Machine Design

Pump Up the Pixels

A new image sensor gives CCDs and CMOS a run for their money.

Pump up the pixels

, Associate Editor

Digital photography is heating up, becoming more popular with consumers and professionals. Falling camera prices, increased pixel counts, and the ability to have a complete, dry photo lab on your desk are making digital photography too good to pass up.

For those still hesitant to enter the world of digital, the big question is whether or not the quality of digital photos equals that of film. Higher-end professional cameras with enough megapixels can give results comparable to film. But such cameras are out of the price range of most consumers. The X3, a new image sensor introduced to the market in early '02, from Foveon, Santa Clara, Calif. (, promises to deliver the sharpest digital images yet at a price that should help crack the consumer market.

Three colors per pixel

All image sensors do the same thing: sense light. Standard sensors, such as CMOS and CCD, are called mosaic image sensors. They use three color filters -- red, green, and blue -- placed over a single layer of photodetectors in a tiled mosaic pattern. The filters let only a narrow band of wavelengths, corresponding to one of the three colors of light, pass through to a picture element, or pixel. This means each pixel captures and records only one of the three primary colors; red, green, or blue. This limitation causes a loss of color accuracy and resolution.

Camera manufacturers make up for this by using a mathematical operation known as interpolation to fill in the gaps. But reconstructing full-color images from incomplete sampling adds complexity to the camera and diminishes color resolution. Also, interpolation can involve up to 100 calculations/pixel, taxing the camera processors and increasing the time for image capture.

The X3 sensor works on the principle that wavelengths of light are absorbed at different depths in silicon. To exploit this phenomenon, the sensor has three layers of photodetectors embedded in silicon. Detecting photocurrents at different depths provides color information. Stacking photodetectors creates full-color pixels. That is, each pixel records varying amounts of red, green, and blue light with no need for interpolation. In this sense, the X3 works a lot like regular color film which has three different light-sensitive layers, each sensitive to either red, green, or blue light. The X3 promises to make camera designs simpler by eliminating the need for interpolation, which should lead to sharper images and truer colors.

Another benefit of the X3 is variable pixel size, or VPS. It lets the complete area of the image sensor be configured with variable resolutions. This could lead to digital cameras that capture high-resolution still images and full-motion video. VPS lets signals from adjacent pixels be combined into groups and read as one larger pixel. For instance, a 2,300 3 1,500 image sensor contains 3.4 million pixels. Using VPS, these pixels can be grouped into 4 3 4 blocks, turning the sensor into a 575 3 375 pixel array. Grouping smaller pixels together increases the signal-to-noise ratio, letting the camera take pictures in low-light conditions with reduced noise.

Increasing pixel size and reducing resolution lets the sensor run at higher frame rates, accelerating image-capture speed. This means cameras can be built to capture both still images and short videos without sacrificing quality. Previously, dual-mode cameras had to give up image quality in, say, the photo capture mode to produce high-quality video. Also, because pixel resizing can be done on the fly, the X3 is said to be able to capture high-resolution images while recording video, another first in digital photography.

A look at image sensing

Foveon's X3 will have some competition from entrenched CCD and CMOS sensors. But the promised benefits of superior color may be enough to give the X3 the edge.CCDs, or charge-coupled devices, are by far the most widely used image sensor in digital cameras. CCDs capture light on photodetectors on the sensor surface. The photodetectors convert photons of light into electrical charge which is proportional to the brightness level at that point.

After an exposure, charges on the first row are transferred to a read-out register then pass to an amplifier, and on to an analog-to-digital converter. The charges on each row are coupled to those on the row above. So after the row that is read moves into the register, the next row moves down to take its place.

The main difference between CCDs and CMOS sensors is how pixel data is read. With CCDs, data is read one row at a time. CMOS, however, has the advantage of being able to read individual pixels. Also, manufacturing costs are less for CMOS. CCDs are fabricated using a highly specialized process and thus lack the economies of scale that CMOS brings with it. The cost of a CMOS wafer can be one-third the cost of a similar CCD wafer because CMOS sensors can be manufactured on just about any standard silicon production line.

CMOS sensors consume little power. CCDs, on the other hand, can consume up to 100 times as much power as an equivalent CMOS sensor. They also give better quality, low-noise images. CMOS sensors, on the other hand, are more susceptible to noise. As a result, CCDs tend to be used in higher-end cameras and CMOS in lower-end cameras. However, recent advances have put CMOS sensors with quality comparable to CCDs on the horizon.

Camera manufacturers using CCD sensors typically buy a host of components and integrate them all together. This raises complex system integration design problems. But both cost and complexity are lowered using CMOS because the sensors can have processing circuits embedded on the same chip. With CCDs, additional processing must be put on separate chips.

The X3 takes advantage of the CMOS manufacturing process and saves on integration costs. Chips come with integrated analog-to-digital converters and data outputs that interface directly to external DSPs with little intermediary logic required. For now, however, CMOS sensors will continue to do well in low-resolution cameras. CCDs will still dominate mid and upper-level cameras. The X3 has made its debut in mid-range cameras, with a 1.3-megapixel chip in the works for lower-end cameras. If nothing else, the world of digital photography will be an exciting and competitive place in the near future.

A photograph taken with an X3 image sensor from Foveon, Santa Clara, Calif., is crisp, sharp, and has accurate colors.

The first in the X3 series, the F7 sensor has a pixel size of 9 microns and over 10.2 million individual photodetectors arranged in three layers to form 3.54 million full-color pixels.

Conventional sensors have a single layer of photodetectors in a tiled mosaic pattern. Filters let only a narrow band of wavelengths (either red, green, or blue) through. Interpolation fills in the gaps. The Foveon sensor, on the other hand, has three separate layers at each pixel location, which capture full color without interpolation.

The X3 has three layers of silicon, each sensitive to a different wavelength of light, like the layers of standard color film.

The first digital camera on the market to sport the X3 is the 3.5-megapixel SD9 SLR from Sigma, Ronkonkoma, N.Y. ( Reports thus far seem to bear out Foveon's claim to sharper images without the color defects associated with conventional CCD sensors.

From pixel to paper

Capturing images on a sensor is only the first step in digital photography. Most shutterbugs want a hard copy of the photograph. This is where printers enter the scene and the need to reproduce stable color images. A lot of work has been done on archival-quality papers and chemicals. The best quality laser jet prints on archival paper are said to last up to 60 years, based on accelerated test conditions. To simplify the printing process even further, a group of camera and printer manufacturers have proposed a standard for connecting printers directly to the camera, bypassing computers all together. The proposed standard, dubbed DPS for direct-print service, would use USB1.0 as the physical layer and the Picture Transfer Protocol (PTP) as the standard for image-file transfer. Future versions may even support protocols such as wireless LAN.
TAGS: Technologies
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