Diffusing-wave spectroscopy beyond the diffusive regime: the influence of short light paths and anisotropic scattering

We study the fluctuations of light multiply scattered by particles under Brownian motion in a fluid. We focus on the behavior of the time correlation function of the field in the non-diffusive regime, in both transmission and reflection. In transmission through optically thin systems, an extended Diffusing-Wave Spectroscopy (DWS) model based on the Radiative Transfer Equation (RTE) is described, which predicts substantial deviations from the standard DWS theory. For backscattered light, experiments using unpolarized light show a clear dependence on the anisotropy factor g. This behavior is not described by the standard DWS theory. A good agreement with the data is obtained using the RTE model, and an approximate model in which the path-length distribution of the standard DWS is corrected by a prefactor which depends on the level of anisotropy. These results should have broad applications in diffuse-light biomedical imaging, and in the field of soft-materials and biomaterials analysis.