Evaluation of a Polarization-Sensitive, Dual-Wavelength Wearable Photoplethysmography Sensor Across a Range of Skin Tones
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Significance
High-quality photoplethysmography (PPG) signals are essential for accurate extraction of cardiovascular metrics such as heart rate, heart rate variability, and perfusion index. However, signal degradation for individuals with dark skin tones can compromise PPG quality and pose challenges for equitable sensing.
Aim
We develop a dual-wavelength, polarization-sensitive PPG device to assess perfusion index (PI) across a range of skin tones.
Approach
To evaluate the impact of polarization on PPG signal quality, we record PI for co-polarized (polarized illumination and parallel-aligned polarized detection), and cross-polarized conditions (polarized illumination and orthogonally aligned polarized detection) at 655 nm and 940 nm in participants representing light, medium, and brown skin tone categories. Skin tone classification are based on the individual typology angle (ITA) values derived from the CIE L*b* color space measurements.
Results
At 940 nm, light from the cross-polarized light channel significantly increases PI (p < 0.05). At 655 nm, cross-polarization presents a statistically significantly enhanced PI (p < 0.05) relative to light from the co-polarized illumination condition, although the magnitude of the improvement decreases with lighter skin tone indication a possible interaction between skin tone and polarization. This improvement is consistent across all skin tones.
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 towards more robust and inclusive physiological monitoring using PPG-based technologies.
