Average path-length parameter of diffuse light in scattering media

We use Monte Carlo simulations to study in detail the propagation of light in a plane-parallel medium containing scattering particles. In particular, we compute the forward and backward average pathlength parameters (FAPP and BAPP, respectively) of four-flux radiative transfer models as functions of the optical depth. Strong dependence on the single scattering albedo and p hase function asymmetry is found for both quantities. In general the values of the FAPP decrease with increasing absorption, whereas the opposite occu rs for the BAPP. A similar effect is produced when changing from isotropic phase functions to phase functions with a large asymmetry in the forward di rection. We present analytical results for the asymptotic values of the FAP P and BAPP as functions of albedo for the particular case of isotropic scat tering. Our results differ markedly from the predictions obtained recently with two multiple-scattering models by Vargas and Niklasson [J. Opt. Sec. A m. A 14, 2243 (1997); Appl. Opt. 36, 3735 (1997)]. The differences found po int out the intrinsic limitations of these models. (C) 1999 Optical Society of America.