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Photo: Lytro

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CONSUMER ELECTRONICS / AUDIO/VIDEO

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A New Angle on Imaging

Capturing the direction of light beams can make for after-the-fact focusing

By NEIL SAVAGE / WED, NOVEMBER 16, 2011

16 November 2011All cameras

capture the intensity of light as it

strikes their imaging chips. Color

filters provide a second set of

data, sorting the rays into

different wavelengths. But new

devicesincluding one produced

commercially and others still in

the labare starting to capture a

third piece of information: angle.

This allows cameras to gobeyond focusing on a single

plane to measuring images at

many different depths of a scene

at once. Cameras that capture

both intensity and angle will allow

for refocusing already-snapped

pictures, lensless cameras, and the creation of 3-D images with a single camera, according to researchers in several

competing groups.

Alyosha Molnar, assistant professor of electrical and computer engineering at Cornell University, in Ithaca, N.Y., has

developed angle-sensitive pixels for CMOS imagers. Each pixel is made up of a photodiode beneath two layers of

diffraction grating, one of which is slightly out of alignment with the other. The top grating creates an interference patternon the grating beneath it. Depending on how that pattern of light and darkness lines up with the second grating, light will

either pass through to the photodiode below or be blocked. So whether the diode sees bright light or dim depends on the

angle the light is coming from.

Forming an image requires having a detector with pixels in many different orientations and gratings with different

amounts of spacing between them. Taken together, all the pixels produce a series of measurements that can be

processed using a mathematical function known as the Fourier transform to create an image with any focus the user

chooses. The chip gives you the transformed version of a standard bitmap, Molnar says. You transform it back. And

because the camera can use the same data to produce two images at different depths, it can also generate a 3-D

image. Molnar says that smartphones may one day make 3-D movies using this technology.

According to Molnar, camera chips with angle-sensitive pixelsknown as light-field imagersshould be cheap to

produce, because the gratings are inscribed in the layers of wires that are already built into the detector. The diffraction

gratings can be added simply by changing the interconnect layout in an image chips design. The result is, its basically

free, he says.

A newer approach, which Molnar is presenting in December at the IEEE International Electron Devices Meeting, would

inscribe the gratings in the glass on the surface of the detector, generating the same diffraction patterns while blocking

less light, thus making the device more sensitive.

Molnar says his approach should be cheaper than a similar technology already hitting the market. Lytro, a start-up in

Mountain View, Calif., has introduced a consumer camera that allows users to snap pictures first and focus later. The

company started taking orders for the cameras in October and promises to start shipping them in 2012. The product is

based on technology developed at Stanford by Lytro founder and CEO Ren Ng.

The heart of the device is an array of microlenses that lie over the detector. The microlenses focus light rays from

different angles on different pixels in the detector, which yields images at many depths but leads to fairly low resolution.

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You lose a lot of resolution, but you get freedom of refocusing afterwards, says Ramesh Raskar, head of the Camera

Culture group at the MIT Media Lab, who is familiar with the technology but not involved with Lytro.

Raskar has come up with a different approach. He places a patterned piece of glass between the cameras lens and the

image sensor chip. Raskar describes it as the effect of light passing through a screen door. The screen attenuates the

light rays entering the lens, with the amount of attenuation depending on the angle of the ray. He says his method is

similar to the microlens technique but with no loss in resolution. However, for consumers, the Lytro approach might be

preferable, Raskar says, because it doesnt require as much computing power as his technique.

Molnar thinks the existence of three technologiesLytros, Raskars, and hiswill help push the concept of

computational photography forward. And Raskar agrees. Its redefining the notion of a camera, he says. When we

went from film to digital, we really didnt see any variations in the technology of photography itself. I think now were

seeing a second generation of camera technologythat you will actually be able to go beyond what a film camera could

do.

About the Author

Neil Savage writes about strange semiconductors and amazing optoelectronics from Lowell, Mass. In October 2011 he

reported on a laser-powered mechanical memory chip.

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