Shading - Canon imageFORMULA P-208 Service Manual With Parts Catalog

2. Shading

This section explains the reading
mechanism of the white reference sheet for
determination of the shading correction
White
reference
sheet
This machine can read the white refer-
ence data at the document reading position,
unlike other scanners having the black
background color, since its background color
is white. Therefore, there is neither need to
feed the shading sheet nor to move the
internal white reference sheet or the reading
units.
For example, when the lower reading
unit reads the white reference data, the LED
emitted from the lower unit is reflected from
the white reference sheet on the upper unit
to be input to the sensor on the CIS PCB.
Since the white reference sheet is
placed under the reading glass, feeding
document does not cause smudges on it.
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value.
The sectional view of the reading unit is
shown below.
Figure 2-202
When this machine is turned ON or
starts scanning, it reads the white reference
data to determine the shading correction
value.
However, the slightly different optical
paths to the light receiving element are used
for the actual document and the white ref-
erence sheet. Therefore this machine needs
fine adjustment of the shading correction
value using the service mode and the
shading sheet. This fine adjustment is nec-
essary after replacing the reading unit or
after replacing the control PCB recording the
shading correction value.
CANON P-208 FIRST EDITION
CHAPTER 2 FUNCTION & OPERATION
White
reference
sheet
2-7