Phase analysis method for characterizing absorption in turbid media: theory, simulations, and TiO₂ phantom experiments

Separating absorption and scattering in turbid media remains a major challenge in optical characterization. This work takes a step toward addressing it by applying non-contact estimation of the absorption coefficient (µ_a) within a phase-analysis framework originally developed for extracting the reduced scattering coefficient (µ′_s). The iterative multi-plane optical properties extraction (IMOPE) technique reconstructs diffuse-reflectance (DR) phase from multi-plane intensity measurements, enabling phase-based analysis for µ′_s extraction. Here, the DR model is extended to incorporate sensitivity to variations in µ_a, establishing a route for phase-driven absorption assessment, demonstrating opposing trends for µ_a and µ′_s increments. Monte Carlo simulations and TiO₂-based phantoms with independently controlled µ′_s and µ_a validate the theoretical predictions. This study experimentally validates a phase-based µ_a extraction scheme at λ₁ = 473 nm, with cross-wavelength verification at λ₂ = 632.8 nm, demonstrating strong agreement, 91%, 95% for λ_1, λ₂, respectively, between extracted and designed absorption values. A detailed TiO₂ phantom preparation protocol is provided to support experimental reproducibility. Finally, we present an initial framework for estimating µ_a from the reconstructed phase, laying the groundwork for future quantitative separation of absorption and scattering in turbid media.