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TOUCH screens, once the preserve of science museums and ticket machines, have become commonly available on mobile phones thanks largely to the popularity of Apple’s iPhone. Now a novel hand-held device has been developed that can turn an inert tabletop into an interactive touch-screen. It could even end up being projected from a mobile phone.
The device developed by Light Blue Optics, a company spun out from Cambridge University in England, embodies a tiny projector and sensors that allow it not only to cast an image onto a flat surface but also to detect when the image is being touched. This makes it possible to press buttons, move and manipulate virtual objects such as photos and navigate between different screens, all just by touching the projected image.
Surprisingly, the touch-sensitive aspect is not the difficult part to achieve. An invisible infra-red beam is superimposed over the image and sensors detect the reflections that are created whenever and wherever the image is touched. The difficulty is that in order to detect these reflections, the device must be very close to the image it is projecting. That is very difficult to achieve without distorting the image, at least when using the traditional optical components—a light source, a digital-image generator and a system of lenses—found in the hand-held projectors currently on the market.
Light Blue Optics’ device, however, uses a technique called holographic projection instead. It employs the principles of constructive and destructive interference of light, normally used to create holograms, to create high-resolution two-dimensional images. When laser light strikes the surface of a tiny reflective display chip, made from liquid-crystal on silicon, the crystals both diffract the beams and alter the phase of the light waves, creating images that are in focus at all distances from the projector. The advantage of this, according to Chris Harris, head of the company, is that you get much brighter images for the energy expended because none of the light is being filtered out or reflected back during the creation of the image.
It also makes it possible for relatively large images to be projected from a short distance away using the same sort of fish-eye lens used in planetariums to cast images at wide angles. But holographic projection avoids the distortion of fish-eye lenses, enabling a perfectly rectangular image to be cast on a surface that is at an angle to the projector. Because the image is generated in software, errors in the way the image is displayed can be corrected, says Dr Harris. All this means it is possible to combine the projector and the infra-red sensors into a single small device.
Light Blue Optics demonstrated a prototype at a meeting held by the Society for Information Display in San Antonio, Texas, earlier this month. Roughly the same size as a small phone, the device produces an image measuring around 20 centimetres by 15 centimetres (or about the same size as an A5 piece of paper) when placed about seven centimetres above a table. In theory it could generate larger images. But for watching a film on a train, for example, this seems sufficient.