When you bought your digital camera, were you told how many million pixels it had? You're always told that more is best: the higher the number of pixels, the better the picture resolution - and the steeper the price. Early digital cameras offered 1m pixels, while the latest boast more than 8 megapixels and counting - expensive models that sales assistants are always keen to sell.

But research being done at Rice University in Texas by Richard Baraniuk and Kevin Kelly promises to turn digital camera technology on its head. Their dream is to have a digital camera with just one pixel, yet lose none of the picture quality.

Compressed data

"Digital cameras are very inefficient in their use of the pixel data they acquire," says Baraniuk. "We wanted to attack some of the inherent inefficiencies in conventional digital photography, in particular the fact that we sample digital images with millions of pixels but then need to compress the resulting data."

Once you've taken a picture, the pixels are represented by millions of numbers - far too many for your camera to store. A battery-hungry microprocessor does some rapid calculations to find fewer numbers to approximate or compress the image. This not only drains your batteries but also throws away most of the original image information. The days of increasing megapixels are now limited, as 10 megapixels roughly equals the resolution of film.

"So you might ask yourself - is there a better way? Well, some new mathematics has popped up over the past two years or so that suggests there is," says Baraniuk.

Over the past year, Baraniuk and Kelly have been testing a new type of digital camera. While the images they've produced appear fuzzy, the way they've been created is nothing short of revolutionary. It involves what they describe as "pretty deep stuff" - new mathematical algorithms for compressed sensing.

As with the JPEG format, compression is the key, but in a bizarre way. To create an image equivalent to that from 5m pixels, you might only need 200,000 measurements. The light levels are recorded and then complex calculations tease the picture out. A weird feature of the maths means the best kind of measurements to take are random ones.

To build their camera, Baraniuk and Kelly used something normally found in digital projectors: a digital micromirror device (DMD). This has 786,432 bacterium-sized metal mirrors which can be electrostatically tilted, and which fit on to a chip about the size of a postage stamp. The number matches the total reconstructed pixels in the final image.

In a digital projector, the microscopic mirrors reflect light to the lens to make a bright pixel on the screen, or are tilted away to leave it dark. The experimental camera's DMD is run backwards by replacing the light source with a photodiode - in effect, the single pixel for the experimental camera - and filters for each colour. A lens focuses the image to be photographed on to the DMD, while another directs the combined mirror reflections to the photodiode.

Only half of the mirrors reflect light onto the photodiode at one time. Mirror selection is entirely random and, says Baraniuk, is like white noise on a untuned television. The light measurements - repeated thousands of times in quick succession - are of the averaged intensity of just the random "on" mirrors and not the whole picture. A computer then extrapolates an image from the data - also working backwards rather like the DMD - thanks to some mathematical magic. "The bottom line is that we can take a picture with potentially millions of pixels but using just a single detector element," says Baraniuk.

So far, their experiments have only involved photographing static objects such as dice, a ball and a coffee mug. The big drawback? Five minutes to take a picture. To reach a stage where their Cyclops camera might challenge an ordinary digital model may take several years of miniaturisation work and the development of faster algorithms.

"The idea is that it would give the same quality pictures but using less power and with potentially less overall complexity," says Baraniuk. "We're not claiming we'll replace conventional digital cameras tomorrow."

Nigel Allinson, professor of image engineering at the University of Sheffield, has concerns about the method because of possible artefacts in image reconstruction, the extra computing power needed and the loss of light (which means less sensitivity in low-light situations). Nevertheless, he reckons that single detectors might be useful for infrared, ultraviolet or x-rays.

Exciting ideas

"Ideas such as the single-pixel camera are exciting. The field has yet to be fully explored since, as always, there will be limitations. The potential of a single detecting element is that you can work much more easily outside the visible part of the spectrum," says Allinson.

The two American researchers are thinking along the same lines, and they're interested in building a terahertz camera for high-resolution imaging. Terahertz radiation will penetrate cloth, paper or plastics to see what lies underneath and has the potential of distinguishing between liquids - something that airport security might find very useful. But should the single-pixel digital camera ever reach the high street, that pushy sales assistant is going to be lost for words.

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