Photoplethysmography

A non-invasive optical technique used to detect blood volume changes in peripheral circulation, commonly implemented in wearable devices like smartwatches and fitness trackers. PPG sensors use light (typically red and green LEDs) to measure how blood flow absorbs and reflects light through the skin, enabling calculation of heart rate, blood volume pulse, and interbeat intervals. In accessibility applications, PPG is used for stress detection, health monitoring, and affective computing systems that can adapt to user physiological states.