Collimated light sources in the diffusion approximation

Collimated light sources in turbid media are difficult to describe within the diffusion approximation, because they do not meet the requirement of near isotropy. For precise calculation of light intensities close to the source, alternative descriptions of the light source are necessary. In this paper the transition of collimated light into diffusivity is studied by Monte Carlo simulations. On the basis of these simulations and the diffusion approximation a hybrid approach is designed and used to analyze approaches based on analytic source terms. The influence of boundaries to air is studied. The benefits of increased approximation orders are investigated. It is shown that, even in the presence of strong absorption, the diffusion approach can give satisfactory results if only the source terms are suitably chosen.