Photoplethysmography (PPG) is an optical measurement method.

Photoplethysmography (PPG) is a non-invasive medical technology that measures changes in blood volume in the microvascular bed of tissues, such as the skin, using light. It works by shining light into the skin and measuring the changes in light absorption as blood flows in and out of the vessels.

Changes in blood volume are related to cardiac activity, including heart rate and blood pressure, which are key indicators of cardiovascular health.

When light is shone into the skin, it is absorbed by the blood in the vessels and scattered by the surrounding tissue. The amount of light that is absorbed and scattered depends on the amount of blood in the vessels. When the heart beats, blood is pumped into the vessels, increasing the volume and hence the amount of light absorbed. As the blood flows back out of the vessels between beats, the volume decreases, and the amount of light absorbed decreases as well.