  We develop a transfer matrix formalism for four-flux radiative transfer
models, which is ideally suited for studying transport through multiple
scattering layers. The model, derived for spherical particles within the
diffusion approximation, predicts the specular and diffuse reflection and
transmission of multilayer composite films for diffuse or collimated incidence.
The model shows remarkable agreement with numerical Monte Carlo simulations for
a range of absorption and film thicknesses, as well as for an example
multilayer slab.
