Working principle of pulse oximeter

The pulse oximeter uses non-invasive infrared measurement technology to measure the oxygen content in fingers, toes and ears, which is the most common place to measure blood oxygen. The more accurate name of the measurement object is oxygen saturation, namely SpO2, and the measured results are expressed in digital form. The main display is the ratio of total oxygen saturation under the actual oxygen content, which is generally expressed in percentage. The typical oxygen saturation of human health is 90~100%, But the minimum can also exceed 60%. The oxygen saturation of the human body depends on many factors, the most important of which is the poor blood supply ability of the patient's body, and the reading value of HbO2 will decrease.
working principle
By sequentially driving a red LED (660nm) and an infrared LED (910nm), the blue line indicates the sensitivity of the receiving tube to reduced hemoglobin when hemoglobin does not carry oxygen molecules. From the curve, it can be seen that the absorption ratio of reduced hemoglobin to 660nm red light is stronger, while the absorption length of 910nm infrared light is weaker. The red line represents the sensing curve of the receiving tube towards oxygenated hemoglobin in red blood cells containing oxygen molecules, with weak absorption of 660nm red light and strong absorption of 910nm infrared light.
In blood oxygen measurement, the difference between the reduced hemoglobin and the oxygenated hemoglobin is the most basic data for measuring oxygen saturation by detecting the difference between the two light absorption of different wavelengths. In blood oxygen testing, 660nm and 910nm are the two most common wavelengths. In fact, to achieve higher accuracy, in addition to the two wavelengths, there must be an additional wavelength, even up to 8 wavelengths. The main reason is that in addition to reducing hemoglobin and oxygenated hemoglobin, there are also other types of hemoglobin in human hemoglobin. What we often see is carbon weighted oxygen hemoglobin, and more wavelengths are beneficial for better accuracy.
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