Principle of non-invasive blood oxygen saturation detection

Functional oxygen saturation is often used clinically to reflect changes in oxygen levels in the blood. The non-invasive blood oxygen saturation measurement is based on the principle that the absorption of light by arterial blood varies with arterial pulsation. Basic research shows that oxyhemoglobin and non-oxyhemoglobin have different absorption rates for incident light at different wavelengths. When monochromatic light illuminates the human body vertically, the absorption of light by arterial blood will change with the pulsation of the arteries in the light-transmitting region, while the absorption of light by other tissues such as skin, muscle, bone and venous blood is constant. When the finger is illuminated with constant light λ1, λ2 of two specific wavelengths, if the incident light wavelength λ1 is appropriately selected (HbO2, Hb has an equal absorption characteristic here, that is, about 805 nm), the Lambert-Bear law is applied and according to the oxygen saturation. The definition can be derived from the approximate formula for arterial oxygen saturation:

SaO2=a bQ

Where: Q is the ratio of absorbance change of two wavelengths (HbO2, Hb), a, b is a constant, and is related to the instrument sensor structure and measurement conditions.

It is important to note that biological tissue is a complex optical medium with anisotropic, strong scattering and weak absorption. Therefore, it cannot be described by a strict formula in actual measurement, so it is generally determined by measuring the ratio of the change of the absorbance of the two beams. The calibration curve finally obtains oxygen saturation. When the wavelength of the two beams is selected, the wavelength of the incident light is generally selected to be 660 nm and 940 nm.