How does it work?

The colour of blood varies depending on how much oxygen it contains. A pulse oximeter shines two beams of light through a finger (or earlobe etc.), one beam is red light (which you can see when a pulse oximeter is used), one is infrared light (which you don't see).

These two beams of light can let the pulse oximeter detect what colour the arterial blood is and it can then work out the oxygen saturation. However there are lots of other bits of a finger which will absorb light (such as venous blood, bone, skin, muscle etc.), so to work out the colour of the arterial blood a pulse oximeter looks for the slight change in the overall colour caused by a beat of the heart pushing arterial blood into the finger.

This change in colour is very small so pulse oximeters work best when there is a good strong pulse in the finger (etc.) the probe is on. If the signal is too low the measured oxygen saturation may not be reliable and lower than this the pulse oximeter will not be able to work.