That’s down to using a sensor on the same side of your body as the light source. You have to do less precise things like measure backscatter. Which is why I doubt Masimo really had significant research into the specific techniques Apple is using, I guarantee all of Masimo’s hardware goes the more conventional route of shining a light through your body. I do wonder how enforceable their patents in general are, though, considering that the basic idea of a pulse oximeter isn’t covered under patent anymore.I find that the oxygen sensor isn’t very accurate in the ultra. I’ve tested it several times with an actual pulse oximeter and the Apple Watch was always off 2 to 3 units which is significant if you’re measuring for hypoxemia.