Impact of optical wavelength on the reliability of photoplethysmography-based heart rate measurements outside of controlled laboratory environments
Authors: Meier Manuel , Holz Christian 
Published: February 2024

Introduction
The effectiveness of heart rate (HR) measurements via photoplethysmography (PPG) depends on the wavelength of light used. Typical PPG sensors use green, red, or infrared light, each penetrating the skin to different depths (Ray et al., 2021). Here, we present a comparative analysis of the reliability of HR measurements using these wavelengths.
Methods
Our study collected a dataset of 16 participants, each wearing four PPG sensing devices placed at the forehead, sternum, ankle (supramalleolar), and wrist. Each device continuously recorded reflective PPG signals, whereas the device at the sternum additionally captured the Lead I ECG for reference. During the 13 hours of capture, participants went on an outdoor trip from downtown Zurich to the Jungfraujoch railway station at 3,460 m above sea level in the mountains. PPG measurements were obtained using a state-of-the-art MAX86141 optical analog front-end (AFE) coupled with an illumination module of red, green, and infrared LEDs (SFH7072), along with two photodiodes. The ECG recording was resolved by a biopotential AFE (MAX30003), affixed on the chest with gel electrodes. All devices were synchronized by aligning recorded signals post-hoc (33ms accuracy, Meier & Holz, 2023). The HR was extracted from the ECG via time-domain peak detection and a quotient filter. HR was separately derived from each PPG signal, both by time-domain peak detection and frequency-domain analysis. The HR was computed every 5 seconds for a window of 30 seconds. For comparing wavelengths, measurements were considered if at least one wavelength yielded HR with less than 10% error which corresponds to 95,000 HR measurements across the whole dataset.
Results
HR derived from green PPG was most accurate (median error of 3.8%), followed by infrared (7.2%) and red PPG (9.1%). Given participants activity and movement throughout the 13 hours of capture, calculating HR from green PPG was most accurate 64.2% of the time compared to infrared (21.8%) and red PPG (15.2%). The latter cases, infrared and red PPG resulting in more accurate HR, occurred during periods of moderate and high motion.
Discussion/Conclusion
The results indicate that wearable sensors that derive HR from green light PPG can improve their calculations by incorporating additional wavelengths. Since HR based on green light PPG is accurate at rest, PPG using infrared and red would be most beneficial during periods of moderate and increased motion. This finding demonstrates the suitability of infrared and red PPG beyond pulse oxygenation measurements (SpO2). Future work should investigate methods to optimally combine multi-wavelength PPG into a single HR calculation.
References
Meier, M., & Holz, C. (2023). BMAR: Barometric and Motion-based Alignment and Refinement for offline signal synchronization across devices. Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies, 7(2), Article 69. https://doi.org/10.1145/3596268
Ray, D., Collins, T., Woolley, S., & Ponnapalli, P. (2021). A review of wearable multi-wavelength photoplethysmography. IEEE Reviews in Biomedical Engineering, 16, 136-1551. https://doi.org/10.1109/rbme.2021.3121476