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 0？Hz. 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 10？Hz 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 nonuniform distribution of average speckle intensity and varies with the modulation frequency. However, the bandwidths measured at different locations are found to be intensity independent. 1. Introduction The frequency spectrum of the dynamic laser speckle pattern, which fluctuates with time in response to the motions of the scattering centers, can be used to reveal the nature of the scatterer dynamics. A well-known example is the Doppler broadening of the scattered light by the Brownian motion of particles in colloidal suspensions. The frequency shifts generated by quasi-elastic light scatterings, however, are too small to be effectively detected by the traditional optical filtering method that is based on diffraction gratings, interferometers, or molecular filters. Optical mixing techniques, on the other hand, work well with the slower dynamic process which has a characteristic time longer than 10？6？sec [1]. Optical mixing techniques, such as photon correlation spectroscopy and laser Doppler velocimetry, have been adopted for a wide array of applications in biomedical and tissue optics [2–10]. Inasmuch as photon correlation spectroscopy and laser Doppler technique are based on the autocorrelation functions or their Fourier transforms, sufficiently fast sampling rates are needed to meet the Nyquist criterion in the frequency domain. In the context of blood flow imaging applications, a minimum frame rate of ~20？kHz is often needed to measure the flows in arterioles, and hundreds and more frames must be recorded to reach a spectral resolution of ~5？Hz [11, 12]. For full-field applications, the requirements on sampling and recording demand the use of high-speed multiple-channel detectors and
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