Carver Mead turns eye to digital camera that rivals film
Aug 24, 1999 at 04:00 GMT
Foveon Inc. has built a high-end digital still camera that aims to rival the quality of analog film. The new startup is backed by Carver Mead, the inventor of the gallium-arsenide transistor, the silicon compiler and the artificial retina. Although Mead is not revealing details of the three analog VLSI image chips that are to be used in place of a charge-coupled device (CCD), he claims the Foveon camera spits out a 48-Mbyte, 4,000 x 4,000-pixel Photoshop file.
"We have worked very hard at Foveon to make our digital camera as good as a medium-format camera like a Hasselblad � but our camera has no moving parts, you can see the images you take with it instantly and it has many refinements . . . like our electronic loupe," said Mead. According to Mead, most digital still cameras interpolate two-thirds of the information they capture because the red, green and blue sensors are next to each other on the same CCD chip. In order to find the value of "red" at the pixel location of a "green" or "blue" pixel, a software interpolation step must be taken that averages the two red pixels on each side of the green or blue one.
Most digital cameras that use this method cannot measure the color of each part of the image directly, so they can't keep their pixels exactly aligned, which may result in artifacts like moire patterns. A few high-end digital cameras have solved that problem by sacrificing fast shutter speeds, either taking three shots per image (one each for red, green and blue) or scanning the image very slowly.
"Foveon's camera saves its images instantly to disk at any shutter speed and with no interpolation whatsoever," said Mead. The one-shot digital cameras that use interpolation also sacrifice three-fourths of their advertised resolution, since they must put green pixels on each side of each blue and red pixel; put red pixels on each side of blue and green pixels; and finally put blue pixels on each side of red and green ones. The most popular pattern for doing this is G-R-G, B-G-B, G-R-G.
That pattern ensures that each pixel of one color has the other two colors on each side of it, so that an average can be taken of the adjacent colors. Unfortunately, color is only directly measured for one of red, green or blue at each pixel, and the other two are interpolated. Thus, two-thirds of the colors are interpolated at each pixel location. In addition, conventional digital still cameras take four pixels in a square to directly measure the three colors in one pixel location.
Thus, of the 2 million pixel images advertised by today's state-of-the-art digital cameras, only one-third of the color is actually measured, since the other two colors are interpolated at each pixel location. In addition, the true resolution of the resulting image is really one-fourth of the total stated, or a half a million pixels, since adjacent red, green and blue pixels are set up in squares for easy interpolation.
Foveon avoids the color-interpolation problem by using a different technique, which splits the incoming light into three beams with a prism. The prism focuses the red light on a red sensor, the blue light on a blue sensor and the green light on a green sensor. That approach uses three times as many sensor chips, plus a multifaceted prism, but eliminates the moire-pattern problem.
Foveon attaches its sensors directly to the prism faces with glue that has the same index of refraction as the optical glass of the prisms, which eliminates the reflection and blurring problems of earlier designs. The company has a patent pending for the process it uses to attach its sensors directly to the prism.
Foveon employs analog VLSI chips modeled on the optical properties of real film. For instance, a magnified Foveon image will show grain, even though there is no film used anywhere in the process. Film has grain because the particles that change color when exposed to light are of varying sizes � big ones for dark areas with little detail and small ones for light areas of greater detail in an image. Foveon's sensors mimic that property of film.
"Our sensor works more like real film that has a gamma curve, which slopes smoothly from low-density to high-density areas, corresponding to the statistical distribution of coarse and fine grains, whereas CCDs have a completely linear curve," said Mead.
Foveon's analog VLSI chip dedicates fewer or greater resources to each particular area of an image, depending on the detail and light levels at that point. Consequently, the company does not say how many pixels per inch or bits per pixel are captured with its three chips because that depends on the part of the image in question. The one specification it does supply, however, hints at the size of its analog VLSI sensor: When it is ready to output a digital image to a printer, the camera produces a 48-Mbyte, 4,000- x 4,000-pixel Photoshop file.
The Foveon camera looks like a Wintel laptop with a squared-off Hula Hoop surrounding it and a Canon 28-mm to 70-mm lens attached to the front. You aim the laptop with attached lens at the subject and you see a continuous image on the left-hand side of the screen. When the user presses the space bar, the external flash is triggered, and a shot is taken and placed on the right-hand side of the screen.
Instead of zooming in on an image to inspect detail, the user can pop up a small "electronic-loupe" window on top of the image. Inside the window is a blown-up version of what's beneath it � just like the way a real loupe works. The electronic-loupe window is also enabled by the same process as the analog VLSI sensor, allowing it to mimic film grain by dedicating more chip resources to busier parts of an image.
When you pop off the skins of the proprietary parts of the Foveon camera � the Hula Hoop and a thin circuit board box beneath the laptop � you find a wealth of custom VLSI. In addition to the three analog VLSI sensors attached to the prism, an electronic shutter chip eliminates the last vestige of "moving parts" from the camera. "The only moving parts in our cameras are the electrons," said Mead.
Most of the custom chips, and there are many, were manufactured by National Semiconductor Corp., which holds a 45 percent stake in the company. Synaptics Inc., which was co-founded by Mead and Z-80 inventor Federico Faggin, holds a 30 percent stake. Synaptics has signed off all its patents and other intellectual property regarding digital imaging to Foveon.
Foveon also claims to have found second sources for its custom chips � even for the analog VLSI sensors. With its patents still pending on the sensors, Foveon is reluctant to reveal details about its design. However, Mead would say that the sensors are produced on standard digital CMOS fabrication lines that he has learned to tweak for analog VLSI.