DIGITAL cameras could be shrunk to half their size if their image sensors were curved, say scientists at Princeton University in New Jersey. By slashing the number of image-correcting lenses needed, the curvy sensors would also make cameras lighter.
Digital cameras produce images by focusing light onto a flat grid made up of hundreds of thousands of tiny light-sensitive components. Photons striking this charge-coupled device (CCD) produce an electrical charge proportional to the energy, and so brightness, of the incoming photons.
To get a high-quality image, the light must first be passed through a complex series of lenses to correct for spatial and colour distortions that arise in the first-objective-lens of the camera, which actually focuses the image. The image becomes curved as it passes through the lenses, so it has to be “flattened out” by subsequent lenses to form an undistorted image on the flat CCD. “It can take between 6 and 12 lenses to correct for this,” says Wagner.
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But Wagner says the system could be much simpler if the CCD could be made curved. So he and his colleagues took a thin plastic sheet of kaptan, a polyamide fluoride polymer, and sprayed a pattern of silicon nitride “islands” onto it. On top of each island, they built up a silicon layer in which they grew standard image sensors. To lay the foundations for electrical connections between neighbouring islands, Wagner then patterned the surface with an elastic polymer mixture called a photoresist.
Next, Wagner took the plastic sheet with its islands of components, flipped it over and strapped it to a container with a hole in it. Forcing air through the hole while heating the sheet inflated it into a curved surface. The photosensitive elements had been laid out on the flat surface to take account of the spreading and distorting effect of inflation, so they formed a regular grid.
To make connections between the separate islands on the inside of the curved surface, Wagner sprayed aluminium onto the sheet. “You can then dissolve the polymer photoresist, which lifts off the metal where you don’t want it and leaves you with metal connections in the right places,” he says.
“With these spherical CCDs, we can get a factor-of-two reduction in every dimension of the camera,” says Wagner. Although the work was funded by the military as a means of making more compact and lighter surveillance cameras, Wagner says the technology could be incorporated into consumer digital cameras.
“This is a revolutionary suggestion-it’s a wide-ranging and wide-impact idea,” says John Maxwell of Cooke Optics in Leicester, which designs lenses for film makers. Getting camera manufacturers to incorporate the new technology will be hard, he adds, because it will mean radical redesigns.
