Ricoh C211 Service Manual page 420

4. CCD (Charge Coupled Device)
The
CCD can be
thought of as many small photocells in a row. One ceil or element will
read one point (about .005" x .005")
in
a
scan line. Internally, we can imagine a rotary
switch that selects the output of each element in sequence. See figure 1. Each element
conducts proportional to the amount of light striking
its face. The CCD'S output, which we
will call video, represents the current scan line. As the scan line changes, these changes
are reflected in the video output.
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Figure
1
A clock is sent to the CCD to "move" the "rotary switch" inside the CCD. The faster the
input clock, the faster the video signal. There is an "off time" where the output of the CCD
is off while the "rotary switch" resets from the last element to the first element. This
"off
time" is the end of our main scan.
We
can say that our scanning is a raster scan, very much
like the picture is created on a television set. If you could slow the electron beam on a TV
picture tube, you would see the beam move across the screen from left
to right and then
disappear momentarily before reappearing at the left for the next scan. This is the main
scan of the picture tube and the distance the beam moved downward for the next main
scan is the vertical resolution. Our CCD scanner uses the same principle. Since it is
stationary, it provides us with our main scan. During CCD "off time" a DC stepping motor
provides drive to rollers that move the original document forward for the next main scan.
The distance that the rollers move the document is our sub-scan for vertical resolution.
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