PPG (Photoplethysmography)

Photoplethysmography (PPG) is an optical measurement technique that detects blood volume changes in the microvascular bed of tissue. It works by illuminating the skin with a light source and measuring the amount of light that is absorbed or reflected. The variations in light intensity correspond to cardiovascular pulse waves, enabling the measurement of heart rate, blood oxygen saturation, and other physiological parameters.

PPG is the technology behind pulse oximeters, smartwatches, and fitness trackers. Contact-based PPG sensors typically use green or red/infrared LEDs and a photodetector placed against the skin, usually on the fingertip, wrist, or earlobe. The pulsatile component of the PPG signal (AC component) reflects cardiac activity, while the baseline (DC component) reflects tissue composition and venous blood.

Remote photoplethysmography (rPPG) extends the principles of PPG to contactless measurement using a camera instead of a contact sensor. While contact PPG provides stronger signal quality, rPPG offers the advantage of zero-friction measurement without any devices or skin contact, making it suitable for telehealth and large-scale screening applications.