Blue as an Underrated Alternative to Green: Photoplethysmographic Heartbeat Intervals Estimation under Two Temperature Conditions

Abstract

Since photoplethysmography (PPG) sensors are usually placed on open skin areas, temperature interference can be an issue. Currently, green light is the most widely used in the reflectance PPG for its relatively low artifact susceptibility. However, it has been known that hemoglobin absorption peaks at the blue part of the spectrum. Despite this fact, blue light has received little attention in the PPG field. Blue wavelengths are commonly used in phototherapy. Combining blue light-based treatments with simultaneous blue PPG acquisition could be potentially used in patients monitoring and studying the biological effects of light. Previous studies examining the PPG in blue light compared to other wavelengths employed photodetectors with inherently lower sensitivity to blue, thereby biasing the results. The present study assessed the accuracy of heartbeat intervals (HBIs) estimation from blue and green PPG signals, acquired under baseline and cold temperature conditions. Our PPG system is based on TCS3472 Color Sensor with equal sensitivity to both parts of the light spectrum to ensure unbiased comparison. The accuracy of the HBIs estimates, calculated with five characteristic points (PPG systolic peak, maximum of the first PPG derivative, maximum of the second PPG derivative, minimum of the second PPG derivative, and intersecting tangents) on both PPG signal types, was evaluated based on the electrocardiographic values. The statistical analyses demonstrated that in all cases, the HBIs estimation accuracy of blue PPG was nearly equivalent to the G PPG irrespective of the characteristic point and measurement condition. Therefore, blue PPG can be used for cardiovascular parameter acquisition. This paper is an extension of work originally presented at the 42nd Annual International Conference of the IEEE Engineering in Medicine and Biology Society.

Remote physiological monitoring of neck blood vessels

“…For each subject, a region of interest (ROI) in the middle of the neck area was manually selected (avoiding hairy areas for males). Green channel has been widely accepted by researchers in PPG signal analysis 27 . In our study, signal processing was performed using the green channel as it exhibited stronger signal strength compared to the blue channel and more excellent stability compared to the red channel in our study.…”

“…In our study, signal processing was performed using the green channel as it exhibited stronger signal strength compared to the blue channel and more excellent stability compared to the red channel in our study. This may be due to half of the Bayer pattern camera sensor being composed of the green channel, which increased intensity and sensitivity for the green channel 27 . Moreover, compared with blue light, green light penetrates deeper into the skin tissue where blood vessels are more evenly spread, which may contribute to its more extensive modulation depth 28 and resistance to motion artifacts 29 .…”