RGB and Near-Infrared Light Reflectance/Transmittance Photoplethysmography for Measuring Heart Rate during Motion

Photoplethysmography (PPG) is a simple method to measure various physiological indices, including heart rate (HR). To prevent motion artifacts, the optimal light wavelength for PPG measurements should be selected. However, this countermeasure has not been examined thoroughly. This study addressed PPG robustness against motion artifacts for different light wavelengths and measuring modes to accurately determine HR. Twelve healthy volunteers underwent motion artifact experiments during PPG measurements, in which they were asked to either remain still or wave their hands horizontally or vertically as fast and rhythmically as possible. Reflectance mode blue (RB), green (RG), red (RR), and near-infrared (RNIR) lights and transmittance mode red (TR) and near-infrared (TNIR) lights were evaluated for PPG signals acquired along with electrocardiogram (for reference HR) and hand acceleration measurements. The analysis revealed that the RB and RG PPG modes increased the signal-to-noise ratio by approximately 8 dB compared to TR PPG, and the HR obtained from both did not exhibit fixed or proportional bias, with a Pearson's correlation coefficient above 0.986. Furthermore, RNIR PPG was superior to TR PPG by approximately 4 dB, and its calculated HR did not show fixed or proportional bias, with a Pearson's correlation coefficient of 0.967. The RR, TNIR, and TR PPG modes showed comparable and inferior performance. Therefore, blue and green lights followed by near-infrared light in reflectance mode are the recommended settings to measure HR using PPG. These findings may serve as guidelines for researchers and engineers to improve PPG measurements and devices.