Brother HL-4200CN Service Manual page 69

1.3.2 Exposure
In the exposure process, the drum surface charged negatively is scanned by laser
beams to form invisible electrostatic latent image on the drum surface.
This process is performed in parallel for yellow, magenta, cyan and black colors.
•
Laser beams are emitted from the laser diode in the scanner ASSY. By the rotating
polygon mirror, fixed mirror and lens attached to the scanner ASSY of the scanner
ASSY, each color of drum surface is scanned from end to end in the axial direction.
Polygon Mirror
Quad Beam Laser Diode
•
The laser beams are emitted based on the print data (image data) from the printer
controller.
When the print data instructs to print pixel points, laser beams are generated and
when the print data instructs not to print, no laser beams are generated. (On the
areas which are developed by toner, the laser beams light up and areas which are
not developed by toner, laser beams go out.) The laser beams emitted on the
drum surface generate a pair (electron <=> hole) in the optical conductive layer.
[Electrons are excited on the conductive zone, causing holes at the valence band.]
Electrons are induced by the electric field, moved toward the inside metallic part
and flow into it. The holes move toward the outer surface of the optical conductive
layer, are combined with the minus charge (electron) on the outer surface again
and decrease negative charge. As a result, on the drum surface where the electric
potential increases, invisible static latent image (print image) is generated.
: Laser beams (Y)
: Laser beams (M)
: Laser beams (C)
: Laser beams (K)
Mirror
Mirror
Mirror
Lens
Lens
Mirror
Mirror
Mirror
Fig.3-7
III-5
Mirror
Drum (Y)
Drum (M)
Drum (C)
Mirror
Drum (K)
Mirror