This paper introduces a mathematical model that integrates the optical and physiological properties of skin reflections to enhance the understanding of the algorithmic principles behind remote photoplethysmography (rPPG). The model is used to explain the choices made in existing rPPG methods for pulse extraction, and to design robust or application-specific rPPG solutions. The authors propose a new method called "Plane-Orthogonal-to-Skin" (POS), which uses a plane orthogonal to the skin-tone in the temporally normalized RGB space for pulse extraction. The POS method is compared with other rPPG methods, including Blind Source Separation (BSS) techniques (PCA and ICA), model-based methods (PBV and CHROM), and data-driven methods (2SR). The analysis shows that the POS method leverages the physiological properties of PPG absorption, making it more adaptable to different lighting conditions and skin tones. The paper also discusses the differences between POS and other methods, highlighting the advantages of POS in terms of robustness and adaptability. The performance of the proposed methods is evaluated using a benchmark, demonstrating the effectiveness of the POS method in rPPG applications.This paper introduces a mathematical model that integrates the optical and physiological properties of skin reflections to enhance the understanding of the algorithmic principles behind remote photoplethysmography (rPPG). The model is used to explain the choices made in existing rPPG methods for pulse extraction, and to design robust or application-specific rPPG solutions. The authors propose a new method called "Plane-Orthogonal-to-Skin" (POS), which uses a plane orthogonal to the skin-tone in the temporally normalized RGB space for pulse extraction. The POS method is compared with other rPPG methods, including Blind Source Separation (BSS) techniques (PCA and ICA), model-based methods (PBV and CHROM), and data-driven methods (2SR). The analysis shows that the POS method leverages the physiological properties of PPG absorption, making it more adaptable to different lighting conditions and skin tones. The paper also discusses the differences between POS and other methods, highlighting the advantages of POS in terms of robustness and adaptability. The performance of the proposed methods is evaluated using a benchmark, demonstrating the effectiveness of the POS method in rPPG applications.