Triple-decker chip provides full color for every sensor pixel: Foveon X3 array offers many advantages for digital camera designs.

A new type of digital imaging sensor was introduced at PMA 2002 by Foveon Inc., Santa Clara, Calif. (www.foveon. com), which delivers higher and more accurate image quality without producing color artifacts. Employing a three-layer sensor, the radical Foveon X3 technology captures red, green, and blue light at each and event pixel point, eliminating the need for color interpolation. It produces better color and sharper images than the side-by-side, checkerboard-style RGB mosaic arrays.

The new CMOS-based sensor stacks sensitive photo detectors that measure color at three levels throughout a single silicon wafer, providing every pixel in the array with the opportunity of recording the subject in full color, without the use of a color mosaic filter or a three-chip multi-shot imaging system.

"The Foveon X3 represents the most significant breakthrough in digital camera technology since the invention of CCDs more than 30 years ago. This new technology combines the immediacy and excitement of the digital photography experience with the quality that exceeds what consumers have previously come to expect from film," comments Jim Lau, Foveon CEO.

"Current image sensor technology has not enabled digital cameras to realize their full potential. We believe the breakthroughs of the Foveon X3 technology will form the foundation of a new generation of digital cameras in all classes," Lau continues.

Nearly all other color sensor designs record only one color per pixel and are significantly more susceptible to unpredictable color artifacts, which produce color moire and the "Christmas tree light" effect in digital images. The innovative Foveon X3 technology is also the first to incorporate Variable Pixel Size (VPS) into an image sensor, which enables the design of a new class of consumer products--a dual-function digital still camera/camcorder that will excel in image quality for both capture modes, according to the company.

One physical property of silicon is that light of different wavelengths is absorbed at different depths in the material. Blue light is absorbed near the surface, green light is absorbed at a lower level, and red light is absorbed at the deepest level. The sensor design takes advantage of silicon's natural color-separating properties. The Foveon X3 sensor has three photo detectors located in each pixel at different depths, and they measure the photo-generated current at each level. From this measurement, color information for the image is obtained after the three signals are converted into digital data and processed by Foveon's image processing software.

The Foveon X3 sensor design provides several unique benefits:

* Image sensors are much less susceptible to color artifacts, compared to single-layer sensors. The Foveon chip captures three colors per pixel, while mosaic image sensors capture only one color per pixel. Current CCD and CMOS color image sensors have one layer of monochromatic photo detectors, with one photo detector per pixel. To capture color, the array is organized in a mosaic pattern resembling a three-color checker-board with each pixel assigned only one color--red, green, or blue. Since only one color is captured, the sensor employs complex mathematical algorithms to interpolate the remaining two-thirds of the colors not captured.

* The new sensor produces sharper images, since color interpolation and the use of the mosaic filter results in loss of image detail. The company reports the Foveon X3 sensor has twice the sharpness of a mosaic image sensor with comparable pixel count.

* The Foveon X3 sensor gathers more light and can operate successfully at lower illumination levels. In a mosaic sensor, each pixel collects only one color of three, or roughly one-third of the available light. The remaining two-thirds of the incident light is absorbed by the color filter and not used to produce a photograph.

* Since the Foveon X3 sensor eliminates the need for image reconstruction, it is possible to manufacture less intricate digital still cameras. For example, color interpolation adds cost and complexity to digital camera design and operation, as well as increasing the time delay between the shutter release and the actual capture of an image. These delays--due to the digital camera's processing requirements--can easily result in the difference between snapping a great picture and missing an important moment. The new sensor will simplify camera designs and improve "click to capture" performance.

* The Foveon X3 chip design also enables the development of a new hybrid class of digital cameras, which combine both still imaging and digital video capture at high resolution, due to the capability of the Variable Pixel Size (VPS) feature. VPS allows several neighboring pixels to be averaged together in "blocks" to obtain the effect of a larger area pixel. Among its virtues, VPS allows higher equivalent ISO sensitivity at lower resolutions without sacrificing low noise levels, as well as offering flexible video capture at a variety of resolutions.

The combination of digital video and sharp, high-quality still photography in the same camera, is a prime application goal for the Foveon X3 technology. "Parents, for example, can use this new type of camera to capture video of their child playing soccer. Midway through video recording, they can press a shutter button to capture a high-resolution photograph, and then seamlessly continue recording video.

Security applications could also benefit from this high-quality, dual-mode capture capability. Foveon X3-enabled airport security cameras could record video for general surveillance and capture high-resolution still photographs targeting suspected individuals or at timed intervals," Lau suggests.

"The photographic detail and color that can be achieved with this new technology is unsurpassed," Lau states. "Pixel for pixel, Foveon X3-based digital cameras will deliver higher-quality images than any other sensor."

Like all CMOS-based sensors, the Foveon X3 sensor has lower power requirements compared to similarly sized CCD sensors. The company describes the chip's power needs as "ultra low," due to advanced CMOS processing technology. For example, the power consumption of the 20.7-by-13.8mm chip is less than 50mW during readout, less than 10mW in standby mode, and less than 100[mu]W in power down mode. Again, as with all CMOS chips, the X3 sensor has substantially greater resistance to specular highlight "blooming" than CCD sensors.

Several different sizes of Foveon X3 chips are in the works. The first production model is a 2,304-by-1,536-by-3pixel array (3:2 aspect ratio) that measures 20.7-by-13.8mm (25mm diagonal). The sensor has an equivalent ISO 100 sensitivity in full-resolution mode.

Sigma Corp. is the first to incorporate this imaging chip into a camera model. The new Sigma SD9, an interchangeable lens digital camera built on a Sigma 35mm SLR body (www.sigma-photo.com), was introduced at PMA 2002. The camera offers a RAW data recording setting, so the highest picture definition and compact file size are possible through lossless compression of RAW data.

Available during the third quarter of this year, a 6.4-by-4.8mm sensor with 1,344-by-1,024-by-3-pixel resolution is scheduled for 0.5-inch optical formats. Other X3 sensors are also being developed.

The company calls the technology "highly scalable," and it could be used in the design of small, mid-range, and large image sensors that would produce smaller file sizes than mosaic-pattern chips for the same size high-quality images.

"Today's CCD and CMOS image sensors detect only one of the three required colors at each pixel and must mathematically estimate the remaining two colors not detected. By capturing three colors at every pixel instead of one, Foveon X3-based cameras have measured information for all three colors. This results in high-quality photographs with fewer pixels, because the pixels are based on real measurements. Fewer pixels result in smaller file sizes, allowing digital camera users to send a higher-quality photograph more quickly through an e-mail message. It also means more digital photos can be placed on a digital camera storage card," a Foveon document explains.

A Foveon press release also states "Photographs from the Sigma SD9 camera can be enlarged to 30 inches, exceeding the quality of 35mm film." This claim was reported by ABC TV news anchor Peter Jennings in a network prime time broadcast, where he also displayed high-quality images produced by the three-deep sensor array.

Dr. Carver Mead, a pioneer in solid-state electronics and VLSI design, founded Foveon in 1997. Recent accomplishments include development and production of an ultra high-resolution professional portrait studio digital camera and the 16.8-megapixel CMOS image sensor, as well as the new Foveon X3 full-color image sensor. The privately held company's investors include National Semiconductor Inc., Synaptics Inc., and New Enterprise Associates.