In pulse oximetry, a light emitter consisting of red or infrared LEDs shining through a fairly translucent site in the body with good blood flow is used to measure oxygenation in the blood. A photodetector that receives light from the site being measured is located across from the emitter. The pulse oximeter principle works by using this red or infrared light to measure the light absorption characteristics of hemoglobin according to Oximetry.org.
Hemoglobin that is oxygenated absorbs and allows more infrared light and allows more red light to pass through. In contrast, deoxygenated hemoglobin absorbs more red light and more infrared light passes through. The emitter for the oximeter is typically attached to a finger, toe, earlobe, or top of the ear in adults and children. In infants, the emitter is usually placed on the foot, big toe, thumb or palm states Oximeter.org.
Two methods used for sending light through the site being measured are transmission and reflectance. The transmission method is most common. The emitter and photodetector are opposite each other passing light through the measuring site in between with the transmission method. The emitter and photodetector are located adjacent to each other on top of the measuring site in the reflectance method. Light is bounced between the emitter and detector to measure oxygenation. Hemoglobin oxygenation levels are derived by calculating the red to infrared light ratios explains Oximeter.org.