Skin layer of diffusive media

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Abstract
The transport of radiation in the bulk of multiply scattering media is well understood within the diffusion approximation. Such a description does not hold in the skin layers, where the transport mechanism crosses over from free propagation to diffusive propagation, or vice versa. In this work, we examine the effects of the skin layers of optically thick slabs on various quantities, including the angularly resolved diffuse reflection and transmission and the shape of the enhanced backscattering cone. This study is based on the ladder approximation to the multiple-scattering expansion. It does not rely on the diffusion approximation and incorporates a systematic treatment of the internal reflections which take place when the random-scattering medium and the outside have two different optical indices, in an arbitrary ratio m. Many exact results are recovered in the absence of internal reflections (m=1). A systematic approach describes accurately the large-index-mismatch regime (m→0 or ∞), where the improved diffusion approximation is shown to become exact asymptotically. The analytic predictions of the large-index-mismatch approach are compared in a detailed way with numerical results and with the outcomes of previous works.