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Single-Pixel Camera Takes High-Resolution Images

Photonics.com
Oct 2006
HOUSTON, Oct. 3, 2006 -- Using new mathematics and a silicon chip covered with hundreds of thousands of bacterium-sized mirrors, engineers have designed a time-multiplexed camera that takes high-resolution images with a single photodiode. The research, which could prove useful for imaging in wavelengths outside the visible spectrum, such as terahertz imaging, will be presented Oct. 11 at the Optical Society of America's 90th annual meeting, Frontiers in Optics 2006, in Rochester, N.Y.

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Rice professors Kevin Kelly (left) and Rich Baraniuk  with their prototype single-pixel camera. (Photo: Jade Boyd/Rice University)
Unlike today's digital cameras -- which contain arrays of millions of photodiodes -- Rice University's single-pixel creates an image by capturing just one point of light several thousands of times in rapid succession.

For all their ease and convenience, there are few things more wasteful than digital cameras. They're loaded with pricey microprocessors that chew through batteries at a breakneck pace, crunching millions of numbers per second in order to throw out up to 99 percent of the information flowing through the lens.

The oddest part about Rice's camera may be that it works best when the light from the scene under view is scattered at random and turned into noise that looks like television tuned to a dead channel. The new mathematics comes into play in assembling the high-resolution image from the thousands of single-pixel snapshots.

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The single-pixel camera. Using new mathematics and a silicon chip covered with hundreds of thousands of bacterium-sized mirrors, Rice University engineers have designed a time-multiplexed camera that takes high-resolution images with a single photodiode.(Photo: Kevin Kelly/Rice University) 
"White noise is the key," said Richard Baraniuk, the Victor E. Cameron Professor of Electrical and Computer Engineering. "Thanks to some deep new mathematics developed just a couple of years ago, we're able to get a useful, coherent image out of the randomly scattered measurements."

Baraniuk's collaborator, Kevin Kelly, assistant professor of electrical and computer engineering, built a working prototype camera using a digital micromirror device, or DMD, and a single photodiode, which turns light into electrical signals. Today's typical retail digital camera has millions of photodiodes, or megapixels, on a single chip.

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The original closeup image of a mandrill is at left; right is the image taken by the single-pixel camera. (Images: Rice University)
DMDs, which are fabricated by Texas Instruments and today used primarily in digital televisions and projectors, are devices capable of converting digital information to light and vice versa. Built on a microchip chassis, a DMD is covered with tiny mirrors, each about the size of a microbe, that are capable of facing only two directions. They appear bright when facing one way and dark when facing the other, so when a computer views them, it sees them as 1s or 0s.

In a regular camera, a lens focuses light, for a brief instant, onto a piece of film or a photodiode array called a CCD. In the single-pixel camera, the image from the lens is shined onto the DMD and bounced from there though a second lens that focuses the light reflected by the DMD onto a single photodiode. The mirrors on the DMD are shuffled at random for each new sample. Each time the mirrors shift, a new pixel value is recorded by the photodiode.

In effect, the lens and DMD do what the power-hungry microchip in the digital camera usually does; they compress the data from the larger picture into a more compact form. This is why the technique goes by the name "compressive sensing."

Single-PixelCameraDiagram.jpg
In a regular camera, a lens focuses light, for a brief instant, onto a piece of film or a photodiode array called a CCD. In the single-pixel camera, the image from the lens is shined onto a digital micromirror device, or DMD, and bounced from there though a second lens that focuses the light reflected by the DMD onto a single photodiode. The mirrors on the DMD are shuffled at random for each new sample, creating random patterns of black and white, or 1s and 0s, as depicted in the schematic above. Each time the mirrors shift, a new pixel value is recorded by the photodiode. In effect, the lens and DMD do what the microchip in a digital camera usually does; they compress the data from the larger picture into a more compact form. (Image: Kevin Kelly/Rice University)
Today, it takes about five minutes to take a picture with Rice's prototype camera, which fills an entire corner of one of the table's in Kelly's laboratory. So far, only stationary objects have been photographed, but Kelly and Baraniuk say they should be able to adapt the "time-multiplexed" photographic technique to produce images similar to a home snapshot because the mirrors inside DMDs can alter their position millions of times per second. However, their initial efforts are aimed at developing the camera for scientific applications where digital photography is unavailable.

"For some wavelengths outside the visible spectrum, it's often too expensive to produce large arrays of detectors," Kelly said. "One of the beauties of our system is that it only requires one detector. We think this same methodology could be a real advantage in terahertz imaging and other areas."

The research is funded by DARPA, the Office of Naval Research, the National Science Foundation, Air Force Office of Scientific Research, and the Texas Instruments Leadership University Program. For more information, visit: www.rice.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...
image
In optics, an image is the reconstruction of light rays from a source or object when light from that source or object is passed through a system of optics and onto an image forming plane. Light rays passing through an optical system tend to either converge (real image) or diverge (virtual image) to a plane (also called the image plane) in which a visual reproduction of the object is formed. This reconstructed pictorial representation of the object is called an image.
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
mirror
A smooth, highly polished surface, for reflecting light, that may be plane or curved if wanting to focus and or magnify the image formed by the mirror. The actual reflecting surface is usually a thin coating of silver or aluminum on glass.
pixel
Contraction of "picture element." A small element of a scene, often the smallest resolvable area, in which an average brightness value is determined and used to represent that portion of the scene. Pixels are arranged in a rectangular array to form a complete image.
white noise
The random noise having a spectral density that is substantially independent of the frequency over a specified frequency range. White noise is widely used in the random vibration testing of devices.
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