Charge
The charge roll places a uniform negative electrostatic charge on the surface of the drum. The drum surface is
made of a photoconductive material that holds an electrical charge as long as the drum remains in darkness.
Light striking the drum discharges the surface charge.
HVPS
(-VDC)
(-VDC)
Ground
Conductive Tube
Photoconductive
Surface
0V
-VDC value
DRUM
Drum Surface
Voltage
Drum Surface
Image
Ground
Charge roll
Exposure
The printhead generates a beam of laser light. Image data received from the system card assembly modulates
this beam, turning it on and o according to image information that is received from the host computer and
software.
Through the use of a series of rotating and stationary mirrors within the printhead, the beam scans the negatively
charged drum surface. Whenever the print controller sends a command to print a black pixel, the laser switches
on long enough to shine onto the drum at a single pixel point. That point is now discharged and slightly less
negative than the surrounding negative charge. The less negative areas are considered positive.
4062
Appendix C: Theory of operation
539
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