Pulse oximetry is used in:
Pulse Oximetry is a non-invasive method to measure the oxygen saturation levels during the administration of anesthesia or other medications with the help of a finger, ear, or foot probes.
The pulse oximeter sensor consists of two light-emitting diodes, one to transmit red light (660 nm) and the other to transmit infrared light (940 nm), and a photodetector on the opposite side of the vascular bed. 
The light-emitting diode transmits red and infrared light through a vascular bed such as the finger or ear.

Ref: Grossman’s endodontics, 13th edition, pg no:76
Blood volume
Blood coefficient
Laser wavelength for estimation of blood flow
Probe with diode is used to measure oxygen saturation