Evaluation of methods and metrics for identifying scattering regime of dielectric particulate medium

Abstract

The norm for analyzing radiative transfer in randomly dispersed particulate medium is to use the independent scattering assumption, where the radiative characteristics of the system are derived from the superposition of the individual particles' radiative characteristics. This assumption is valid when the particles are sparsely distributed. There are established regime maps that are used to identify if the independent scattering assumption is valid. However, these maps which are based on experimental data have been recently questioned and updates are proposed by studies relying on direct solution of Maxwell's equations, in which scattering coefficients, cross-sections and/or efficiencies of particle ensembles are considered to identify presence of dependent scattering effects. Notably, both directionality of the scattering phenomenon and the presence of coherent scattering occurring on the free-space - particle cloud boundary are overlooked while identifying transition between independent and dependent scattering regimes. This study provides a critical overview of recent numerical approaches and shows that a regime map cannot be drawn relying solely on scattering cross section estimated by electromagnetic field solvers. The dependent scattering effects for dielectric particulate medium can be well represented using static structure factor including the directionality of scattering. Transport scattering coefficient that accounts for both magnitude and directionality of the scattering can be used to represent the radiative behavior adequately. Therefore, it can be used as the single intrinsic characteristic for identifying the transition between dependent and independent scattering, rather than the scattering coefficient and the asymmetry parameter alone.