ChApter 1. digitAl CAmerAs & imAges
30 For more on textbooks in digitAl photogrAphy, visit http://www.photoCourse.Com
http://www.photocourse.com/itext/CCD/
image sensors—Types...
When using a film camera you can insert any kind of film you want. It’s the
film you choose that gives photographs distinctive colors, tones, and grain.
If you think one film gives images that are too blue or red, you can change to
another film. With digital cameras, the “film” is permanently part of the cam-
era so buying a digital camera is in part like selecting a film to use. Like film,
different image sensors render colors differently, have different amounts of
“grain,” different sensitivities to light, and so on. The only ways to evaluate
these aspects are to examine some sample photographs from the camera or
read reviews written by people you trust.
Initially, charge-coupled devices (CCDs) were the only image sensors used in
digital cameras. They had already been well developed through their use in
astronomical telescopes, scanners, and video camcorders. However, there is
now a well-established alternative, the CMOS image sensor. Both CCD and
CMOS image sensors capture light using a grid of small photosites on their
surfaces. It’s how they process the image and how they are manufactured
where they differ from one another.- CCD image sensors. A charge-coupled device (CCD) gets its name from
the way the charges on its pixels are read after an exposure. The charges on
the first row are transferred to a place on the sensor called the read out regis-
ter. From there, they are fed to an amplifier and then on to an analog-to-digi-
tal converter. Once a row has been read, its charges in the readout register
row are deleted, the next row enters, and all of the rows above march down
one row. With each row “coupled” to the row above in this way, each row of
pixels is read—one row at a time. - CMOS image sensors. Image sensors are manufactured in factories
called wafer foundries or fabs where the tiny circuits and devices are etched
onto silicon chips. The biggest problem with CCDs is that they are created in
foundries using specialized and expensive processes that can only be used to
make other CCDs. Meanwhile, larger foundries use a different process called
Complementary Metal Oxide Semiconductor (CMOS) to make millions of
chips for computer processors and memory. CMOS is by far the most com-
mon and highest yielding chip-making process in the world. Using this same
process and the same equipment to manufacturer CMOS image sensors cuts
costs dramatically because the fixed costs of the plant are spread over a much
larger number of devices. As a result of these economies of scale, the cost of
fabricating a CMOS wafer is significantly less than the cost of fabricating a
similar wafer using the specialized CCD process. Costs are lowered even far-
ther because CMOS image sensors can have processing circuits created on the
same chip. With CCDs, these processing circuits must be on separate chips.
Despite their differences, both types of sensors are capable of giving very
good results and both types are used by major camera companies. Canon
and Nikon both use CMOS sensors in their high-end digital SLRs as do many
other camera companies.
This photo shows the
pixels on an image
sensor greatly enlarged.
Courtesy of IBM.
A silicon wafer used to
make image sensors.
Courtesy of IBM.
Click to see where the
name “charge-coupled
device” comes from.