ROCHESTER, NY
One constant challenge for video camera makers has been sensitivity, the ability to capture a quality, or at least usable image in low-light conditions.
Techniques such as amplifying the gain in a sensor to up its sensitivity, have usually come at the expense of increased noise or a lower resolution image. Videographers have had to accept that image degradation because a bad picture was often better than no picture at all.
However, Eastman Kodak has introduced what it terms a next generation color filter pattern that it claims will increase imager sensitivity by two to four times, a one or two f-stop advantage, with no image degradation.
FRONT FILTERING
Color cameras, video or still, whether single or three-imager models, have long divided an image into the three additive primary colors, red, green and blue. Pixel information from each of these colors was used to determine the colors (chrominance) in the image, and weighted information from all three imagers was also used to determine the brightness (luminance) of the image.
But dividing the image into the three primary colors required filtering in front of the pixels, which was like putting sunglasses on them. Filtering lowered the overall amount of light reaching the pixels.
(click thumbnail)Fig 1: The original Bayer pattern (left) used twice as many green pixels as red or blue ones in order to mimic the human eye's greater resolving power with green light. The new Kodak technology (right) substitutes half of Bayer's filtered pixels with unfiltered, or polychromatic pixels. Of the remaining pixels, half are green filtered, with one-quarter each red and blue filtered.
To understand Kodak’s new technology, it’s helpful to understand that current video cameras commonly arrange their pixels in the Bayer pattern, invented by Kodak’s Dr. Bryce E. Bayer in the 1970s. Bayer used twice as many green pixels as red or blue ones in order to mimic the human eye’s greater resolving power with green light, (Fig. 1). The new Kodak technology substitutes half of Bayer’s filtered pixels with unfiltered, or polychromatic pixels. Of the remaining pixels, half are green filtered, with one-quarter each red and blue filtered.
“Because there’s no color filter associated with panchromatic pixels, more light goes into them and they’re able to detect more of the photon that come through the lens,” said Mike DeLuca, product marketing manager for Kodak’s Image Sensing Group.
The unfiltered pixels supply luminance information, and the red, green and blue filtered pixels supply color information. DeLuca said it wasn’t as easy as adding the polychromatic pixels and pushing the product out the door. “There are about 15 patents we’ve applied for, that cover not only the clear pixels, but the specific patterns [the pixels are placed in] as well as for the software.”
While the clear pixels allowed more light to enter, one of the challenges Kodak faced was increasing sensitivity of the remaining color filtered pixels. This was done by using software to combine pairs of like color filtered pixels together, (Fig. 2). “We end up with a lower resolution color record,” said DeLuca, “but we use that for the chrominance part of the image, which your eye is not as sensitive to.”
(click thumbnail)Fig 2: While clear pixels allowed more light to enter, one of the challenges Kodak faced was increasing sensitivity of the remaining color filtered pixels. This was done by using software to combine pairs of like color filtered pixels together.
FOUR-IMAGER CAMERAS?
DeLuca said the new panchromatic pixel technology will work equally well on CCD and CMOS sensors. He said one of the appeals of the CMOS sensors is their ability to carry other processing circuits in addition to pixels.
“In the future we have an opportunity to take [our] software and put it directly on a CMOS chip, so that one chip not only would have a sensor with these patterns on it, but also have the right image processing software on it, to be able to provide [the finished color picture] as a final product directly off the sensor.”
As noted earlier, tests that Kodak has done pitting a sensor employing the polychromatic pattern against a comparable sensor employing the Bayer pattern show the polychromatic technology yields from two to four-times more sensitivity. This not only allows imaging in lower light, but use of higher shutter speeds to freeze a moving subject.
While the tests that Kodak has done to this point with the technology have been in the still photography application, DeLuca said there is no reason it would not work equally as well for video.
Does this forbode the appearance of four-imager video cameras (RGB-P)? “I’ll be honest,” said DeLuca. “I don’t know what to think in terms of a four-imager camera, and I don’t know what work, if any, our guys have done about that.”
But a four-imager camera is not unprecedented. Alan Keil, vice president and director of engineering for Ikegami, pointed out that one of Ikegami’s first high-definition CCD cameras had two green imagers offset one-half pixel-width in order to gain resolution.
“At that juncture the highest pixel count was 1.2 or 1.4 million pixels [on a sensor], and we felt that was insufficient,” Keil said. “So we used the dual-grid approach to match the performance of the previous tube-based HD camera.”
Keil said the Ikegami-built camera that NHK used for its Ultra-HD exhibition seen at the past two NAB conventions also uses two offset green imagers to achieve its 16-times HD resolution.
And looking into television’s deep past, in their early color camera development, both GE and RCA built four-imager tube cameras in which one tube was used for polychromatic pickup, and the other three for red, green and blue.
DeLuca said the first of these next-generation imagers will be available for manufacturers to test in early 2008. But he cautioned against looking for them in professional gear right away. “Our initial target here is for consumer markets, for camera phones, digital still cameras and large markets such as that,” he said.
Still, it would seem that professional video camera makers could benefit from the increased sensitivity of Kodak’s polychromatic pixel pattern technology, particularly with the low light challenges presented by the smaller imagers some manufacturers have deployed in their new HD camcorders. It wouldn’t be the first time professional camcorders picked up technology first introduced for consumer equipment.
