Diffusion approximation revisited.

We study the diffusion approximation (DA) to the radiative transport equation (RTE) in infinite homogeneous space. Different definitions of the reduced intensity I(r) that satisfy a simplified RTE (without accounting for scattering) and that are often used in the derivation of the DA are examined. By comparing the results of the DA with exact solutions to the RTE, we come to the conclusion that the best accuracy in the DA is achieved if we choose the definition of the reduced intensity (from a family of possible definitions) that results in I(r)=0. Thus, the separation of the specific intensity into reduced and diffuse components is unnecessary. We also discuss the conditions under which the DA is applicable.