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Electronic Imaging & Signal Processing

CCDs charge ahead

Eye on Technology - electronic imaging

From oemagazine May 2001
30 May 2001, SPIE Newsroom. DOI: 10.1117/2.5200105.0001

In 1969 when George Smith and Willard Boyle invented the CCD detector at then Bell Labs, no one could have foreseen the impact these devices would have on electronic appliances. Today, CCD applications run the gamut from security cameras to factory control systems. The digital-camera market gives an indication of the CCD growth overall. Worldwide shipments of digital still cameras increased by 109.5% to 10.82 million units in 2000, according to a recent report from Nikkei Market Access (Tokyo, Japan). The report also predicted that growth would slow slightly in 2001 but still jump 71% to 18.54 million units shipped.

Most high-performance digital cameras use CCDs, although some small units such as those for cellular telephones use CMOS image sensors instead. Because digital cameras must compete with lenses and film, the emphasis has been on the development of devices with ever-higher resolution. The higher the pixel count on a CCD, the higher the resolution. Sony (Tokyo, Japan), for example, fields a digital camera with 5.07 X 106 pixels. In Japan, Fuji Photo Film (Tokyo, Japan) is a formidable presence in digital cameras, holding a 15.2% market share. But the company is weak in the United States, which keeps it from garnering top honors. Still, Fuji's latest digital camera boasts a resolution equivalent to 6.03 million pixels. Although the actual chip only contains 3.3 million X 106 pixels, the device uses image interpolation firmware to effectively double the picture element data.

On the R&D front, the NHK Broadcast Technology Research Institute (Tokyo, Japan) and a Hiroshima University (Hiroshima, Japan) research team headed by Zenko Hirose have created a film made of silicon nanocrystals. Tests show that the film has photoelectric characteristics. While experiments have established the basic function of the material, Hirose and his team also found that controlling the size of the silicon crystals determined the wavelength of the light detected.

According to Hirose, the new material may pave the way for much more compact high-resolution cameras for broadcast use. Conventional color cameras have three CCD elements for every pixel—one for red, one for green, and one for blue—and use a prism to separate the colors. Hirose's nanocrystals could enable production of a single element with three film layers composed of different-sized crystals, with one layer for each color. This would eliminate the need for a triad of CCD elements, as well as the prism. Processes used in the semiconductor industry can be used to create the nanosized silicon films, Hirose says.

Yoshinori Takeuchi and colleagues at the Matsushita Advanced Materials Research Laboratory (Tokyo, Japan) have developed a smart pixel array. Unlike conventional detectors, the device does not just convert light into electricity, but it also can create light output from light input. A smart pixel has a photoreceptor to sense light, an electronic processing element, and a tiny companion light- emitting diode (LED) that emits light as the data signal. In short, the smart pixel array is a light-to-light sensor, a characteristic that makes it potentially many times faster than conventional CCDs. CCDs and their attendant circuitry take 30 ms to process images. Smart pixel array devices take less than 1 ms to process input.

"Our original objective was to create a light-based computer," says Takeuchi. "The smart pixel array is the result of our efforts to speed up image processing."

With a package size of 18 mm2, Takeuchi's experimental sensor is currently too large to be practical. The 6 mm2 chip only has 64 pixels. Nevertheless, he says, "We now understand the principles, and the experimental sensor's performance exceeded our expectations." And he also sees practical possibilities in the not-too-distant future. -- Charles Whipple

The smart pixel array incorporates photodiodes to sense light and LEDs to emit light in response.