CCD vs. CMOS
Sensors
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How
big are the sensors?
The
current generation of digital sensors are smaller than film.
Typical film emulsions that are exposed in a film-based camera
measure 24mm x 36mm. If you've look at the specifications of a
typical 1.3-megapixel camera, you'll find that it has a CCD
sensor that measures 4.4mm x 6.6mm. As you'll see in a later
section, a smaller sensor means smaller lenses.
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Once the light is
converted into electrons, the differences between the two main sensor
types kick in. The next step is to read the value (accumulated charge)
of each cell in the image. 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 completely standard manufacturing processes to create
the chip -- the same processes used to make most
microprocessors.
Because of the manufacturing differences, there are several noticeable
differences between CCD and CMOS sensors.
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CCD sensors, as
mentioned above, create high-quality, low-noise images. CMOS
sensors, traditionally, are more susceptible to noise.
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Because each pixel
on a CMOS sensor has several transistors located next to it, the
light sensitivity of a CMOS chip is lower. Many of the photons
hitting the chip hit the transistors instead of the photodiode.
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CMOS sensors
traditionally consume little power. Implementing a sensor in CMOS
yields a low-power sensor. CCDs, on the other hand, use a process
that consumes lots of power. CCDs consume as much as 100 times more
power than an equivalent CMOS sensor.
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CMOS chips can be
fabricated on just about any standard silicon production line, so
they tend to be extremely inexpensive compared to CCD sensors.
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CCD sensors have
been mass produced for a longer period of time, so they are more
mature. They tend to have higher quality pixels, and more of them.
Based on these
differences, you can see that CCDs tend to be used in cameras that focus
on high-quality images with lots of pixels and excellent light
sensitivity. CMOS sensors usually have lower quality, lower resolution
and lower sensitivity. However, CMOS cameras are less expensive and have
great battery life. |