Canon multipass c3000 Service Manual (en)

Contact Sensor
The Contact Sensor used in the Document Feed Section scans Letter-width
8.43" (214 mm), documents at a horizontal resolution of 8 dots/mm (203.2
Resolution in the vertical direction may be selected by the 
button on the operation panel.  Resolution is 3.85 lines/mm (97.8 lpi) in
Standard mode and 7.7 lines/mm (195.6 lpi) in Fine mode (for fax
For copying, the read resolution is automatically set to following.
Direct copy :
360 dpi 
360 dpi
Memory copy :
8 dot/mm 
7.7 line/mm
The structure of the Contact Sensor is shown in 
Figure 2-5
.  Light from the two LED cells,
located at either end of the Contact Sensor, is directed by the optical guide, gathered by the
lens, and used to illuminate the surface of the document.  Light reflected back from the
document surface is captured by 1728 amorphous silicon photosensor chips.  The captured
light is amplified as an analog signal, and sent as serial data to the System controller IC on
the SCNT board.
The System control IC (include UHQ 6.5 unit) on the SCNT board performs image processing
(halftone) in order to cleanly transmit image documents.
Shading Correction
To compensate for variations in Contact Sensor output, this model has
shading correction data stored in its control SRAM and image processing
SRAM, when shipped from the factory.  Thus, it does not perform a prescan of
each page, as performed by previous models.  Conformity of the shading data
is maintained by comparing the data between the two previously mentioned
SRAM blocks before reading each page.  If for some reason, there is an error
in the shading data, the message 
will be displayed to
indicate the shading data error.  In this case, the shading correction data
constant from the image processing IC SRAM is used.
When using the constant from the image processing IC SRAM, there is a
possibility of black spots appearing when in Auto Halftone Mode.  When this
type of image defect occurs, try re-registering the shading data.
For details on the shading data re-registration process, please see 
Page 3-6
Part 1: Facsimile