Pulse oximeters in smart devices measure blood oxygen saturation based on which physical principle? Do they work by light absorption, or is there another method? What is their typical accuracy rate?
How does a pulse oximeter sensor work?
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A pulse oximeter sensor actually relies on a simple yet clever physics principle: photoplethysmography (PPG). Essentially, light from LEDs (usually red and infrared wavelengths) passes through your skin and is detected by a sensor after reflecting off the blood vessels underneath. Deoxygenated hemoglobin (deoxy-Hb) absorbs more infrared light, while oxygenated hemoglobin (oxy-Hb) absorbs more red light. The sensor calculates SpO₂ (oxygen saturation) by comparing the absorption rates of these two wavelengths.
While accuracy is typically in the 95–100% range, proper sensor contact, minimal movement, and clean nails are crucial. In my experience, comparing the Pixel 7’s sensor with the Fitbit Sense yielded readings between 97–99%. However, cold weather or motion (like running) can cause slight fluctuations. If you're aiming for precise measurements, make sure your finger fully covers the sensor—otherwise, the results might be misleading.
Infrared and red light (typically at wavelengths of 660 nm and 940 nm) are reflected onto your skin, and the amount of light that passes through is measured. This works on the principle that oxygenated and deoxygenated hemoglobin absorb light at different rates. In my own measurements, readings in the 95-99% range are usually reliable, but errors can occur with moving hands or cold extremities.
Pulse oximeter sensors actually use the principle of light absorption to measure oxygen saturation in the bloodstream. They employ two different wavelengths— infrared (IR) and red light— to compare the light absorbed by different components of the blood. Oxygenated (oxygen-rich) and deoxygenated (oxygen-poor) hemoglobin absorb these lights at different rates. The sensor measures the changes in light passing through your finger (or earlobe) and converts this data into an SpO2 (oxygen saturation) reading using an algorithm.
The accuracy is typically between 95-99%, providing results close to professional medical devices. In my experience, high-quality smartwatches with good sensors (like the Garmin Venu 2 or Polar H10) can sometimes give inaccurate readings due to wrist thickness, but fingertip models usually deliver consistent results. I tested it a few times and was impressed to see SpO2 values changing in sync with my pulse— really impressive stuff!