While wearing an Apple Watch, you may notice that it suddenly glows green. It’s caused by the optical heart sensor, which measures your heart rate. But why green?
Well, it’s because of a technology called photoplethysmography. And while that may sound complicated, the concept is fairly simple. Blood appears red since it reflects red light and absorbs green light. The heart sensor takes advantage of this by using green LED lights along with light‑sensitive photodiode sensors to measure the amount of blood flowing through your wrist. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. And the green LED light doesn’t glow, it flashes hundreds of times per second.
At that speed, our eyes can’t discern individual flashes and the light simply appears to be solid green. But each flash of the LED serves as a snapshot of your blood flow. Put hundreds of these snapshots together, and the Apple Watch can figure out when your heart is beating and calculate your heart rate. But if you’ve ever felt like the green light is way brighter than usual, there’s a reason for that too. If the heart sensor can’t detect your blood flow due to a weak signal, it’s designed to brighten the green light and increase the flashing speed to compensate.
Making the glow more noticeable than usual. This typically happens if you’re wearing a looser band, or if your wrist is in a certain position that prevents the watch from making contact with your skin. Now, you may notice that the green light isn’t on constantly, despite the apple watch recording your heart rate throughout the entire day. That’s because Apple included a second way to measure heart rate using infrared sensors. These use much less battery than the green LEDs and are designed to take measurements in the background every ten minutes. The problem is they’re not as accurate. So if the watch detects an issue with the infrared sensor’s accuracy, it’ll switch to the green sensor instead.
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