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If you own a smartwatch, chances are you’ve taken the device off your wrist and noticed the underside flashing green for a second or two. It doesn’t matter if it’s an Apple Watch Ultra 3 or a budget alternative device like the Amazfit Active Max; they all use green lights because it makes it easier to detect your pulse.
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. Smart watches shine a light on the skin, recording the amount of light returned and how quickly the amount changes, and calculate numbers to give 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 do we see hemoglobin in 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 surface of the body allows smartwatches to easily analyze data and provide more accurate measurements.
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, they use a process called pulse oximetry, which shines red light and infrared light onto your skin. However, unlike the photoplethysmography feature of the device and its green light, pulse oximetry is not concerned with how quickly the amounts of light returned change, but rather with the amount of light returned in general. 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 uses 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.
