Wavelength-Dependency of PPG Morphological Features for Camera-Based Blood Pressure Estimation

Blood pressure (BP) monitoring has become a daily necessity for well-being management. Camera-based BP monitoring has attracted much attention due to its comfort and convenience. It extracts photoplethysmographic (PPG) signals from the facial skin remotely and calculates morphological features for BP estimation. Although green light with strong pulsatility is commonly used, its penetration depth in skin tissues is shorter than that of the near infrared (NIR) light, indicating that the NIR light may contain more distinct PPG features (i.e., dicrotic wave) that could be more beneficial for BP estimation. In this study, we investigated the wavelength-dependency of the camera-PPG waveform for BP measurement. In particular, two morphological features (K-value and Augmentation Index) from the narrow-band green (550nm), red (660nm), and NIR (850nm) channels were measured and compared for BP calibration. Experimental data were collected from 20 healthy adult subjects using the ice water stimulation protocol. The results show that the camera-PPG waveform features have clear wavelength-dependency. The NIR channel that has deeper skin penetrability (than the green channel) but higher pulsatility (than the red channel) gives more descriptive morphological features related to BP. The conclusions drawn from this study inspire the selection of wavelength for PPG morphologv-based BP measurement.