The backscattering of polarised light from turbid media - An analysis of the azimuthal intensity variations and its implications for the position of the source of diffusing radiation

TL;DR

This study investigates azimuthal intensity variations in backscattered polarized light from turbid media, developing a model that matches experimental data and offers insights into the source position of diffusing radiation.

Contribution

The paper introduces the first quantitative model combining Mie theory and Monte Carlo simulations to match experimental backscattering patterns in turbid media.

Findings

Good agreement between model and experimental contours

Reflected snake photons originate within about 4 times the transport mean free path

Patterns can probe the assumptions of the diffusion approximation

Abstract

We study the azimuthal variation in the backscattered intensity that is seen when polarised light is scattered by a turbid medium. We present experimental observations of these intensity variations in colloidal suspensions over a wide range of optical densities. For a medium composed of spherical scatterers, we have developed using Mie scattering theory and Monte Carlo simulations of photon transport, a model which calculates the constant intensity contours of the backscattered intensity. Comparisons of calculated and experimentally obtained contours show very good agreement. To our knowledge, this is the first model to provide a quantitative comparison with experimental data. Close to the exact backscattering direction, where we have made our intensity measuements, we show that the patterns are formed by what we have called `reflected snake photons'. These are photons that have been backscattered once and have maintained their direction of propagation thereafter until they exit the medium. We also find that the reflected snake photons originate within a depth of about $4l^{*}$ from the point of entry of the incident beam, where $l^{*}$ is the photon transport mean free path. Further, in a novel approach, we have used these patterns as a probe of the assumptions underlying the diffusion approximation and present new results on the position of the apparent source of diffusing radiation within the medium. Possible applications are also discussed.