Pictures in the air: 3D printing with light

Daniel Smalley has long dreamed of building the kind of 3D holograms that pepper science-fiction films. But watching inventor Tony Stark thrust his hands through ghostly 3D body armour in the 2008 film Iron Man, Smalley realized that he could never achieve that using holography, the current standard for high-tech 3D display, because Stark’s hand would block the hologram’s light source. “That irritated me,” says Smalley, a physicist at Brigham Young University in Provo, Utah. He immediately tried to work out how to get around that.

Smalley’s team has taken a different approach — using a technique known as volumetric display — to create moving 3D images that viewers can see from any angle. Some physicists say that the technology comes closer than any other to recreating the 3D projection of Princess Leia calling for help in the 1977 film Star Wars. “This is doing something that a hologram can never do — giving you an all-round view, a Princess Leia-style display — because it’s not a hologram,” says Miles Padgett, an optical physicist at the University of Glasgow, UK.

The technique, described in Nature on 24 January1, works more like a high-speed Etch a Sketch: it uses forces conveyed by a set of near-invisible laser beams to trap a single particle — of a plant fibre called cellulose — and heat it unevenly. That allows researchers to push and pull the cellulose around. A second set of lasers projects visible light — red, green and blue — onto the particle, illuminating it as it moves through space. Humans cannot discern images at rates faster than around 10 per second, so if the particle is moved fast enough, its trajectory appears as a solid line — like a sparkler moving in the dark. And if the image changes quickly enough, it seems to move. The display can be overlaid on real objects and viewers can walk around it in real space.

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