Evaluation of a polarization-sensitive, dual-wavelength wearable photoplethysmography sensor across a range of skin tones.

Significance: High-quality photoplethysmography (PPG) signals are essential for accurate extraction of cardiovascular metrics such as heart rate, heart rate variability, and perfusion index (PI). However, signal degradation for individuals with dark skin tones can compromise PPG quality and pose challenges for equitable sensing. Aim: We developed a dual-wavelength, polarization-sensitive PPG device to assess PI across a range of skin tones. Approach: To evaluate the impact of polarization on PPG signal quality, we recorded PI for co-polarized (polarized illumination and parallel-aligned polarized detection) and cross-polarized conditions (polarized illumination and orthogonally aligned polarized detection) at 655 and 940 nm in participants representing light, medium, and brown skin tone categories. Skin tone classification was based on the individual typology angle values derived from the CIE L*b* color space measurements. Results: At 940 nm, light from the cross-polarized light channel significantly increases PI ( Conclusions: Our results suggest that the cross-polarized condition improves PPG signal quality by reducing the influence of superficial scattering and enhancing deeper vascular signals. This approach may be especially beneficial for individuals with darker skin tones and offers a promising path toward more robust and inclusive physiological monitoring using PPG-based technologies.