  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.
