All medical-grade blood oxygen sensors that I have seen in hospitals are put on the tip of the finger. This arrangement allows them to beam light through the hu

All medical-grade blood oxygen sensors that I have seen in hospitals are put on the tip of the finger. This arrangement allows them to beam light through the human flesh and measure how much of it is absorbed - the light source is on one side of the finger and the sensor on the other and there is no bone between them at the tip of the finger, just tissue with lots of blood in it. Smart watches are carried on the wrist so they can only measure light reflected back which gives much lower sensitivity and accuracy.