Canon imageCLASS MF8450c, Service Manual

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Chapter 7
7-3
7.1.3 Electrostatic Latent Image Formation Block
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imageCLASS MF9170c / imageCLASS MF9150c / imageCLASS MF8450c
In this block, an electrostatic latent image is formed on the photosensitive drum by following three steps.
When a laser beam scans the photosensitive drum charged with a negative potential, an image with negative charge is formed on the photosensitive drum. This
image is called "an electrostatic latent image" because it is invisible to human eyes.
Step 1: Pre-exposure
As a preparation for the primary charging, the LED light of the pre-exposure LED is exposed on the photosensitive drum surface. This eliminates the residual charge
on the drum surface in order to avoid image density unevenness.
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Step 2: Primary charging
As a preparation for the latent image formation, the surface of the photosensitive drum is changed with a uniform negative potential.
The machine charges the photosensitive drum with negative charge directly from the primary charging roller, which is interlocked with the drum.
The primary charging roller is made of a conductive rubber, and a negative DC bias is applied to the roller.
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Step 3: Laser beam exposure
The latent image is formed on the photosensitive drum by a laser beam.
When a laser beam is exposed on the photosensitive drum, the potential on the exposed area (light area) is neutralized and the negative potential is removed from
the drum surface. This area forms electrostatic latent images. A negative potential remains in the area where a laser beam was not exposed (dark area).
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Photosensitive drum
LED
Photosensitive drum
Primary charging roller
DC bias
Laser beam
dark area
light area