Gaussian and non-Gaussian speckle fluctuations in the diffusing-wave spectroscopy signal of a coarsening foam

All previous applications of diffusing-wave spectroscopy to aqueous foams have relied on the assumption that the electric field of the detected light is a Gaussian random variable and hence that the Siegert relation applies. We test this crucial assumption by simultaneous measurement of both second- and third-order temporal intensity correlations. We find that the electric field is Gaussian for typical experimental geometries equivalent to illumination and detection with a plane wave, both for backscattering and transmission through an optically thick slab. However, we find that the Gaussian character breaks down for point-in-point-out backscattering geometries in which the illumination spot size is not sufficiently large in comparison with the size of the intermittent rearrangement events.