Photoplethysmography (PPG) measures the volumetric change of an organ by measuring light transmission or reflection. It can be used to measure heart rate.
As blood is pumped into the arteries of the body, this causes them to swell slightly before once again returning to their previous state.
By simply shining a light on a patch of skin with an LED light source, the increased pulse pressure will cause a measurable difference in the amount of light reflected back onto or transmitted through to a light sensor. The LED light should be placed in an area where arteries are close to the skin, such as a fingertip, or an earlobe.
The amplitude of this signal is directly proportional to pulse pressure – the higher the peak, the stronger the pulse. Even though the signal can be less apparent if measured on the skin at a point far from the heart (such as a toe), the change in pressure is still enough to expand these arteries to a degree that is measurable.
Each peak in the resulting signal can be identified by a heart rate algorithm that can ultimately determine the amount of time that occurs between each successive peak – giving another measure of heart rate.
Maxim's optical sensors such as the MAX30102, MAX30101, MAX30112, MAX86141 and MAX30110 mesure PPG signals.