  The present paper discusses the diffusion approximation of the linear
Boltzmann equation in cases where the collision frequency is not uniformly
large in the spatial domain. Our results apply for instance to the case of
radiative transfer in a composite medium with optically thin inclusions in an
optically thick background medium. The equation governing the evolution of the
approximate particle density coincides with the limit of the diffusion equation
with infinite diffusion coefficient in the optically thin inclusions.
