Now, the fitness tracker can directly measure heart rate and oxygen saturation, further expanding the possibility of self-observation. In addition, pulse measurement on the wrist or finger is more convenient than wearing a chest strap.
An optical sensor suitable for measuring heart rate and blood oxygen saturation utilizes the absorption of light by blood, more specifically, the absorption of light by hemoglobin contained in blood.
The fitness tracker measures blood oxygen saturation when using both infrared and red light. The so-called pulse oximetry is based on the fact that the absorption behavior of hemoglobin (Hb) combined with oxygen (oxygenated hemoglobin HbO2) changes. The concentration of these two hemoglobin variants can be determined by measuring the absorption at two different wavelengths. This will give you a hemorrhagic oxygen saturation.
In this application, red light (660 nm) and infrared light (940 nm) are ideal because at these two wavelengths, the absorption behavior of the two hemoglobin molecules is the largest. In contrast to pulse measurements that only consider changes in light absorption, in this case the absolute value of the light absorption of the arterial blood must be measured. In practice, blood oxygen saturation can be expressed by a ratio function (Imin/Imax) of the minimum and maximum detector signals at the respective wavelengths.