RGB-Only rPPG for Contactless SpO2 Estimation: A Critical Review of Optical Constraints, Calibration Dependence, and Clinical Validation

This review focuses exclusively on RGB camera-based remote photoplethysmography (rPPG) for contactless estimation of peripheral oxygen saturation (SpO2). Unlike heart-rate estimation, which mainly depends on recovering pulse periodicity, SpO2 estimation requires oxygenation-sensitive optical contrast, stable channel-wise AC/DC information, and calibration and validation against a physiological reference. The reviewed literature is organized around the main stages of an RGB-only SpO2 pipeline. Existing methods are further discussed as ratio-based or Beer–Lambert-inspired approaches, classical regression models, deep learning and spatiotemporal methods, and hybrid or physics-informed designs. This critical review shows that RGB-based systems can estimate oxygenation-related signals in controlled settings. However, current evidence is limited by device-dependent camera responses, narrow oxygen saturation ranges, weak validation, limited demographic reporting, and insufficient hypoxemia testing. The main conclusion of this review is that RGB-only SpO2 estimation is useful under controlled conditions, but it should not be treated as an absolute, clinically interpretable SpO2 measurement method without stronger validation. Future work should focus on wider SpO2 distributions, transparent calibration procedures, cross-device testing protocols, subgroup reporting, and a clear distinction between oxygenation trend monitoring and clinically interpretable absolute SpO2 measurement.