Usually, a photon is either absorbed or reflected at the surface,
with a probability defined by the surface albedo. If
mc_surface_reflectalways is specified, each photon is reflected and
the albedo is considered by reducing the photon weight. In case of BRDF,
mc_surface_reflectalways is switched on automatically because the
other method is no longer implemented for non-Lambertian BRDFs, due to
implementation and numerical problems. For small albedos, the computational
time is increased if mc_surface_reflectalways is used; however,
the accuracy of the upward radiance (reflected by the surface) is increased
considerably. In case of clouds, however, computational time might be
increased considerably without gaining accuracy.
Arve Kylling
2010-03-10