Time-domain diffuse correlation spectroscopy in samples composed of particles moving at different speeds. (a) The light emerging from the source is multiply scattered from slowly (orange), and rapidly (red) moving particles inside the sample, and then reaches the detector. As the scattering particles move, the photon light paths change over time (solid and dashed lines), leading to intensity fluctuations at the detector (b). (c) The larger the particle speed, the more rapid intensity fluctuations. Thus, the intensity autocorrelation function (ACF) of the rapidly moving particles decays faster (red line) than that of slow particles (orange line). However, the total ACF (black line) contains contributions from all photons. We disentangle these contributions using a multi exponential fitting. As the time-of-flight increases (late time gate), we register lower number of correlated photons. Hence, the intensity autocorrelation contrast decreases.