A modified form of the linear-transport equation is derived and applied to situations in which the scattering function is strongly peaked in the forward direction. This important case, which includes multiple scattering of light by biological suspensions, is very difficult to handle by use of ordinary linear-transport theory, but quite tractable with the modified equation. The modified equation is a very good approximation to the usual transport equation throughout the scattering medium except in the close vicinity of a δ function (i.e., a unidirectional) source. In the case of no absorption, the modified equation describes the statistics of stiff polymer chains.
Richard P. Hemenger, "Solutions of a modified transport equation for multiple scattering in suspensions of highly anisotropic scatterers," J. Opt. Soc. Am. 64, 503-509 (1974)