Diffuse Beam : Coherent Backscattering and Localization

On the basis of reciprocity, interference between two opposite paths ean be argued to be constructive in the backscattering direction of, for instance, a slab geometry, and exactly as large as the conventional diffuse background calculated from (1.8). At backscattering, the equation of radiative transfer is thus 100 % wrong! As always, the width of an interference effect is roughly given by the wavelength divided by the typical distance between two typical points of scattering, in this case the mean free path, giving !:1() ::::J l/kf [113]. One can still argue as to what mean free path should be used here: the transport or the scattering mean free path. Although a physical argument favors the first (recall Fig. 1.2), a rigorous eonfirmation for anisotropie scatterers (for which both mean free paths differ) has only been given recently [114, 115]. Thus