Digital cameras have become extremely common as the prices
have come down. One of the drivers behind the falling prices has been the
introduction of CMOS image sensors. CMOS sensors are much less expensive to
manufacture than CCD sensors.
Both CCD (charge-coupled device) and CMOS (complementary
metal-oxide semiconductor) image sensors start at the same point -- they have
to convert light into electrons. One simplified way to think about the sensor
used in a digital camera (or camcorder) is to think of it as having a 2-D array
of thousands or millions of tiny solar cells, each of which transforms the
light from one small portion of the image into electrons. Both CCD and CMOS
devices perform this task using a variety of technologies.
In a CCD device, the charge is actually transported across
the chip and read at one corner of the array. An analog-to-digital converter
turns each pixel's value into a digital value. In most CMOS devices, there are
several transistors at each pixel that amplify and move the charge using more
traditional wires. The CMOS approach is more flexible because each pixel can be
read individually.
CCDs use a special manufacturing process to create the
ability to transport charge across the chip without distortion. This process
leads to very high-quality sensors in terms of fidelity and light sensitivity.
CMOS chips, on the other hand, use traditional manufacturing processes to
create the chip -- the same processes used to make most microprocessors.
Because of the manufacturing differences, there have been some noticeable
differences between CCD and CMOS sensors.
CCD sensors, as mentioned above, create high-quality,
low-noise images. CMOS sensors, traditionally, are more susceptible to noise.
Because each pixel on a CMOS sensor has several transistors
located next to it, the light sensitivity of a CMOS chip tends to be lower.
Many of the photons hitting the chip hit the transistors instead of the
photodiode.
CMOS traditionally consumes little power. Implementing a
sensor in CMOS yields a low-power sensor.
CCDs use a process that consumes lots of power. CCDs consume
as much as 100 times more power than an equivalent CMOS sensor.
CMOS chips can be fabricated on just about any standard
silicon production line, so they tend to be extremely inexpensive compared to
CCD sensors.
CCD sensors have been mass produced for a longer period of
time, so they are more mature. They tend to have higher quality and more
pixels.
CMOS sensors are just now improving to the point where they
reach near parity with CCD devices in some applications. CMOS cameras are
usually less expensive and have great battery life.