Back in the 1940s, a Dutch physician called John Poel invented a device that changed the nature of heart monitoring. His device is based on the idea that the color of skin changes according to the amount of blood it contains. That’s easy to see in the way people quickly become red faced when exerting themselves,
But it turns out more subtle color changes occur at rest as the heart pumps blood around the body. The ebb and flow of blood through capillary vessels changes the amount of light reflected from the skin. Poel’s device beamed light into the skin and monitored changes in the amount reflected to determine a person’s heart rate. He called the technique photoplethysmography and it has since become a standard in medical settings the world over, with cheap blood oximeters that clip onto a finger now widely available over the counter.
But in recent years, physicians have begun developing remote techniques to measure heart rate—analyzing video recordings of a person’s skin to detect the tiny color changes as the heart beats. In this way, they have learned to measure heart rate in a wide range of lighting conditions using relatively cheap cameras with images taken from a distance.
