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 (SpO_2,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. SpO_2,cam estimates in Vis were based on the normalized ratio of camera-based PPG amplitudes in red over green light (RoG). SpO_2,cam in Vis was validated against contact SpO₂ (reference) and compared with SpO_2,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 SpO_2,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.