Note the bright red light coming from one side of the probe in Fig 1.

Question: What is this red light for?

Answer:

All pulse oximeter probes have light emitting diodes (LEDs) which shine two types of red light through the tissue.

The sensor on the other side of the tissue picks up the light that is transferred through the tissues.

The oximeter can then determine the SpO₂ of arterial blood in the peripheral circulation.