MONITOR

Chips that see in colour
Mar 14th 2002


A new kind of optical chip that detects colours directly is set
to transform the digital camera business

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.

Quelle: Econimist.com (nur mit Abo)

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