Apple Watch promises to be the most personal device you’ve ever strapped on by keeping track of everything from your movement to your heart rate, 24 hours a day.
Mixing high tech with high fashion can be tricky, but in a new support document, Apple details how it uses what’s known as “photoplethysmography” to track your heart rate by flashing green light at your veins.
Here’s how Apple Watch’s heart rate tech works:
“Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate.”
Apple Watch can also used infrared light to measure pulse. The watch switches to infrared checks every 10 minutes when you’re not working out. If the readings aren’t accurate it switches back the green LEDs. It can also increase the brightness and sampling rate if signals are low.
To get an optimal reading, Apple recommends you should fit the device snugly but comfortably. If your Apple Watch band is too loose the sensors won’t be able to detect your pulse.
Other factors that could affect heart rate measurements include skin perfusion, which is how much blood flows through your skin. Skin perfusion varies significantly from person to person. Working out in the cold and motion can also affect measurements.