The Apple Watch (or any other smart watch) should have a built-in sun measuring sensor, noting when people have not enough sun exposure to produce the necessary amount of Vitamin D to be healthy. Once AR is there, they could make a game out of it, showing progress bars at the peripery of the eyes. This could make reaching the targets much easier for a lot more people.
That's fine. As long as the sensor is available the user can expose it. I think this is a pretty cool idea, and would yield more accurate measurements than estimating based off of latitude and cloud coverage alone. Albeit it'll all but guarantee tan lines.
