Discriminating between Absorption and Scattering Effects in Complex Turbid Media by Coupling Polarized Light Spectroscopy with the Mueller Matrix Concept

Abstract

The separation of the combined effects of absorption and scattering in complex media is a major issue for better characterization and prediction of media properties. In this study, an approach coupling polarized light spectroscopy and the Mueller matrix concept were evaluated to address this issue. A set of 50 turbid liquid optical phantoms with different levels of scattering and absorption properties were made and measured at various orientations of polarizers and analyzers to obtain the 16 elements of the complete Mueller matrix in the VIS-NIR region. Partial least square (PLS) was performed to build calibration models from diffuse reflectance spectra in order to evaluate the potential of polarization spectroscopy through the elements of the Mueller matrix to predict physical and chemical parameters and hence, to discriminate scattering and absorption effects, respectively. In particular, it was demonstrated that absorption and scattering effects can be distinguished in the Rayleigh regime with linear and circular polarization from the M₂₂ and M₄₄ elements of the Mueller matrix, correspondingly.

Keywords: Mueller matrix; PLS; absorption; complex media; polarization; scattering; spectroscopy.

Figure 1

A set of 50 liquid optical phantoms obtained by mixing methylene blue (MB; absorber), intralipid 20% (IL; scatterer) and distilled water (dilution agent) prepared in this study.

Figure 2

Schematic representation of PoLiS system in reflectance with PSG (polarization state generator), PSA (polarization state analyzer), LP (linear polarizer), QWP (quarter-wave plate) and AL (aspheric lens).

Figure 3

A matrix array showing the polarization measurements necessary to obtain each Mueller matrix element. A two-letter combination corresponds to one measurement. For example, the combination (I_VI_P) means that the PSG is adjusted to obtain linear polarization along the vertical axis (y axis) for incoming light and PSA is adjusted to recover linear polarization with a +45° offset for reflected light. I₀ corresponds to unpolarized light.

Figure 4

Results of r₁₁ (a), r₂₂ (b) and r₄₄ (c) diffuse reflectance spectra for the M₁₁, M₂₂ and M₄₄ elements of the Mueller matrix diagonal, respectively. The yellow arrows (1) and (2) in figures (a) and (b) indicate the two absorption peaks of methylene blue at 609 nm and 668 nm, respectively.

Figure 5

Predicted vs measured absorber concentrations (c_abs) for (a) r₂₂ and (b) r₄₄, and scatterer concentrations (c_dif) for (c) r₂₂ and (d) r₄₄.

Figure 5

Predicted vs measured absorber concentrations (c_abs) for (a) r₂₂ and (b) r₄₄, and scatterer concentrations (c_dif) for (c) r₂₂ and (d) r₄₄.