How Your Smartwatch Reads Your Heartbeat With GREEN Light

Your watch counts your heartbeat without ever touching your pulse. It does it with a flash of green light.

How can a light shining on your wrist possibly know when your heart beats?

The trick is that blood soaks up green light. The watch shines green LEDs into your wrist and a sensor measures how much light bounces back. Every time your heart beats, a fresh wave of blood floods the vessels under your skin. That extra blood absorbs more green light, so less returns to the sensor. Between beats the blood drains, and more bounces back. The sensor reads that flicker, brighter, darker, dozens of times a second. Each dip is one beat, and counting the dips gives your heart rate.

Green is not random. Blood absorbs green light far more strongly than red or infrared, so the beat-to-beat change stands out. Your watch is not feeling your pulse at all. It is watching your blood blink.

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