  Collective effects in atom-light interaction is of great importance for
cold-atom-based quantum devices or fundamental studies on light transport in
complex media. Here we discuss and compare three different approaches to light
scattering by dilute cold atomic ensembles. The first approach is a
coupled-dipole model, valid at low intensity, which includes cooperative
effects, like superradiance, and other coherent properties. The second one is a
random-walk model, which includes classical multiple scattering and neglects
coherence effects. The third approach is a crude approximation only based on
the attenuation of the excitation beam inside the medium, the so-called "shadow
effect". We show that in the case of a low-density sample, the random walk
approach is an excellent approximation for steady-state light scattering, and
that the shadow effect surprisingly gives rather accurate results at least up
to optical depths on the order of 15.
