laser speckle patterns. The method is free from the limitation imposed by the Nyquist criterion on the minimum sampling rate. The temporal modulation shifts the intensity fluctuation spectrum by an amount that is equal to the modulation frequency. The subsequent integration of the intensity signal, performed at each detector pixel, acts effectively as a low-pass filter allowing the extraction of the spectral component at the modulation frequency. Within the speed range from 1 to 10 mm/s investigated in our experiment, the result demonstrates that the moving speed of the diffuse object can be determined with a good accuracy.