  The diffuse intensity propagating in turbid media is sensitive to the
presence of any kind of object embedded in the medium, e.g. obstacles or
defects. The long-ranged effects of isolated objects can be described by a
stationary diffusion equation, the effect of any single object being
parametrized in terms of a multipole expansion. An absorbing object is chiefly
characterized by a negative charge, while the leading effect of a non-absorbing
object is due to its dipole moment. The associated intrinsic characteristics of
the object (capacitance $Q$ or effective radius $R_{\rm eff}$, polarizability
$P$) can be evaluated within the diffusion approximation for large enough
objects. The situation of mesoscopic objects, with a size comparable to the
mean free path, requires a more careful treatment, for which the appropriate
framework is radiative transfer theory. This formalism is worked out in detail
for spheres and cylinders of the following kinds: totally absorbing (black),
transparent, and totally reflecting.
