One of the challenges in rPPG is the very small signal because only a limited amount of the skin-reflected light has interacted with pulsatile blood. The cardiac-synchronous periodicity of the PPG waveform is used to isolate the absorption of arterial blood from the other absorbers. In contrast to SaO 2 where arterial blood samples have to be taken, SpO 2 enables a non-invasive and continuous measurement using low-cost hardware, a pulse-oximeter, attached to a peripheral such as finger or earlobe. Peripheral oxygen saturation (SpO 2) is the optical measurement of arterial oxygen saturation (SaO 2). the fraction of hemoglobin saturated with oxygen. For (preterm) infants, COPD patients and for the detection of sleep apnea it is important to also monitor the oxygen content of arterial blood, i.e. So far, most attention in the field of rPPG has been given to heart (pulse) rate and derived features, and respiration. Furthermore, compared to RF-based techniques (e.g., radar, WiFi) that only measure the motion signal induced by breathing and beating of the heart, remote photoplethysmography (rPPG) is based on blood absorption and therefore can be used to measure the blood oxygenation level. Using consumer-grade cameras is particularly interesting as it allows a cost-efficient and convenient measurement of physiological information, movements and context information simultaneously. Over the years, various contactless methods for physiological monitoring have been proposed including radar, sonar, thermal imaging, WiFi and regular cameras. Monitoring of physiological parameters without touching the skin is highly attractive, e.g., for people with burned or fragile skin, when there is a serious risk of cross-infection transmission such as with COVID-19, or where the conventional contact-based measurement causes discomfort which could affect its diagnostic value. This study provides important insights on the possible applications and use cases of remote pulse-oximetry with current affordable and readily available cameras. Extrapolation of the results suggests that the error during the most challenging motion scenario can be reduced to approximately 2 percent when using a parallax-free single-optics camera. We found that the amount of parallax has an important effect on the accuracy of the SpO 2 measurement during movement and that the proposed local image registration reduces the error by more than a factor of 2 for the most common motion scenarios during screening. We estimate oxygen saturation values with a previously used global frame registration method and with a newly proposed adaptive local registration method to further reduce the parallax-induced image misalignment. In this paper we investigate the effect of parallax quantitatively by creating a large dataset consisting of 150 videos with three different parallax settings and with realistic and challenging motion scenarios. The introduced parallax by these cameras could however jeopardise the SpO 2 algorithm assumptions, especially during subject movement. Because of the lack of readily available and affordable single-optics multi-spectral cameras, custom-made multi-camera setups with different optical filters are currently mostly used. Camera-based pulse-oximetry enables contactless estimation of peripheral oxygen saturation (SpO 2).
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