Herman Peek, a senior scientist at the company, presented a paper on the prototype device at this week's International Electron Devices Meeting. "This is the largest number of pixels and the smallest pixel size for a digital still camera ever published in the world," Peek said during the presentation of his paper.

Although the prototype sensor is monochrome, and the use of red, green and blue filters would reduce resolution to 2 million tricolor pixels, Peek said the small size of individual light sensing elements, 3-micron x 3-micron, would allow such a sensor to rival small format film photography.

"We are approaching the grain size of film, which is about 2 microns diameter, but image quality is also determined by the quality of the lens. The same thing effects CCD."

Images produced using the sensor and displayed by Peek, albeit through an overhead projector system, were indistinguishable from images obtained from film. Individual hairs could been seen on the heads of head-and-shoulder portraits, which prompted congratulations from the audience.

Peek said he expects to see a similar high-resolution CCD sensor in production within about two years, although it might include fewer pixels. "I don't think we will go to more pixels," he said. "We are already asking, 'Is six million too much and are we adding cost unnecessarily?' Perhaps five million or four million is the right amount."

For a 2/3-inch format, Peck showed that the pixel size was on the limit of lens resolution, although the device could also be used for lens-less applications.

The Philips CCD is built using a 0.5-micron CCD process operating at 14 volts and has an active area of 2,048 active lines and 3,072 active measuring 9.11 mm x 6.07 mm. The sensor has a dark current of 800 picoamperes/centimeter² and a dynamic range of 63 dB. In his paper, Peek also discussed how novel techniques had been used to overcome problems with dark current suppression.

The development of a high-resolution sensor using CCD technology represents a fight back by the technology against a challenge it faces from lower cost CMOS image sensors, which also allow the possibility of including signal processing and logic circuits on the same die.

"CCD will always be expensive in comparison with CMOS image sensors," Peek said. "CCD can't compete with CMOS on price but it can on quality, and image quality is very important to consumers."