How smart watch measures heart rate?

Heart rate monitors in smartwatches typically use photoplethysmography (PPG) technology to measure the wearer's heart rate.

PPG is a non-invasive method that uses light to measure changes in blood volume and flow within the body. In smartwatches, this is usually done by shining light from an LED onto the skin, and then detecting the changes in light that result from the pumping of blood through the veins.

The smartwatch typically has sensors that pick up the light reflected from the skin and convert it into an electrical signal, which is then processed to determine the wearer's heart rate. This measurement can be taken continuously, or at specific intervals, to provide a real-time reading of the wearer's heart rate.

Why green LED: Green light has a longer wavelength than red light, which makes it easier to detect changes in blood volume and flow through the skin. This is because the longer wavelength of green light is better able to penetrate the skin and reach the blood vessels, where it can be used to measure changes in blood flow.

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