Accuracy of the diffusion equation with extrapolated-boundary condition   for transmittance of light through a turbid medium
  The linear intensity profile of multiply scattered light in a slab geometry extrapolates to zero at a certain distance beyond the boundary. The diffusion equation with this "extrapolated boundary condition" has been used in the literature to obtain analytical formulas for the transmittance of light through the slab as a function of angle of incidence and refractive index. The accuracy of these formulas is determined by comparison with a numerical solution of the Boltzmann equation for radiative transfer. 
J.C.J. Paasschens, M.J.M. de Jong, C.W.J. Beenakker