On applicability of the far-field approximation to the analysis of light scattering by particulate media

- Problems of the light scattering theory for densely packed media are analyzed.
- A model of a semi-infinite medium containing randomly oriented clusters is considered.
- The radiative transfer and weak localization are taken into account.
- The numerical modeling results are compared to the laboratory measurements available.
- The far-field concept correctness for scattering by particulate media is discussed.

Problems of the theory of light scattering by densely packed discrete random media are analyzed with the model, considering such medium as a semi-infinite layer composed of randomly oriented clusters. Each of the clusters is assumed to be in the far zones of all other clusters. Under this approach, the numerical solution of the radiative transfer equation and the equation for weak localization of waves yields the characteristics of radiation reflected by the medium. Since in these equations an elementary volume of the medium is assumed to be represented by randomly oriented clusters, the near-field effects, as well as the irregular shape and heterogeneity of the scatterers, are partially taken into account. The model results are compared to the currently available laboratory measurements of the intensity and the degree of linear polarization of nonabsorbing samples (MgO, Al2O3, and SiO2) composed the scatterers smaller than the wavelength in size. The scattering characteristics of the samples with different (though not very high) packing densities are considered. In the frames of the applied model, some of the calculated phase profiles of polarization well agree with the measured ones. This allowed us to estimate the relative concentration of scatterers in the media and their sizes. At the same time, the measured phase dependences of intensity are poorly fitted with the models. This suggests that some scattering mechanisms remained beyond the frames the considered model; these mechanisms noticeably influence the intensity of radiation reflected by the medium, while their effect on the linear polarization is negligible.