Every teenager's dream mobile app? Spots. We have all had them and we all hate them. Particularly when we are in those delicate teenage years when discovering a zit on the end of your nose can feel like the end of the world (or at least the end of your chances at the school disco). One start-up thinks it has something that can help. Scan Z is a device that uses light to analyse skin, taking in various clues about the state of a person's face. That information is then fed into a smartphone app, which its designers say will give an accurate estimate of whether a person is likely to get a spot - and once they do, how long it will be around for. From here, advice is given on the best treatments. The BBC's technology reporter Dave Lee - complete with his own spot - tried out the technology, which comes with a hefty $249 price tag.