Abstract
For in vivo determination of optically active (chiral) substances in turbid media, like for example glucose in human tissue, the backscattering geometry is particularly convenient. However, recent polarimetric measurements performed in the backscattering geometry have shown that, in this geometry, the relatively small rotation of the polarization vector arising due to the optical activity of the medium is totally swamped by the much larger changes in the orientation angle of the polarization vector due to scattering. We show that the change in the orientation angle of the polarization vector arises due to the combined effect of linear diattenuation and linear retardance of light scattered at large angles and can be decoupled from the pure optical rotation component using polar decomposition of Mueller matrix. For this purpose, the method developed earlier for polar decomposition of Mueller matrix was extended to incorporate optical rotation in the medium. The validity of this approach for accurate determination of the degree of optical rotation using the Mueller matrix measured from the medium in both forward and backscattering geometry was tested by conducting studies on chiral turbid samples prepared using known concentration of scatterers and glucose molecules.
Highlights
Studies on polarization properties of scattered light from a turbid medium like human tissue have received considerable attention because the depolarization of scattered light can be used as an effective tool to discriminate against multiply scattered light and can facilitate imaging through tissue [1,2,3]
We show that the change in the orientation angle of the polarization vector arises due to the combined effect of linear diattenuation and linear retardance of light scattered at large angles and can be decoupled from the pure optical rotation component using polar decomposition of Mueller matrix
The method developed earlier for polar decomposition of Mueller matrix was extended to incorporate optical rotation in the medium. The validity of this approach for accurate determination of the degree of optical rotation using the Mueller matrix measured from the medium in both forward and backscattering geometry was tested by conducting studies on chiral turbid samples prepared using known concentration of scatterers and glucose molecules
Summary
Studies on polarization properties of scattered light from a turbid medium like human tissue have received considerable attention because the depolarization of scattered light can be used as an effective tool to discriminate against multiply scattered light and can facilitate imaging through tissue [1,2,3]. A major problem in determining the concentration of chiral substances in a turbid medium, is the fact that unlike dilute solutions, the incident polarized light gets strongly depolarized due to multiple scattering and only a small fraction of the light coming out from the medium retains its initial state of polarization [15,17] Techniques such as polarization modulation and synchronous detection methods have been developed to extract the surviving polarization fraction of polarized light scattered from chiral turbid medium and analyze this for quantification of the degree of optical rotation [19 -21]. Recent studies [22] based on this approach have shown that in the backscattering geometry, the relatively small rotation of the polarization vector arising due to the optical activity of the medium is totally swamped by the much larger changes in the orientation angle of the polarization vector due to scattering This effect was observed to be minimal in the forward scattering direction. It was proposed that in order to avoid errors in concentration determination of chiral molecules in turbid samples, the scattered light in the forward direction should be used [22]
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