Smartwatches use red and infrared light to measure your heart rate and blood oxygen levels. Here's how it works in simple terms:
Red and infrared light react differently when they hit your skin. Oxygenated blood absorbs more infrared light and reflects more red light, while deoxygenated blood absorbs more red light and reflects more infrared light.
When you wear a smartwatch, it shines these lights onto your skin and measures how much light is reflected back. By analyzing these reflections, the device can estimate your heart rate and blood oxygen levels.
As your heart beats, blood flow increases and decreases. The smartwatch detects these changes in blood flow by monitoring the light reflections. This is how it tracks your pulse.
How does a pulse oximeter measure?
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While rummaging through the drawer for a regular charging cable, I stumbled upon my Raspberry Pi sensor kit. Deciding to give it a try since it was going to be a long night anyway, I set up a mini pulse oximeter and clipped it onto my finger. As I monitored the data on the serial monitor, I noticed an interesting graph: the absorption of the red light changed with each heartbeat. Here's how it actually works: the blood in the capillaries absorbs more light with each heartbeat, and the sensor detects this. Infrared light is used as a reference, making it easier to distinguish changes in blood flow.