Digital noise 63
incorporated into a CMOS sensor, and the tiny differences in
performance from pixel to pixel, you’re going to fi nd noise in the
output image. To overcome this problem, Canon, for example,
developed on-chip technology to record the noise of each pixel
before exposure, and automatically subtract such noise from the
image when it is created. The incorporation of noise reduction on
the sensor enables the reading of a noise-free signal. This on-chip
circuitry can be added only to CMOS sensors, not to CCDs,
because of the differences in the way the two are manufactured.
To counteract noise, Canon added a fourth transistor that acts as
a transfer gate. Kodak designed four-transistor CMOS pixels in
their own proprietary confi guration, whereas Nikon’s LBCAST
(Lateral Buried Charge Accumulator and Sensing Transistor
array) is a completely new type of image sensor that’s different
from CCD and CMOS. LBCAST saves more power and achieves
less dark noise while increasing image- processing speed and
improving sensitivity, contrast, and color reproduction.
Fixed-pattern noise can be suppressed with noise-reduction and
on-chip noise-reduction technology. A typical approach is CDS
(correlated double sampling), having one light signal read by
two circuits. First, only the noise is read. Next, it is read in
combination with the light signal. When the noise component is
subtracted from the combined signal, the fi xed-pattern noise can
be eliminated. Canon’s method for suppressing random noise is
called complete electronic charge transfer, or complete charge
transfer technology, and ensures that the sensor resets the pho-
todiodes that store electrical charges.
NMOS vs. CMOS?
The chips that Olympus calls Live MOS are actually NMOS technology chips used by Olympus and other members
of the Four Thirds consortium. They are CMOS chips that combine some of the advantages of CCD with those of
CMOS. NMOS (negative-channel metal-oxide semiconductor) is a type of semiconductor that is negatively
charged so its transistors are turned on or off by the movement of electrons. By contrast, PMOS (positive-
channel metal-oxide semiconductor) works by moving electron vacancies. NMOS is faster than PMOS.
Noise-reduction software
Both Adobe Photoshop and Lightroom have built in noise-
reduction features; for some shooters, that may be all they need.
If you need more, it’s time to bring out the power tools. These
are small bits of software that can be Photoshop-compatible
plug-ins, Photoshop Actions, or graphics utilities that make a
digital photographer’s life a little easier for creating practical or
special effects. In the same way that an electric screwdriver
makes household projects faster than using an old-fashioned
hand tool, software power tools let you produce imaging projects
quicker and with less fuss than doing it “the hard way.”
