The ultraviolet part of the sunlight can damage the human skin by sunburn depending upon the radiation intensity. For people who are heavily exposed to sunlight (beach tourists, sailors, mountain hikers), it would be useful to know whether the current UV radiation they are exposed to can cause sunburn. Ideally, these people would carry a small measuring device with them that gives them continuous information about the intensity of UV radiation. It can be shown that open source hardware and software, 3D printing and a smartphone app are already sufficient to realize such a UVI measuring device in the form of a wristwatch.
