Pulse oximetry based on photoplethysmography imaging with red and green light

Remotely measuring the arterial blood oxygen saturation (SpO2) in visible light (Vis) involves different probing depths, which may compromise calibratibility. This paper assesses the feasibility of calibrating camera-based SpO2 (SpO2,cam) using red and green light. Camera-based photoplethysmographic (PPG) signals were measured at 46 healthy adults at center wavelengths of 580 nm (green), 675 nm (red), and 840 nm (near-infrared; NIR). Subjects had their faces recorded during normoxia and hypoxia and under gradual cooling. SpO2,cam estimates in Vis were based on the normalized ratio of camera-based PPG amplitudes in red over green light (RoG). SpO2,cam in Vis was validated against contact SpO2 (reference) and compared with SpO2,cam estimated using red-NIR wavelengths. An RoG-based calibration curve for SpO2 was determined based on data with a SpO2 range of 85–100%. We found an \(A^{*}_{rms}\) error of 2.9% (higher than the \(A^{*}_{rms}\) for SpO2,cam in red-NIR). Additional measurements on normoxic subjects under temperature cooling (from \(21\,^{\circ }{\text{C}}\) to \(<15\,^{\circ }{\text{C}}\)) evidenced a significant bias of − 1.7, CI [− 2.7, − 0.7]%. It was also noted that SpO\(_{\text{2,cam}}\) estimated at the cheeks was significantly biased (− 3.6, CI [− 5.7, − 1.5]%) with respect to forehead estimations. Under controlled conditions, SpO\(_{\text{2,cam}}\) can be calibrated with red and green light but the accuracy is less than that of SpO\(_{\text{2,cam}}\) estimated in the usual red-NIR window.