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The Spectral Analysis of Dynamic Laser Speckle Patterns Generated by Brownian Particle Suspensions: a Stroboscopic Effect Based Filtering Technique
The power spectrum of the time-varying intensity in the dynamic laser speckle patterns is determined by passing the shifted power spectrum through a low-pass filter which is implemented via the signal integration. The light intensity is modulated sinusoidally to induce the stroboscopic effect which shifts the resonant frequency component of the spectrum to 0Hz. The homodyne dynamic laser speckles generated by the quasi-inelastic scattering of the Brownian motions in colloidal suspensions are investigated.Within the frequency range from 10Hz to 10 kHz used in this work, the bandwidth of the Lorenztian power spectrums is shown to be inversely proportional to the particle size, which is in agreement with the prediction of the dynamic light scattering theory of diffusing particle. The spatial variation observed in the full-field power spectrum maps is caused by the nonuniformdistribution of average speckle intensity and varies with the modulation frequency. However, the bandwidths measured at different locations are found to be intensity independent.
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