Effective phase function for light scattered by disperse systems - the small-angle approximation

We focus on the quasi-ballistic regime of the scattering process induced in a diffusive medium by a collimated laser beam. The main goal is to find an approximate analytic solution for the radiative transport equation. Particles with characteristic dimensions larger than the light wavelength and relative refraction indices close to unity have a scattering phase function strongly peaked in the forward direction. This specific angular dependence was used to approximate, for any scattering order, the angular dependence of the scattered light fluxes and to obtain simple analytic expressions. Even more, in the small-angle approximation the scattered far field can be described by an effective phase function given by the same analytic formula as the single-particle phase function.