The use of a pulse oximeter allows pilots to monitor the oxygen saturation of hemoglobin in their blood. How a pulse oximeter works is pretty ingenious. Rather than directly measuring hemoglobin concentration, the small device, which costs about $40 and can be purchased online, takes constant measurements of the absorption of specific wavelengths of light in oxygenated hemoglobin. Blood is red due to the quality of oxyhemoglobin it contains, causing it to absorb light of certain wavelengths. The pulse oximeter incorporates two LEDs, one red and one infrared. Infrared light is absorbed by the oxyhemoglobin and red light by the reduced hemoglobin. The oxygen saturation of the blood determines the degree of light absorption, and the result is shown on a digital display of oxygen saturation on the pulse oximeter's screen.