Imaging in turbid media using quasi-ballistic photons

TL;DR

This paper investigates how quasi-ballistic photons can be used to image through turbid media by combining experiments, Monte Carlo simulations, and a novel definition of photon memory, surpassing diffusion theory limits.

Contribution

It introduces a new model for photon propagation based on angular correlation, enabling deeper imaging in scattering media than traditional diffusion theory predicts.

Findings

Polarisation-preserving photons penetrate deeper than diffusion theory suggests.

Monte Carlo simulations validate the new photon definition and model.

Experimental images reveal shadow formation at greater depths than expected.

Abstract

We study by means of experiments and Monte Carlo simulations, the scattering of light in random media, to determine the distance upto which photons travel along almost undeviated paths within a scattering medium, and are therefore capable of casting a shadow of an opaque inclusion embedded within the medium. Such photons are isolated by polarisation discrimination wherein the plane of linear polarisation of the input light is continuously rotated and the polarisation preserving component of the emerging light is extracted by means of a Fourier transform. This technique is a software implementation of lock-in detection. We find that images may be recovered to a depth far in excess of what is predicted by the diffusion theory of photon propagation. To understand our experimental results, we perform Monte Carlo simulations to model the random walk behaviour of the multiply scattered photons. We present a new definition of a diffusing photon in terms of the memory of its initial direction of propagation, which we then quantify in terms of an angular correlation function. This redefinition yields the penetration depth of the polarisation preserving photons. Based on these results, we have formulated a model to understand shadow formation in a turbid medium, the predictions of which are in good agreement with our experimental results.