In addition to the invasive tests in hospitals, most blood oxygen tests are carried out by pulse oximeter. The principle of oximeter is to emit light with specific wavelength through red LED+ infrared LED.
Then the device receives reflected or projected light, which means that the clip-on oximeter uses projection, while wrist devices such as smart watches and smart bracelets naturally do not use projection, so they all use reflection.
But the accuracy of the two is still different. Let's take Apple's Apple Watch smart watch as an example. When we look at various parameters, we can see that Apple's heart rate monitoring function has been certified by the US FDA, but its blood oxygen saturation monitoring has not been certified, and most smart wearable devices have not been certified by the FDA.
This can also be seen in domestic equipment. For example, the ECG analysis of Huawei watch GT 3 Pro has passed the national NMPA Class II medical device certification, but almost no wrist oximeter has passed the certification, even if there is one, it has nothing to do with smart wearable devices. It can be seen that the smart wearable device scheme is still difficult to achieve medical-grade blood oxygen measurement.