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New CMOS sensors catching on in cameras

Mainstream CMOS chipmaking technology is becoming more competitive for capturing light in digital cameras. It's swept the high-end SLR market.

LAS VEGAS--You may not know it from the outside, but digital cameras are getting something like an eye transplant.

Deep within every digital camera is a sensor chip whose job it is to capture light. Most camera sensors today use CCD (charge-coupled device) technology, but a newer approach called CMOS (complementary metal oxide semiconductor) is catching on, particularly at the high end of the market.

Pentax's K20D, the company's new top-end camera, is the first SLR from the company to employ a CMOS sensor. Pentax

CMOS advantages can include lower noise, lower power consumption, lower price, and faster response times. In the prestigious and fast-growing digital SLR (single-lens reflex) camera market, Canon and Olympus have used CMOS sensors for years, but high-profile new arrivals on the CMOS bandwagon include Sony, Pentax, Samsung, and most notably Nikon.

CMOS itself has been around for decades--it's the method used to manufacture the vast majority of computer processors--but its use as an image sensor rather than an information processor is a relatively new development. In recent years it's begun making inroads against CCD, a technology with many more years of refinement in image sensor technology.

In compact cameras, CCD still dominates. Where CMOS has caught on most widely is videocameras, mobile phone cameras, and notably, SLR cameras. In this latter category new CMOS-based cameras include Nikon's D3 and D300, Sony's Alpha A700, and Pentax's K20D, and Samsung's GX20, which is derived from Pentax's K20D. All these cameras top the companies' respective lines, and the Pentax and Samsung cameras are being shown off here at the Photo Marketing Association trade show here.

CCD today leads CMOS when it comes to performance and a wider bright-to-dark range, said Fas Mosleh, CMOS market segment manager for professional and applied imaging at Eastman Kodak, but because CMOS sensors can ride the coattails of the rest of the chipmaking business, CMOS outdoes CCD in one very important domain: price.

"Because it's a standardized process, with high-volume production, the pricing is very competitive. It's better than CCD and getting better," Mosleh said. Kodak, a digital imaging pioneer, builds its own CCD sensors and and more recently started designing CMOS sensors to be built by Taiwan Semiconductor Manufacturing Corp. and IBM, so it's relatively neutral in the debate over which technology is superior.

Pentax makes the move to CMOS
John Carlson, Pentax' product manager for imaging systems, is outspoken on the CMOS advantages for SLRs. "Lower power is the key thing," he said; it enables more shots per battery, smaller batteries, or more energy for image-processing tasks. Pentax buys its K20D's CMOS sensor from Samsung.

CMOS also sensors can power a live view of the scene on the camera's LCD, a feature that's universal in compact cameras but still a relative novelty among SLRs. CCDs get too hot and consume too much power for live view on the large sensors used in SLRs, Carlson said.

Sony, like Canon, builds its own CMOS sensors. Using CMOS means that some processing can be done on the sensor chip, including the conversion of analog information produced by the light being photographed into digital signals. Sony's 12-megapixel A700 sensor has more than 4,000 analog-to-digital converters, said Mark Weir, Sony's technical prod manager for digital cameras.

Because that conversion happens earlier in the image-handling pipeline, before image data is transferred off the sensor, there's less opportunity other camera electronics to sully the image with noise. In digital photography, noise takes the form of colored speckles, and it's a major bane, especially when shooting in dim conditions.

CCD sensors are still widely used, though, in part because many more years of work have been invested into milking the most out of the process, said Mike DeLuca, Kodak's CCD market manager for professional and applied imaging.

Where CCD still has the edge
"One problem with CMOS is it's difficult to get the manufacturing process optimized both for the imaging part and the processing part," DeLuca said. In contrast, "CCD technology was built for imaging. The architecture was set up to optimize the imaging characteristics available on the silicon."

Kodak has begun selling a 5-megapixel CMOS sensor--and the company's camera division is the first customer, using the chip in the low-end $99 Easyshare C513. But the company also has a business selling some of the biggest image sensors around: 39-megapixel CCDs used by medium-format camera companies such as Hasselblad and Phase One. These measure a whopping 48x36mm, twice the surface area of a full frame of 35mm film (though not as large as medium-format film).

In this rarefied atmosphere, where camera equipment costs tens of thousands of dollars, CCD still rules the roost. In part that's because a camera doesn't need to shoot at high speeds, and in part because consuming a lot of battery power isn't a top-level problem.

"For those customers, the first, second, and third priority is the image quality the sensor provides," DeLuca said.

Canon builds its own CMOS sensor. Shown here is a silicon wafer with high-end "full-frame" image sensors Canon

Phase One, which uses Kodak CCD sensors, agrees. "For the 50- to 80-megapixel sensors on the horizon, we still feel the CCD will be the best way forward," said PhaseOne Chief Executive Henrik Hakonsson. "We are carefully monitoring CMOS all the time, but for the customers we working for we have not found the quality we're looking for."

CMOS's reputation in digital imaging has suffered from inflated expectations.

"It has been for some time generally held that CMOS technology in image sensors will overtake CCD at some point. I would say that three or four years ago, the predictions were that by the time 2007 or 2008 rolled around, CMOS would be done replacing CCD," Weir said. "History has shown those predictions were premature."

But in the long run, Weir still gives CMOS the edge. "Are there long-term advantage suggest that transition will take place? Probably."