DIGITAL cameras are getting cheaper and more capable all the time, yet they still rely on an inelegant technological kludge. The sensor at the heart of a modern digital camera consists of a rectangular array of millions of individual picture elements or "pixels". But each pixel is sensitive only to the brightness, not the colour, of the light that falls on it. To produce a colour image, a carefully designed mosaic of red, green and blue filters is positioned over the sensor array, so that 50% of the pixels are covered by a green filter, 25% by a red filter, and 25% by a blue filter.
Each pixel thus "sees" in only one colour. But for a full-colour image, the brightness of each colour at each pixel must also be known. So once the picture has been taken, the camera's software fills in the missing information through mathematical guesswork, or interpolation, based on the values of neighbouring pixels. This is clever, but it causes a few problems. Take a picture of a shiny object, such as a silver teapot, and you may notice strange coloured speckles, or artefacts, in the brightest parts of the image where the interpolation has been fooled. Interpolation also reduces the image's sharpness.
A new kind of image sensor devised by Foveon, an imaging company based in Santa Clara, California, cunningly solves these problems. The X3 sensor, as it is known, is the first such device to capture red, blue and green light at every pixel in the sensor array. It does this by cleverly exploiting silicon's ability to absorb light of different colours at different depths. Red light penetrates deepest, whereas green and blue are absorbed nearer the surface. So by stacking three detectors on top of each other at different depths within a silicon wafer—the blue sensor on top, green in the middle and red at the bottom—it is possible to measure the intensity of red, green and blue light in exactly the same place. Each pixel in the X3 sensor in effect "sees" in all three colours. The result is a full-colour image without the need for interpolation.
This has a number of advantages. For a start, the images are sharper, and do not suffer from colour artefacts. Also, the X3 sensor should be cheaper to make than existing sensors, because there is no need to go to the trouble of laying down a mosaic on top of it--a process that can require as many as 24 separate manufacturing steps. Another benefit is that a camera with an X3 sensor does not need to waste time and energy carrying out the interpolation—a process that typically requires 100 calculations per pixel. So it should respond faster and run for longer between battery charges.
In conjunction with its partner, National Semiconductor, Foveon has produced the X3 on a chip made using a 0.18-micron CMOS (complementary metal oxide semiconductor) process, which should make it possible to exploit the same economies of scale that other CMOS chips enjoy. The CMOS process is used for making memory chips and microprocessors.
But the best thing about the X3 sensor, says Jim Lau, chief executive of Foveon, is that it should make possible a new kind of hybrid camera. At the moment, digital video cameras take poor quality stills, and digital still cameras take poor quality video (largely because they have to compromise quality in order to perform all that interpolation on several frames per second). "We see a new class of cameras that can do still and video with no compromise in either mode," says Mr Lau.
The first X3 sensor consists of 3.5m pixels (3.5 megapixels) and will appear in a high-end consumer camera. Foveon will not manufacture X3 cameras itself, but is licensing the technology to other firms. Mr Lau claims a "high level of interest" among the big camera manufacturers, whose products will sport the X3 logo (just as "Intel Inside" appears on many PCs). A second X3 sensor, which will appear in mass-market digital cameras in the summer of 2003, will have a resolution of 1.3 megapixels. That may not sound much, but Mr Lau points out that it actually has 3.9m sensors, so that it comfortably outperforms a conventional 2.0-megapixel sensor.
Foveon may find selling its products on the basis of quality, rather than raw pixel count, a marketing challenge. But in time, the aim is to make "X3" synonymous with sharper digital pictures.
Mar 14th 2002