As we know oxygen enters the lungs and then it gets is passed on into blood. The blood carries oxygen to the various organs in our body. The main way oxygen is carried in our blood is by means of haemoglobin. During a pulse oximetry reading, a small clamp-like device is placed on a finger, earlobe, or toe.
Small beams of light pass through the blood in the finger, measuring the amount of oxygen. It does this by measuring changes in light absorption in oxygenated or deoxygenated blood. Oxygenated blood absorbs more infrared light and passes more red light while deoxygenated blood absorbs red light and passes more infrared light. The light emitted gets absorbed by the oxygenated blood and the rest of the light is reflected through the finger and falls over the detector whose output data is then processed( I2C communication protocol) and read through a microcontroller.