In
this document:
CCD
defined 
A
CCD (Charge Coupled Device) converts an optical image into an
electrical signal. The image photons that strike the surface of
the CCD are converted into an electrical charge pattern in the
silicon. Each pixel functions like a separate charge bucket. The
charge pattern is transported to the output by using electrical
signals to move the charge buckets in a manner similar to the
passing of buckets in a bucket brigade. At the output of the CCD,
the charge is converted to a voltage.
Comparing
CCD with film
In
a digital camera, a single CCD is used to collect every image. In
a film camera, every image is recorded on a separate frame of
film. When film is exposed to light, silver grains and dye clouds
are formed that give the printed image a granular appearance. Fast
(more sensitive) films are grainier than slower, less sensitive
films. In a CCD, however, enough photons can be captured to obtain
grain-free prints. A CCD also captures light more efficiently than
film. A color CCD captures about 10 percent of all visible
photons, while film captures only about 1 percent.
Comparing
the professional and the consumer CCD
Virtually
all digital cameras designed for professional photographers, and
many broadcast television cameras, use a frame transfer CCD. A
different CCD architecture, known as interline transfer, is used
in almost every consumer digital camera and camcorder. The
principal advantages of frame transfer are large dynamic range and
charge capacity. The principal advantages of interline transfer
are fast electronic shuttering and low smear. An interline
transfer CCD provides the best video performance and a frame
transfer CCD provides the best overall picture quality. This is an
especially important benefit in still photography.
The
benefit of a professional frame transfer CCD
The
use of a professional frame transfer CCD offers a couple of
benefits. The first is a more pleasing rendition of high
dynamic-range scenes, which can be captured effectively with the
extended dynamic range of the frame transfer CCD. The greater
charge capacity of the frame transfer CCD yields the second
benefit, lower noise in the captured image.
Charge
capacity defined
Charge
capacity is the maximum potential charge capability of energy
wells to store charge without overflowing. Charge capacity
increases with the area of the pixel, and is much greater in a
frame transfer CCD than in an interline CCD.
The
importance of charge capacity
The
charge capacity determines the dynamic range of the CCD. It also
determines how much noise will be present in the image. The random
arrival of photons is accompanied by shot noise, which is a
natural process present in any method of light capture. As the
level of signal charge increases, the relative level-of-shot noise
decreases. If the charge capacity is low, captured images will be
noisy, especially in the dark areas. If the charge capacity is
high, the captured images can be free of perceptible noise.
Dynamic
range defined
Dynamic
range is the range of light that can be faithfully captured in a
single scene, from the darkest shadows to the brightest highlight.
The high end of the dynamic range is limited by the charge
capacity of the CCD. The low end of the range is limited by the
noise of the electronics.
The
importance of dynamic range
Many
photographic scenes have a very wide dynamic range. If the dynamic
range of the camera is smaller than the dynamic range of the
scene, highlight texture and shadow detail will be lost.
Benefits
of using a professional frame transfer CCD instead of a consumer
CCD
For
the same resolution, a professional 2/3-inch CCD will have larger
pixels than those found on a 1/2-inch consumer CCD. Large frame
pixels provide greater charge capacity and dynamic range, which
yields images with lower graininess, and a more pleasing rendition
of high dynamic-range scenes as evidenced in better highlights and
shadow details. In addition, they provide greater sensitivity,
because the sensitivity varies directly with the pixel area. These
factors coupled with HP imaging technology provide higher-quality
images at low light levels.
Copyright
Hewlett-Packard Co. 2002
This
information is subject to change without notice and is provided
"as is" with no warranty. Hewlett-Packard shall not be
liable for any direct, indirect, special, incidental or
consequential damages in connection with the use of this
material.
