Ads in your eye
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.
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?
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