Prize in the eyes
It's a fact of optics that camera-makers have fought for more than a century: simple lenses project a curved image onto the back of the camera, but cameras (both digital and film) use a flat light detector, which causes distortion around the edges. Good camera lenses reduce this distortion with complex, expensive designs, but they are clunky compared to the one-piece lens in a mammal's eye.
Nature long ago figured out that the simple solution is a curved detector, and that's why the retina in your eye is curved. Now, in the journal Nature, we read about an electronic eye -- a digital camera -- that follows nature's path, using a curved light detector.
John Rogers, a professor of materials science and engineering at the University of Illinois, first joined light-sensitive silicon detectors into a small, flat grid, which he then distorted into a hemispheric shape, which closely matches the image from a one-piece lens.
That simple morphing changed the whole picture, says Rogers, who worked with others at Illinois and Northwestern University to make the camera and analyze its performance. Using a hemisphere, he says, "improves the image quality at the edges, reduces the spatial distortion, and sharpens the focus."
Rogers was not the first to see the advantages of curving the light detector, but the snafu, which his group finally appears to have side-stepped, has been the need to fabricate a curved digital "film" from unbending silicon.
Make photodetectors as individual parts;
Join the detectors with flexible "wires" made of metal-coated plastic;
Attach the detectors to a stretchy plastic sheet while it is stretched flat like a drumhead; and
Bond the assembly -- with all the electronics in place -- to a rigid, hemispheric shape, then attach wires going to the computer.
Using a stretchy platform is key. Anybody knows you can't wrap an apple smoothly with a sheet of paper, Rogers says, "but you can wrap an apple with latex because latex can stretch. We use this hybrid silicon-plastic system in order to go from the plane to the hemisphere; you can't do that with a brittle silicon wafer," which is the standard construction of electronic components.
For 20 years, Rogers says, the theoretical advantages of using a curved detector have attracted researchers and research money. "There have been a number of efforts to put a CCD [charge-coupled device -- which converts photons of light to electrical signals] on a hemispheric surface, but to our knowledge, ours is the first went all the way to making a camera."
The existing detector contains just 256 pixels, but Rogers is confident it can be scaled up, although not necessarily in a university lab.
The new techniques, commented Takao Someya in the same issue of Nature, "succeeded in eliminating these fundamental limitations of conventional artificial-vision systems." Someya, who works on something called quantum-phase electronics at the University of Tokyo, added that the Rogers group "delivered an outstanding contribution by showing how progress in electronics can be made by overcoming the constraints of flat silicon wafers."
Now that it's possible to make curvy, solid-state electronics, camera-makers may shift their emphasis from complex, heavy and expensive lenses to molding their digital film, Rogers says.
Shaped electronics may have other uses, he adds. Using flexible sheets of plastic and flexible connections "allows us to put electronics in places where we couldn't before. We can now, for the first time, move device design beyond the flatland constraints of conventional wafer-based systems."
The human body, for example, does not have planar structures, so curves would seem more appropriate for electronic gadgets that must interface with it. "We are a good distance down the path of putting conformable electronics onto the brains of mice, where we can measure the EEG [the brain's electrical waves]," Rogers says. Shaped electrodes could also be used to measure the activity of heart muscle, he adds. "It could be anything electronic, it does not have to have the photodetector capability."
- David J. Tenenbaum
A hemispherical electronic eye camera based on compressible silicon optoelectronics, Heung Cho Ko et al, Nature, Vol 454|7 August 2008| doi:10.1038/nature07113.
Electronic eyeballs, Takao Someya, NATURE|Vol 454|7 August 2008.