Working principle of nail oximeter: By driving a red LED(660nm) and an infrared LED(9 10nm) in turn, the blue line shows the response curve of the receiving tube to reduced hemoglobin when hemoglobin has no oxygen molecules. As can be seen from the figure, reduced hemoglobin has a strong absorption rate of 660nm red light, but a weak absorption length of 9 10nm infrared light. When the red line represents hemoglobin and red blood cells containing oxygen molecules, the absorption of 660nm red light is weak, and the absorption rate of 9 10nm infrared light is strong. In blood oxygen measurement, the difference between reduced hemoglobin and oxygenated hemoglobin is the most basic data to measure blood oxygen saturation by detecting the difference of light absorption at different wavelengths. In the blood oxygen test, the two most common wavelengths, 660nm and 9 10nm, actually need to achieve higher accuracy. In addition to two wavelengths, it needs to be increased, even up to eight wavelengths. The main reason is that besides reduced hemoglobin and oxygenated hemoglobin, there are other hemoglobin in human body, and what we often see is carboxyhemoglobin. More wavelengths are good for your accuracy.
Image source: Kangshang wrist oximeter