■ In addition, the inaccurate sensors used with a Bayer mask require ”blur fil-
ters” to reduce color artifacts. The random appearance of artifacts and the
requisite blur filters force the software to reduce sharpness.CCD Competition: Low-Cost CMOS Image Sensors
The CMOS (Complementary Metal Oxide Semiconductor) image sensor is an inex-
pensive sensor replacement for CCDs that does away with a few disadvantages of
CCDs, but introduces its own issues.
CMOS sensors produce medium-quality images that are more susceptible to noise
than a CCD sensor. Each pixel on a CMOS sensor can be read individually, however,
which makes it more flexible in manufacture and cost than CCD sensors.
Another benefit of CMOS sensors: They are built using the same equipment as CPU
chips, such as the Pentium 4 chip. Because they use common equipment, CMOS sen-
sors are less expensive to manufacture and benefit from innovations in CPU tech-
nology.
The flexibility of CMOS sensors is also the reason why they aren’t as precise as
CCDs. Each pixel on a CMOS sensor has several transistors located next to it, mak-
ing it possible for photons to hit the transistors instead of the photo-diode. This
makes the CMOS sensor less sensitive to light than the CCD sensor.
CMOS technology consumes much less power than CCD chips: as little as 1/100th of
the power needed by a CCD sensor. This is why CMOS sensors appear on mobile
phones. On professional digital SLRs however, which often have large high-power
batteries, the CCD sensor is the best choice.What About Foveon?
A CMOS-based technology called Foveon was announced in 2001 that uses layers of
sensors to capture red, green, and blue at every pixel location (see Figure 2.5). This
technology relies on a triple-layer CMOS sensor array that captures blue light first,
then green, and finally red. The technology takes advantage of light’s penetrating
power. Red light can penetrate silicon much more easily than green or blue. As a
result, the red layer is on the bottom of the three-layer stack.
Blue light, which has the shortest wavelength and combines with electron “holes” in
silicon, is captured first. The green layer is in the middle.
Every sensor in the Foveon technology is essentially an eye that captures exactly the
color and brightness of the light it receives. Currently Sigma makes two cameras
with this technology. By the time you read this, more camera manufacturers may
have licensed Foveon in their cameras.20 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY