How your smartwatch reads your pulse with light

Your smartwatch's green LEDs shine into the top millimeter of your wrist, where hemoglobin in tiny blood vessels soaks up some of the light. Each heartbeat briefly adds a little more blood, and the light that gets back to the photodiodes dips, usually by under one percent. Software times the peaks: 0.8 seconds apart means 75 beats a minute. It's your pulse the watch reads, not your heart's electrical signal, so movement, a loose strap, some tattoo inks and cold skin can fool it.

1. 0:00 The hook
2. 0:09 The sensor
3. 0:20 Into the skin
4. 0:28 Blood soaks up green
5. 0:39 The heartbeat
6. 0:56 Light becomes a number
7. 1:09 Pulse, not electricity
8. 1:20 What fools it
9. 1:35 Why green
10. 1:50 The payoff