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Silicon Camera Mimics Eye

Photonics.com
Aug 2008
CHAMPAIGN and EVANSTON, Ill., Aug. 6, 2008 -- The human eye's ability to use a curved surface to capture images has stumped research groups trying to reproduce it for the last 20 years. But now engineers report they have solved the problem by using stretchable optoelectronics to create a working eye-like camera.
EyeCamera.jpg
Close-up view of the completed electronic eye camera, which integrates a transparent hemispherical cap with a simple, single-component imaging lens. (Images courtesy John Rogers, University of Illinois at Urbana-Champaign)
The new camera's design uses an array of single-crystalline silicon detectors and electronics configured in a stretchable, interconnected mesh instead of the flat microchips used as light sensors in digital cameras. It is based on the design of the human eye, which has a simple, single-element lens and a hemispherical detector, said the researchers at the University of Illinois and Northwestern University.

The camera integrates such a detector with a hemispherical cap and imaging lens, to yield a system with the overall size, shape and layout of an eye. Because it's stretchable, the new imaging device can be conformed to a curved surface and is being touted as the next step toward creating artificial retinas for "bionic" eyes.

"Conformally wrapping surfaces with stretchable sheets of optoelectronics provides a practical route for integrating well-developed planar device technologies onto complex curvilinear objects," said John Rogers, the Flory-Founder Chair Professor of Materials Science and Engineering at Illinois. "This approach 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 advantages of curved detector surface imaging have been understood by optics designers for a long time, and by biologists for an even longer time," said Yonggang Huang, Joseph Cummings Professor of Civil and Environmental Engineering and Mechanical Engineering at Northwestern's McCormick School of Engineering and Applied Science, who collaborated with Rogers. "That's how the human eye works -- using the curved surface at the back of the eye to capture an image."
EyeCameraSchematic.jpg
Schematic illustration of steps for using compressible silicon focal plane arrays and hemispherical, elastomeric transfer elements to fabricate electronic eye cameras.
On a normal camera, such electronics must lie on a straight surface, and the camera's complex system of lenses must reflect an image several times before it can reflect on the right spots on the focal plane.

But exactly how to place those electronics on a curved surface to yield working cameras has stumped scientists, despite many different attempts over the last 20 years. The electronics lie on silicon wafers, which can only be compressed 1 percent before they break and fail.

Rogers and Huang got around this by creating an array of photodetectors and circuit elements that are so small -- approximately 100-µm square -- they aren't as affected when the elastomer pops back into its hemispheric shape. Think of them like buildings on the Earth -- though flat-bottomed buildings are built on the curved Earth, the area they take up is so small that the curve isn't felt. 

They also designed the array so that the silicon component of each device is sandwiched in the middle of two other layers, the so-called natural mechanical plane. That way, while the top layer is stretched and the bottom layer is compressed, the middle layer experiences very small stress.

When tested, more than 99 percent of the devices still worked after snapping the elastomer back to its hemispherical shape. Researchers found that the silicon in the devices was only compressed .002 percent -- far below the 1 percent compression where silicon fails.

The array package is then transfer printed to a matching hemispherical glass substrate. Attaching a lens and connecting the camera to external electronics completes the assembly. The camera has the size and shape of a human eye.
EyeCameraOnBoard.jpg
The electronic eye camera mounted on a circuit board.
Early images obtained using this curved array in an electronic eye-type camera indicate large-scale pictures that are much clearer than those obtained with similar, but planar, cameras, when simple imaging optics are used.

"In a conventional, planar camera, parts of the images that fall at the edges of the fields of view are typically not imaged well using simple optics," Huang said. "The hemisphere layout of the electronic eye eliminates this and other limitations, thereby providing improved imaging characteristics."

Huang and Rogers will continue to optimize the camera by adding more pixels.

"There is a lot of room for improvement, but early tests show how well this works. We believe that this is scalable, in a straightforward way, to more sophisticated imaging electronics," Huang said. "It has been a very good collaboration between the two groups."

Researchers are testing the same design principles in a range of other applications, including as a thin, conformable monitor to detect electrical signals traveling across the undulating surface of the human brain.

The research is the cover story of the Aug. 7 issue of Nature; Rogers is corresponding author.

For more information, visit: www.uiuc.edu or www.northwestern.edu


GLOSSARY
camera
A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
elastomer
Any material of a macromolecular nature that can stretch at room temperature to more than twice its length and return to approximately its original shape when stress is released.
eye
The organ of vision or light sensitivity.  
glass
A noncrystalline, inorganic mixture of various metallic oxides fused by heating with glassifiers such as silica, or boric or phosphoric oxides. Common window or bottle glass is a mixture of soda, lime and sand, melted and cast, rolled or blown to shape. Most glasses are transparent in the visible spectrum and up to about 2.5 µm in the infrared, but some are opaque such as natural obsidian; these are, nevertheless, useful as mirror blanks. Traces of some elements such as cobalt, copper and...
optoelectronics
A sub-field of photonics that pertains to an electronic device that responds to optical power, emits or modifies optical radiation, or utilizes optical radiation for its internal operation. Any device that functions as an electrical-to-optical or optical-to-electrical transducer. Electro-optic often is used erroneously as a synonym.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
retina
1. The photosensitive membrane on the inside of the human eye. 2. A scanning mechanism in optical character generation.
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