SpO2 Sensor

SpO2 sensors use the principle that oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) absorb light differently at specific wavelengths. Oxygenated hemoglobin absorbs more infrared light (940nm) and allows more red light (660nm) to pass through, while deoxygenated hemoglobin absorbs more red light. The sensor alternates red and infrared LEDs at high frequency (typically 100-1000Hz), and a photodetector measures transmitted or reflected light. The ratio of pulsatile (AC) to non-pulsatile (DC) components at each wavelength is calculated, and this R-value is converted to SpO2 using empirically-derived calibration curves validated against arterial blood gas analysis. Normal SpO2 ranges from 95-100% at sea level. Clinical pulse oximeters must meet ISO 80601-2-61 accuracy standards (Arms ≤3% for SpO2 70-100%). Wearable accuracy can be affected by low perfusion states, dark skin pigmentation, nail polish, motion artifacts, ambient light, and peripheral vasoconstriction from cold.