Figure 1 from Measurement of arterial oxygen saturation (SpO2) using PPG optical sensor | Semantic Scholar
Sensors | Free Full-Text | Overestimation of Oxygen Saturation Measured by Pulse Oximetry in Hypoxemia. Part 1: Effect of Optical Pathlengths-Ratio Increase
How pulse oximeters work explained simply.
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Pulse oximetry: Understanding its basic principles facilitates appreciation of its limitations - ScienceDirect