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CHAPTER 2
B. Scanning Exposure
The simplest example of exposure of the
photosensitive drum by the Laser beam is
to imagine that both the drum and beam
are stationary. In this case the laser beam
illuminates a single point on the drum.
V i d e o slgnal '
Scanmng mlrlor
R@re 2-8
When the scanning mirror is rotating and
the laser beam strikes one of its faces, the
reflected laser beam scans from one end of
the photosensitive drum to the other
(horizontal
scan).
A broken line is produced by switching
the laser beam on and off during the
horizontal wan according to the VIDEO
signal I/VDO)
V i d e o signal \
Scanning mirror
Eyeure 2 - 9
W h e n
t h e
p h o t o s e n s i t i v e
in addition to the horizontal scan,
rotating.
the laser beam also advances around the
photosensitive drum (vertical scan). The
photosensitive drum rotates at constant
speed while the laser beam scans across it.
In the time that the beam takes to sweep
2-8
Photosensitive
d r u m
i s
a l s o
across the drum and return to its original
position, the drum surface ratates about 43
)_trn (600 raster lines/inch). Successive
horizontal scans are therefore separated by
this interval.
Scanning mirror
Video signal
11
12
Figure 2-10
The surface of the photosensitive drum is
charged to a negative potential by the
primary charging roller before printing
starts. When laser light strikes the
photosensitive drum during the horizontal
and vertical scans described above, the
electrostatic charge at that location is
neulralized. producing an electrostatic
latent image on the photosensitive drum
surface.
There is a small fixed beam-detect (BD)
mirror in the laser beam path. As the laser
beam is swept horizontally toward the
starting point for printing a line of data
lthe scanning start position), the beam
strikes the BD mirror and is reflected to
the sensor on the BD PCB. The laser beam
is converted to an electrical signal by the
sensor and the signal is sent to the DC
controller as the BD INPUT signal (/BDI).
The DC controller produces the BEAM
DETECT signal (/BD) in response to the
/BDI signal. The /BD signal is sent to the
video controller as a differential signal (BD,
/BD).
The laser is ON whenever the laser
beam is not scanning the drum surface to
allow detection of the /BD signal. This
-
Photosensitive
t3
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