How does a pulse oximeter work?
Lower the oxygen saturation. Watch how the red signal changes compared with infrared.
Try moving from 98% to 80%. Which signal changes more?
No pulse signal: no saturation estimate
Light reaching the detector
Why the dip? More arterial blood during a pulse → more absorption → less light reaches the detector.
Solid line: your setting. Faint dashed line: fixed 98% reference. Both signals are enlarged equally, relative to their own steady-light baseline.
Why two colours, and why a pulse?
Two colours: deoxygenated haemoglobin absorbs more red light at 660 nm than oxygenated haemoglobin. At 940 nm, oxygenated haemoglobin absorbs more than deoxygenated haemoglobin. Their different responses allow an estimate of oxygen saturation.
The pulse: arterial blood volume changes with each heartbeat. More arterial blood absorbs more light, so less reaches the detector. Tissue and non-pulsating blood contribute to the steady background.
The device compares the size of the pulsating change with the steady signal at each wavelength, then compares these two fractions. Real devices convert this ratio into SpO₂ using calibration data.
What the number means: SpO₂ estimates arterial haemoglobin oxygen saturation. It does not directly measure oxygen concentration in tissue, total blood oxygen content or carbon dioxide.
Teaching simulation: illustrative absorption coefficients and an ideal pulse at 72 beats/min. The displayed estimate is calculated from the model’s signal ratio; this is not a validated clinical calibration. Signal changes are enlarged for visibility. Blood vessels are shown through the skin as a schematic cutaway; their pulse is exaggerated and synchronised with the signal dips. The finger and light paths are schematic; the travelling waves do not represent optical wavelength to scale. Red and infrared channels are shown together; real sensors alternate the LEDs. Infrared is invisible and is shown in blue. Real readings can be affected by motion, poor circulation, skin pigmentation and abnormal haemoglobins. Measurement principles · Device limitations.