  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.
