A Reflectance–based multimodal wearable photoplethysmography (PPG) sensor

This paper presents ChromaSense, a reflectance-based wearable photoplethysmography (PPG) sensor designed for robust monitoring of vital signs, including blood oxygen saturation (SpO2), pulse rate, and respiration rate across diverse skin tones. Conventional PPG-based sensors are susceptible to melanin-induced optical attenuation, which can contribute to measurement error, particularly in individuals with darker skin pigmentation. To address this limitation, ChromaSense integrates spectral colorimetric sensing to estimate skin reflectance and adaptively adjust LED drive current. The system was evaluated on a cohort of 50 participants spanning the Fitzpatrick skin tone scale. For SpO2, the device achieved a root-mean-square accuracy of 1.36% relative to a Food and Drug Administration-cleared reference pulse oximeter (Masimo MightySAT), satisfying the ISO 80601-2-61:2013 standard requirement of ≤3.5%. Bland–Altman analysis showed a mean bias of −0.27% with 95% limits of agreement from −2.91% to 2.36%. For pulse rate estimation, 84% of measurements were within ±10% of the reference values, and for respiration rate estimation, 90% of measurements were within ±5 breaths per minute of the reference values. A one-way analysis of variance (ANOVA) showed no statistically significant differences in SpO2 measurement error across Fitzpatrick skin types (p = 0.518). These results suggest that the proposed multimodal sensing strategy mitigates skin-tone-dependent variability within the evaluated cohort and enables consistent physiological measurement performance across diverse users.