（Photoplethysmography （PPG））

When a beam of light of a certain wavelength shines on the fingertip skin surface, the contraction and dilation of blood vessels with each heartbeat affects either the transmission or the reflection of the light. As the light passes through the skin tissue and then reflects back to the photosensitive sensor, there is some attenuation of the light. While the absorption of light by muscles, bones, veins and other connected tissues is essentially constant (provided there is no significant movement at the measurement site), arteries are different, and because they are pulsating with blood, the absorption of light will naturally vary. When we convert the light into electrical signals, it is because the arterial absorption of light has changed while other tissues to the absorption of light is basically unchanged, the signal obtained can be divided into DC signal and AC signal. By extracting the AC signal from them, the characteristics of blood flow can be reflected.