An electronic pulse oximeter, which can be clipped on the bridge of the nose, on the forehead, on the earlobe, or to the tip of a finger, determines the concentration in arterial blood.

An electronic pulse oximeter, which can be clipped on the bridge of the nose, forehead, on the earlobe, or to the tip of finger, determines the oxygen concentration in arterial blood. Hemoglobin is the protein complex in blood that is responsible for carrying oxygen around the body. The pulse oximeter actually measures the concentration of hemoglobin in the blood and determines oxygen concentration from this value.

It's cell theory. easy name i would think.