You Can Touch it, Turn it, but it isn't Really There


Virtually Yours
by Duncan Graham-Rowe

Source: New Scientist

December 16, 2000.

SURGEONS and designers could soon be manipulating 3D moving images floating in mid-air rather than on computer screens, say engineers at DERA, Britain's soon-to-be-privatised defence research lab. They say designers will be able to extend a car's bodywork just by waving a stylus, and almost instantly see what it looks like--or surgeons could twist a brain scan around to locate an injury.

"We can create a real image floating in 3D space," says Chris Slinger, head of holography at the Defence Evaluation and Research Agency in Malvern, Worcestershire. Working in a joint venture with the Ford Motor Company, DERA says it plans to have its first products based on advanced computer generated holography (CGH) on the market in 2003.

Unlike other techniques, such as stereography or virtual reality, CGH doesn't require people to wear cumbersome headgear to see the image, and prolonged use doesn't lead to any ill effects. Users manipulate images using tools that exist partly as real objects and partly as virtual tools. It is like nothing else we have seen before, claims Slinger.

CGH is based on the same principle as the holograms invented by Dennis Gabor in 1949. A hologram is essentially an interference pattern generated from the object being depicted. When light strikes the hologram it is diffracted, forming a series of wavelets. Interference between these wavelets produces wavefronts that simulate the light that would have come from the original object.

In a normal hologram, the image appears to be "inside" the hologram that's producing it. But with a computer generated hologram it is possible to produce interference patterns that simulate the waves from an object hanging in empty space. This means an image can be projected in front of the screen, Slinger says (see Diagram). In the DERA/Ford device, a mirror focuses the image so that it can be seen from a range of angles. There is a another key difference, too: as well as displaying images of real objects, the CGH system can create 3D images of imaginary objects.

The main problem with previous computer-generated holograms has been that they don't have enough pixels to produce an image of a useful size, says Stephen Benton of the Massachusetts Institute of Technology. Benton says roughly a billion pixels are needed to produce a 3D image. "You very quickly get into gigabytes of image data that you have to compute, store and display. It's a gargantuan feat," he says.

DERA developed the screen on which the hologram is formed. Called an "active tiling modulator", it uses ferro-liquid crystals to create vast numbers of pixels that form a hologram. The system is modular and can be scaled up or down to the required image size. Previously, this had been too complex and computationally demanding, says Benton.

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