Glasses-Free 3-D From Almost Any Angle

Grid of gratings could create 3-D displays for smartphones and other small devices

A relatively simple way of controlling the direction of light could lead to effective, inexpensive glasses-free 3-D screens for cellphones or tablets that function over a wide field of view, say researchers who developed the technique.

“We designed this technology to be particularly well-suited for mobile applications,” says David Fattal, a researcher in the Information and Quantum Systems Lab at Hewlett-Packard Laboratories, in Palo Alto, Calif. Fattal was speaking during a telephone press briefing on the project, which was described in a paper published in Nature yesterday.

Three-dimensional displays work by delivering images with a slight spatial offset to each of the viewer’s eyes. In the days of movies like Creature From the Black Lagoon, that meant showing overlapping red and blue images, each of which was blocked from one eye by glasses with a red filter and a blue filter. Modern autostereoscopic—or glasses-free—displays use a variety of techniques to deliver dual images, but they often require the viewer to be in just the right spot, and they’re too bulky for something as small as a cellphone screen. Holography can also produce a 3-D image, but currently that technology works too slowly for video.

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Video: Fattal et al, Nature

The HP team started with a standard LED backlight, in which light from LEDs arrayed along the side of a device are delivered through light guides to a plane behind a liquid crystal display. On top of the light guide, the team added a grid of diffraction gratings, each with one of 192 different combinations of pitches and orientations, to act as individual pixels. A portion of the light traveling through the waveguide exits through each of the gratings and is sent in a slightly different direction. By modulating the light going to each grating, the researchers produced a series of images each angled to produce a 3-D picture from a different point of view.

Other displays, such as that of the Nintendo 3DS, produce 3-D images from just two points of view. But this new technique can produce images from 64 points of view. The images are spaced close enough together to allow the viewer to experience a smooth transition from one to another and to see them through a 90-degree field of view. “It’s like having 64 displays running in parallel,” Fattal says. “You actually see the object really extruded up to a centimeter either in front [of] or below the screen of the display.” Someone viewing an image of the globe could see all of the continents simply by moving her head, he says.

“We do want to re-create the feeling that you have a virtual object coming out of the screen in front of you, so we don’t want it to disappear as soon as you tilt your head,” Fattal adds.

The actual prototype the researchers built, which they used to display a 3-D HP logo as well as a video of a turtle swimming, produced still images with 64 points of view and video images for 14. Fattal says it shouldn’t be technically difficult to increase the number of images.

The display was also dim, because the light from each LED is divided among the gratings, so each individual image receives only a fraction of it. Fattal says they used inexpensive LEDs in their prototype, but better LEDs, in which the light is distributed over a narrower range of angles, will help increase brightness. The design allows red, green, or blue light to be delivered to each pixel, removing the need for a typical LCD’s color filters, which would further dim the output. The lack of filters also means the display can be transparent. The team is exploring applying their technique to other systems besides LCDs, although Fattal wouldn’t say what those are.

Raymond Beausoleil, another researcher on the project, says he can’t speculate on when or if such a display would be commercially available. Some sort of static, decorative display might be possible in the short term, he says. “Something that would work at, let’s say, the tablet level would require a fairly significant investment and a lot of engineering,” he points out.

“This is a really exciting technology,” says Gordon Wetzstein, a postdoctoral researcher in MIT’s Media Lab who also devises 3-D displays. He says the HP technology may work best when combined with a sophisticated computer algorithm for creating moving images. “I believe it to be the first step toward practical, glasses-free 3-D displays in small form factors.”

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Video: Fattal et al, Nature

About the Author

Neil Savage, based in Lowell, Mass., writes about strange semiconductors, unusual optoelectronics, and other things. In the April 2013 issue he reports on a breakthrough that could lead to a way to combined CT scanners and MRI machines.

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