Melanin-corrected absolute tissue oxygen saturation estimation via hybrid transmittance-reflectance spectroscopy

Accurate absolute tissue oxygen saturation (StO₂) measurement is clinically important, yet conventional approaches are often degraded by scattering-induced optical pathlength variability and epidermal melanin absorption. We propose a compact hybrid spectroscopy framework that combines transmittance and diffuse reflectance measurements to address both limitations. Transmittance is acquired at thin anatomical sites (e.g., the finger web or earlobe), where tissue thickness defines the measurement geometry and reduces pathlength uncertainty. In parallel, the epidermal melanin volume fraction is independently estimated from diffuse reflectance and incorporated as an explicit correction term in a modified Beer-Lambert-law model for absolute StO₂ estimation. We evaluated the method using Monte Carlo simulations across wide ranges of melanin volume fraction and StO₂. The proposed hybrid approach suppresses melanin-induced bias and improves robustness to pathlength variability, enabling robust absolute StO₂ estimation. This framework offers a practical route to reliable absolute StO₂ assessment across diverse skin pigmentation, particularly when pulse oximetry is unavailable or unreliable.