The intensity fluctuations of scattered laser light are reinvestigated. The objective is to show that useful spectral information can be obtained by correlation of the recurrence rate of a fixed, arbitrary intensity level. In developing the concept, we first analyze the arrival and departure statistics of the scatterers in certain light-scattering experiments. Kolmogorov’s forward equation is solved for the transition probability of the scatterer occupation number in the detection volume. It then becomes apparent that in the absence of configuration periodicities the scattered light intensity of a many-scatterer system is a process with statistically independent increments. It reduces to a sequence of independent pulses when the occupation number fluctuates between zero and one. All intensity levels recur at exponentially distributed time intervals, the mean recurrence rate depending on the level. Furthermore, the recurrence rate autocorrelation function becomes an important and easily accessible observable. In the absence of scatterer configuration periodicities it equals the scatterer velocity correlation function times a constant factor. Several experimental examples utilizing commerical digital correlators are presented which demonstrate the application of recurrence rate correlation. A case of continuing interest is the measurement of turbulence spectra in fluid flow.
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