FORWARD-SCATTERING RATIOS AND AVERAGE PATHLENGTH PARAMETER IN RADIATIVE-TRANSFER MODELS

Optical properties of films containing spherical particles in a non-ab sorbing matrix have been modelled by a four-flux radiative transfer th eory. In this paper we demonstrate methods to calculate all parameters in this model. Scattering and absorption coefficients can easily be c omputed from Lorenz-Mie theory if the particle concentration is not to o high. Forward-scattering ratios for collimated and diffuse radiation , sigma(c) and sigma(d), respectively, are in general not equal. We es tablish a method to evaluate the forward-scattering ratios from Lorenz -Mie theory and an improved version of Hartel theory, both for perfect ly diffuse isotropic radiation as well as for anisotropic conditions. We also give an explicit way to calculate the average pathlength param eter in terms of particle refractive index, particle concentration, si ze parameter and distance from the illuminated film interface. The cha racterization of forward-scattering ratios and the average pathlength parameter leads to an improved understanding of the applicability of t he standard four-flux model.