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Kodak doubles light sensitivity of digicam image sensors

Posted: 19 Jun 2007 ?? ?Print Version ?Bookmark and Share

Keywords:CMOS image sensors? light sensitivity? digicam image sensors?

Eastman Kodak Co. last week unveiled what it says are "next-generation color filter patterns" designed to more than double the light sensitivity of CMOS or CCD image sensors used in camera phones or digital still cameras.

The new color filter system, offered in a family of new patterns depending on applications and system architecture, are a departure from the widely used standard Bayer patternan arrangement of red, green and blue pixelsalso created by Kodak.

While building on the Bayer pattern, the new technology adds a "fourth pixel, which has no pigment on top," said Michael DeLuca, market segment manager responsible for image sensor solutions at Eastman Kodak. Such "transparent" pixelssensitive to all visible wavelengthsare designed to absorb light.

'Next milestone'
DeLuca claimed the invention is "the next milestone" in digital photography, likening its significance to ISO 400 color film introduced in the mid-1980's. The earlier innovation enabled consumers to take pictures in low light conditions.

The new color filter pattern approach, called "panchromatic pixels," allows "a [black and white] image to be detected with high sensitivity," according to Kodak. The remaining RGB pixels then collect color information, which is combined with the information from the panchromatic pixels to generate the final image.

"The technique is admirably simple: open the window to let in more light," said Tony Henning, editor of The Mobile Imaging Report. "It's almost inconceivable that nobody else thought of, or acted on this idea, until now."

In the age of mega-pixel warfare among camera vendors, image sensor companies have scrambled to pack more pixels into sensors. The move to higher resolution in the same form factor, however, requires smaller pixels with a reduced ability to gather light.

"Much of the effort to improve sensor sensitivity has been to boost the gain in the signal from the pixels. But this introduces noise [and] grain in the images," said Ed Lee, director of consumer services and digital photography trends service at InfoTrends. "So the trade-off has been sensitivity versus signal noise."

Noting that such a trade off is not new, many major manufacturers of CMOS image sensors "claim at least a degree of success in compensating for smaller pixel sizes and reduced light sensitivity," said Henning. Some reduce the number of transistors per pixel, leaving more room for the photodiode, or they reduce the size of metal layers to make more room for the photodiode, explained Henning. "Others reduce the distance between the photodiode and the microlenses above them."

But what may potentially set the fourth-pixel photo approach apart is that "Kodak is looking at it from a different direction" by using new color filter pattern technology, said Jon Erensen, senior research analyst at Gartner. Other image sensor companies are also experimenting with color filters based on different arrays of pixels, Erensen said, but he hasn't heard of anyone implementing it.

The advance may be exactly what Kodak needs to compete in the CMOS image sensor market against rivals such as Micron or OmniVision. Kodak introduced its first CMOS image sensors for digital cameras and camera phones two years ago, but has made little headway in the market, said Gartner's Erensen.

Kodak said its color filter pattern technology can be used in both CMOS or CCD image sensors, or even on future image sensors with advanced pixel architectures. Kodak's primary target, however, is its homegrown CMOS sensors designed for consumer cameras. The first Kodak sensor using the technology is expected to sample in the first quarter of 2008, according to DeLuca.

Technical challenges
To reconstruct full-color images, Kodak has also developed new software algorithms specifically designed to process raw data generated from image sensors using new color filter patterns. In theory, the algorithms use more sensitive panchromatic pixels as a luminance channel to create the final image. They also derive chrominance information from color pixels on the sensor.

DeLuca said Kodak will offer several different color filter pattern options, optimized for specific applications and system designs and with different levels of processing power.

There is one concern, however. Several analysts cited the additional processing overhead that may be needed as image information based on the new color filter patterns is compiled. Henning noted, "It does introduce an extra layer of complexity or artificiality in the algorithms necessary to reconstruct the RGB color at every pixel."

Unlike the well-understood Bayer pattern, "with these new patterns where you have pixels with no color information...it has to be harder to make an educated guess," Henning said.

InfoTrends' Lee agreed. "Kodak has not said whether this [will require] more or less processing than today's Bayer pattern," Lee said.

Kodak's DeLuca acknowledged that its new code still needs more optimizing.

While industry observers are enthusiastic about Kodak's approach, Henning said its technology remains unproven until it delivers a product with a noticeable improvement in picture quality. "The only thing that matters is the quality of the pictures," Henning said.

- Junko Yoshida
EE Times




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