How does it work?
Because of an 18th century principle known as Beer’s Law: the relationship between the absorption of light and the concentration of the absorbing substance.
A small probe, attached to a separate computerized unit, is clipped to a spot on the body with good blood flow – typical sites are the finger or the earlobe. Inside the probe is a light emitter that sends out two streams of light, one red and one infrared. Opposite the emitter is a photodetector which receives and measures these streams of light as they pass through.
Different frequencies of light are absorbed in different volumes. Oxygenated haemoglobin absorbs a greater amount of infrared light while allowing more red light to pass through. Deoxygenated haemoglobin absorbs a greater amount of red light while allowing more infrared light to pass through. By calculating the absorption by haemoglobin in the patient’s blood stream at these two different wavelengths, the oximeter can calculate the level of oxygenation present.
This principle is also called spectrophotometry. You can learn more here.