Breakdown of light transport models in photonic scattering slabs with strong absorption and anisotropy

The radiative transfer equation (RTE) models the transport of light inside photonic scattering samples such as paint, foam and tissue. Analytic approximations to solve RTE fail for with strong absorption dominant anisotropic predict unphysical negative energy densities diffuse flux in wrong direction. Here we fully characterize regions three popular a slab, namely $P_1$ approximation (or diffusion approximation), $P_3$ approximation, modification that corrects forward approximation. We find delta function correction eliminates range scattering. In addition, compare predictions these analytical methods exact Monte Carlo simulations physical regions. present maps relative errors albedo anisotropy scatterers realistic index contrast typical polymer slab air optical thickness. error provide guideline accuracy interpret experiments on slabs. Our results show is significantly inaccurate extract parameters unless sample scatters purely isotropic elastic. exceeds terms its moderate absorption, most accurate considered here