Depolarization of light in a multiply scattering medium: Effect of the refractive index of a scatterer

Abstract

We report the results of a study carried out to investigate the influence of the refractive index and size parameter of a scatterer on the depolarization of linearly and circularly polarized light in a turbid medium. The results show that for a given refractive index of the surrounding medium, the influence of the refractive index of the scatterer on the depolarization of both linearly and circularly polarized light is rather weak for samples with smaller-sized scatterers (Rayleigh scatterers, radius a<<lambda , anisotropy parameter g < or =0.2 ). For a given value of optical thickness (tau= mu(s) xd , mu(s) being the scattering coefficient, d the physical thickness), the depolarization of circularly polarized light was observed to be higher than that of linearly polarized light for these samples. In contrast, for samples prepared using larger-sized scatterers (Mie scatterers, a > or =lambda , g > or =0.7), linearly polarized light was observed to depolarize much faster than circularly polarized light when the refractive index of scatterers was large (n=1.59) but no appreciable difference in depolarization of linearly and circularly polarized light was observed when the refractive index of scatterers had a lower value (n=1.37) . Further, for scattering samples having Mie scatterers, for comparable values of tau and g , depolarization of polarized light was much higher for samples with scatterers of lower refractive index.

Turbid medium polarimetry in biomedical imaging and diagnosis

“…This particular scheme of measurement has been employed by us and many others to measure Stokes parameters of light transmitted (or back-scattered) from tissue and tissue-like turbid medium [20][21][22]. Note that this method does not employ any polarization modulation scheme.…”

“…Suspension of Intralipid or aqueous suspension of polystyrene microspheres has usually been used to prepare tissue phantoms for polarimetric studies [18][19][20][21][22]. The usual approach has been to use a chosen size of scatterers that would give a value of anisotropy parameter (g) comparable to that of tissue.…”

“…We shall begin with a simple single scattering theoretical treatment based on Mie theory, developed previously by us [20,21] to understand the mechanism of depolarization of light due to scattering. Mie theory provides exact solution of the vector wave equation for scattering of incident plane wave (light) by any spherical scatterer embedded inside a homogeneous medium [24].…”

“…Since it is difficult to quantify the morphological parameters of a complex system like tissue, in most of the experimental depolarization studies, the individual influence of these morphological parameters has been systematically investigated on well-characterized tissue simulating phantoms [6,[18][19][20][21][22]. Suspension of Intralipid or aqueous suspension of polystyrene microspheres has usually been used to prepare tissue phantoms for polarimetric studies [18][19][20][21][22].…”

“…These are supplemented by suitable theoretical treatments (based on radiative transport theory, diffusion theory and Monte Carlo techniques) for modeling polarized light propagation in random medium [10][11][12][13][14][15][16][17]. Numerous studies have also addressed depolarization studies on tissue simulating phantoms (whose constituent scattering and polarization properties are known and user-controlled a priori) to understand the polarization properties of multiply scattered light (and the mechanism of depolarization) in tissue for practical implementation of the polarization gating scheme for tissue imaging [18][19][20][21][22]. Note that for the realization of the polarimetric approaches for quantitative tissue diagnosis also, one needs to develop similar experimental turbid polarimetry systems, theoretical treatments for forward modeling of complex tissue polarimetry events, inverse analysis methods for extraction and quantification of the constituent intrinsic tissue polarimetry characteristics.…”