Smartwatches measure your pulse using a process called photoplethysmography. This technique detects the extent of blood volume changes in tissue beds, particularly in the skin. Capillaries expand and contract as they transfer blood to your organs, including your skin, and these movements coincide with your heart rate. Smartwatches shine a light on the skin, record the amount of light reflected and how quickly it changes, and calculate your pulse. But why do smartwatches use green light? Because blood is red. No, seriously.
We all know that blood contains hemoglobin, iron-carrying molecules that turn red when exposed to oxygen. But why does hemoglobin appear red? Because it absorbs most of the light and only reflects light in a specific wavelength: red. Red objects absorb green light like crazy, so when the capillaries are full of blood, they gobble up the green light. This makes it easier for smartwatches to differentiate capillaries that are swollen with blood from capillaries that are not. Additionally, green light doesn’t penetrate your skin as deeply as other colors. On the surface, this seems like a drawback, but that’s exactly the problem: keeping measurements on the body’s surface allows smartwatches to analyze data more easily and provide more accurate readings.
Smartwatches use different lights for different measurements
Depending on your smartwatch model and its features, you may notice some other light colors on the underside. Devices like Apple Watches can glow red, and the reason is more or less the same as why they glow green. Well, a little.
For smartwatches to measure your blood oxygen levels, they use a process called pulse oximetry, which shines red and infrared light onto your skin. However, unlike the device’s photoplethysmography feature and its green light, pulse oximetry is not concerned with how quickly the amount of light returned changes, but rather with the overall amount of light returned. This is why red and infrared lights are so important.
As stated previously, hemoglobin consumes wavelengths of green light and reflects wavelengths of red light. However, the strength of a hemoglobin’s red hue and how it interacts with different wavelengths of light depends on the amount of oxygen it contains. Oxygenated hemoglobins have an intense red color that absorbs more infrared light and less red light, while deoxygenated hemoglobins have the opposite behavior. Smartwatches calculate how much red and infrared light is absorbed versus how much is not, and use that data to calculate your blood oxygen level. If you have an Apple Watch that doesn’t include a blood oxygen monitoring app, it’s probably because the device only has green lights capable of photoplethysmography, not the red and infrared lights needed for pulse oximetry.
