The light sensors on the phones and watches work great but most incident light meters use a diffuser to average the light from different angles; most apps use the camera to calculate UV from pixels. I made an app for android called "Incident light meter" that uses the front light sensor on the phone and the way it averages the light is by having the user move the phone in a circular motion around an imaginary Y axis, while pressing a "measure" button. It works very well but it needs both hands I can't picture trying to average the light surrounding the object with the watch in a wrist. Perhaps someone has a better idea about the ergonomics, the code is very simple. I was working on a prototype for an inexpensive bluetooth device, the size of a dice, that can send the data to the phone but unless I do one of those kickstarter campaigns, social media, etc, I don't see a commercial future to it. One important aspect is the type of battery needed to run the device, and to be honest I would prefer something that runs on AA or without batteries at all rather than to rely on something that needs to be charged every night.
