Photon migration in a two-layer turbid medium a diffusion analysis

We analyse the diffusion of photons in a two-layer turbid medium, paying specific attention to parameters suggested by the use of lasers for diagnostic purposes in a medical setting. The data produced by such experiments consists of an intensity profile measured on the surface of the medium, which is measured either as a function of time at a fixed distance from the input laser beam, or else as a function of distance keeping the measurement time fixed. In both cases we demonstrate that the presence of a second layer can be detected provided that physical properties of the two layers differ by a sufficient amount, and also show up in calculated values of such parameters as the mean travel time of a photon to the surface of the medium, and the distribution of the maximum depth reached by an arbitrary photon.