In this day and age, it’s easy to become blasé about technology. Understanding why a common device works has become less and less important as our gadgets become integrated into our lives and are more reliable than ever but occasionally my curiosity just gets the best of me.
I remember from when I was younger going to visit people in the hospital and seeing the little white plastic clothespin-like thing clamped to a finger with a glowing red light emanating from it that fed heart rate data to the machine that went blip-blip-blip with each beat of the heart.
Fast-forward a few years and now I wear that same technology on my wrist under my Apple Watch. Some sort of sensor sits against my skin and does the same thing. But how does it actually work?
Short answer, long word: they use photoplethysmography, a method of detection which measures differences in light absorption.
The bottom of those watches have a mixture of infrared and green LEDs and light-sensitive diodes that pulse back and forth hundreds of times per second and measure how much light is absorbed or reflected.
Most of the time in order to sense your heartbeat the IR LEDs, which are just on the edge of our visual spectrum, are activated. If they aren’t doing the trick because of sweat or other interference the green LEDs are activated. Why green? On the color spectrum, green is the exact opposite of red and therefore is a more efficient color to use to detect the presence of blood in your veins.
When blood is in your wrist after a heartbeat, it absorbs the light and signals to the watch that a heartbeat is occurring. In between beats, there is less light absorbed due to the absence of blood.
That’s why it’s important that your watch fits snugly to your wrist and is unobstructed.
How accurate is this method? A 2017 study showed that the displayed result was within 5 BPM of an ECG reading 95% of the time so reliable enough to track your activity and workout effectiveness for sure.