Radiative Transfer and Flux Theory
  The fundamental notions of radiative transfer, e.g., Lambert's cosine rule, are studied from the point of view of flux and stress theory of continuum mechanics. For the classical case, where the radiance is distributed regularly over the unit sphere, it is shown that Lambert's rule follows from a balance law for the transfer of radiative power in each direction $\norb$ of the sphere, together with the appropriate Cauchy postulates and the additional assumption that the corresponding flux vector field $\radvec_{\norb}$ be parallel to $\norb$. An analogous theory is presented for the irregular case where the distribution of radiance on the sphere is given as a Borel measure. 
Reuven Segev and Joe Goddard
math-ph math.MP