The solar-powered waterproof device is less than 2 mm thick, 9 mm in diameter, and is designed to be worn for up to two weeks at a time (additional adhesives can be used to reapply it after that). It contains no battery or moving parts, and is claimed to incorporate the world's most accurate UV dosimeter. To get a reading from it, the user just runs an iOS/Android app on their smartphone, then swipes the phone over the NFC (Near Field Communication)-enabled device. The app will tell them whether they've exceeded safe limits for UVA and UVB exposure, either for that day or over time – the device can store up to three months of user data. [Source]