article: It was the famous logo for the Rolling Stones and was part of an experiment by tech start-up Lightvert. Its technology can produce images that appear to be 200m (656ft) high, but which only exist in the eye of the viewer for a fraction of a second. So could we be on the verge of seeing giant digital ads in our cities, similar to those featured in the seminal 1982 sci-fi film Blade Runner? Lightvert certainly hopes so. Its tech, called Echo, works by employing a narrow - no more than 200mm - strip of reflective material fixed to the side of a building. A high-power projector mounted below or above the strip beams light off the reflector directly into the viewer's eye. The image appears momentarily, exploiting what's called the "persistence of vision" effect - the way sparklers seem to leave a trail of light when you wave them around quickly. "Echo technology is being well received by those who have seen it to date," says Daniel Siden, Lightvert's chief executive. "Viewers are curious as to how it works and engage with it in a way you just don't see with traditional screen media." He is hoping that landlords will grab the opportunity to turn their buildings into revenue-generating digital billboards that are huge, yet physically unobtrusive. These are very early days for the firm, but Mr Siden believes outdoor advertising generally is overdue a leap forward in innovation. But would you want a billboard to target adverts at you specifically? That's what happened in Moscow recently. Drivers approaching a digital billboard were shown ads for Jaguar's new SUV (sports utility vehicle), but only if they were driving a different make of SUV. A machine-learning camera recognised the vehicle's make and changed the ad to target those drivers. The billboard image was adaptive, too, showing the Jaguar against a dark background if it was night-time, or manoeuvring through snow if the weather was bad. The ads were developed by Synaps Labs and founder Aleksey Utkin says there are plans to test similar ads in the US this summer. But won't such ads be dangerously distracting? A study on driver distraction by the Swedish National Road and Transport Research Institute found that digital billboards attracted the gaze of drivers for much longer than other signs. Researchers found drivers often took their eyes off the road for more than two seconds - a threshold which other studies have found to be dangerous. Some bricks-and-mortar retailers, such as video games specialist Game, are using augmented reality (AR) tech to bring their shop windows to life. Overlaying moving digital images on to the real world, as seen through a smartphone camera, caught the public imagination after the release of Nintendo's Pokemon Go game. "AR is no longer just a nice-to-see technology - it has a far more extensive commercial application for brands, where results can be tracked and measured," says Richard Corps, co-founder of Ads Reality. "We're seeing a much wider industry use of AR tech in different environments, such as in education as well
summary: Passers-by on a London street were recently amazed to see a fleeting image of a pink tongue protruding from fruitily plump lips, seemingly suspended in mid-air.