  Radiation dominated accretion disks are likely to be subject to the ``photon
bubble'' instability, which may lead to strong density inhomogeneities on
scales much shorter than the disk scale height. Such disks -- and magnetized,
radiation-dominated atmospheres in general -- could radiate well above the
Eddington limit without being disrupted. When density contrasts become large
over distances of order the photon mean free path, radiative transfer cannot be
described adequately using either the standard diffusion approximation or
existing prescriptions for flux-limited diffusion. Using analytical and Monte
Carlo techniques, we consider the effects of strong density gradients deep
within radiation- and scattering-dominated atmospheres. We find that radiation
viscosity -- i.e., the off-diagonal elements of the radiation stress tensor --
has an important effect on radiative transfer under such conditions. We compare
analytical and numerical results in the specific case of a plane-parallel
density wave structure and calculate Eddington enhancement factors due to the
porosity of the atmosphere. Our results can be applied to the study of
dynamical coupling between radiation forces and density inhomogeneities in
radiation dominated accretion disks in two or three dimensions.
