​The tissues and organs in the human body are maintained by the oxygen provided by the breathing and circulatory system. Through the detection of the blood oxygen concentration in vitro, it is possible to indirectly know whether the body's oxygen supply is sufficient, especially for middle-aged and elderly patients who are chronically ill. As long-term lack of oxygen leads to rapid deterioration of body functions, the detection of blood oxygen concentration can be used as a reference value to provide important vital signs.