Laser/Scanner; Scanning Mirror; Exposure, Image Formation; Scanning A Rotating Drum - Canon LBP-CX Series Service Manual

II. LASER/SCANNER
A. Introduction
The laser driver (part of the DC controller PCB)
produces a LASER DRIVE signal (LD) based on the
video signal (VDO, VDO) transmitted from the video
controller (through the video interface), which causes
the semiconductor laser to generate the laser beam
that exposes the rotating photosensitive drum.
The laser driver stabilizes laser beam intensity
according to the signals generated by the photodiode
in the laser unit, and adjusts beam intensity accor-
ding to the sensitivity of the drum based on signals
from the drum sensitivity identification micro-
switches.
As the drum rotates the stationary laser scans the
drum by reflecting a beam off a rotating scanning
mirror. The result is a raster scan, very much like that
which forms the pictures in a television set. The
arrangement is shown below.
Scanning mirror
Focusing lenses
Beam position
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sensor
Optic fiber '-
Beam-to-drum
Scanner motor
mirror
Figure 2-4
The collimating lens aligns the laser light into a
thin, parallel beam which strikes the six-faced scann-
ing mirror. Reflecting from the mirror, the laser beam
is focused to a tiny spot on the drum surface. If the
laser is ON, it exposes the drum at this point. How
this exposed point is developed into a visible dot on
paper is explained in Section III.
The focusing lenses focus the laser beam onto the
drum.
B. Exposure, Image Formation
The simplest example of image formation is an
unmoving laser beam striking an unmoving drum in a
single spot. If the image were developed the result
would be a white print with a single, tiny, black dot.
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Scanning mirror
Figure 2-5 Scanning mirror
This printer uses the rotating, six-faced scanning
mirror shown in Figure 2-5 to move the laser beam
across the drum.
Print
density of
developed
line
LO signal
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Drum
Black level White level
(Laser ON) (Laser OFF)
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Figure 2-6 A single scan
Figure 2-6 shows a non-rotating drum which is
scanned once by the laser beam. The result of a laser
pulse train is shown above. A laser exposure results in
a black trace when developed.
Actual scanning is accomplished with a rotating
drum, so that the resulting scan resembles the raster
technique used in television.
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LO signal
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Image
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Figure 2-7 Scanning a rotating drum
2-4
COPYRIGHT
©
BY CANON INC. CANON LBP-CX SERIES REY.2 JULY 1985 PRINTED IN JAPAN (IMPRIME AU JAPONJ