Photoplethysmography (PPG) is a non-invasive optical technique used to measure blood volume changes in the microvascular bed of tissue. It works by shining a light, usually from a LED, onto the skin and detecting the amount of light that is reflected back. Variations in blood flow cause changes in the light absorption, which can be analyzed to monitor heart rate and other cardiovascular parameters. PPG is commonly used in wearable devices, such as fitness trackers and smartwatches, to provide real-time health monitoring. It is valued for its simplicity and effectiveness, making it a popular choice for both clinical and personal health applications.