Guiding Light

"Three-dimensional moving pictures have not changed much over the decades. Even Disney’s latest contribution, “Meet the Robinsons”, relies on the audience wearing the same kind of polarising glasses that were used more than 50 years ago. Television has moved on a little more than cinema, but not much. Designs exist for screens that feed separate images to a viewer’s right and left eyes, thus achieving the same effect as the polarising specs. Unfortunately, only one viewer at a time is able to benefit, because he has to sit in line with the centre of the screen in order for it to work.

That may be about to change. A new form of display being developed by Ian Sexton and his colleagues at De Montfort University, in Leicester, England, could allow people to watch 3DTV from anywhere in the room, without having to resort to glasses. Better still, it can be modified to allow a whole family to view, rather than just a lone individual—and it might even let different people watch different channels at the same time.

All three-dimensional representations of moving images work by fooling the brain into constructing a 3D perception from two slightly different images arriving at different retinas. The brain can be fooled this way because this is how it perceives depth when confronted with the real world rather than a flat screen.

Existing 3DTV (at least, the sort that does not need glasses) creates the separate images by showing them as alternating lines across the screen. The light from the lines is directed to the appropriate eye using either tiny lenses or diffraction gratings over each pixel (the dots of which the picture is composed). These are arranged so that one set of rows can be viewed only from the right-hand side, while the other can be seen only from the left.

It sounds clumsy, but it works—for a single viewer. What Dr Sexton and his colleagues have done is to adapt the technique to steer each image in several directions at once. The result is that moving images can be seen in 3D by up to four people, even as they squabble about who should sit where on the sofa.

To make this possible, Dr Sexton has added two new gizmos to the equipment. One is a system that tracks the whereabouts of the audience’s heads. The other is a novel diffraction technique known as holographic projection.

The head-tracking system, which was developed by the Fraunhofer Heinrich-Hertz Institute in Germany, works by calculating how far away a head is, based on its apparent size (the direction is obvious, of course). It is also able to identify and monitor the position of the eyes, so it knows exactly where each image needs to be beamed and passes this information on to the holographic-projection system.

The term holographic projection does not, in this case, refer to a holographic image of the sort most people would understand by the term. That would be the ultimate in 3DTV, but is still some way off. Rather, it is a way of projecting beams of light wherever they are needed.

This is done using a special type of liquid-crystal array developed by a Cambridge-based firm called Light Blue Optics, with which Dr Sexton has been collaborating. The alignment of the liquid crystals in the array controls the direction in which light is reflected from them. That alignment is, in turn, controlled by the tracking system. The upshot is that the crystals can steer light from a single pixel towards four separate points corresponding to the places where the viewers’ right or left eyes are. If that pixel were receiving signals from different channels, more than one programme could be shown at once (although the viewers would obviously need headphones for the sound tracks).

It will probably be a few years before you can have one of these at home. Mass production, Dr Sexton reckons, is a decade away. But specialised applications—particularly in medicine—should be much closer. It would help doctors a lot, for example, if the images from endoscopes could be shown in 3D. But that is something you would probably not want to see in your living room."

Source: The Economist