The colour of blood is examined by the sensor to understand the oxygen levels present in it. As Fitbit explains, “Deoxygenated blood, which is returned to your lungs via your veins, is a slightly darker red color than the fully oxygenated blood in your arteries. The sensors measure the relative reflection of red and infrared light from your blood via your wrist, and seeing how it varies as your heart beats, the device estimates your SpO2 value.”
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