Shading Correction - Canon GP160F Service Manual

B.

Shading correction

Even when reflected light from a document with even density is detected by the
CCD, the CCD output will not be consistent for the following reasons.
Reason 1: There is variation in the photosensitivity of the individual CCD pixels.
Reason 2: The volume of light filtered through the lens center and the peripheral
areas is different.
Reason 3: The volume of light from the scanning lamp centre and ends is different.
Reason 4: The scanning lamp's quality gradually deteriorates. Shading is performed
in order to correct the above.
There are two types of shading correction; shading correction for every copy and the
automatic shading (the value of which is set in the service mode).
a. Shading target value setting
The automatic shading correction is operated and executed by TEST MODE 2:CCD
in the service mode.
This operation measures and records the density of the white reference plate
(behind the copyboard glass). The memorized data are arithmetically processed and
used as the 'target value' when correcting the shading.
b. Shading correction
In this operation, the scanning lamp light irradiates the white reference plate, (refer
to Figure 4-302) and the volume of that reflected light is digitized by the analog image
processing assembly. The digitized reflected light volume is input as shading data into
the image processor PCB's shading circuit.
The memorized target value and the shading data are arithmetically processed. The
result is then set as the shading correction value.
The shading correction value corrects the unevenness in the CCD's pixels at the
time of document scanning, and is used to create image density uniformity.
Shading correction takes place at the following times:
• When the power is switched ON.
• When the number of pages in a document is between 1 and 19, shading correction
occurs before the first page is scanned.
• When performing continuous copying (copying 20 pages or more), shading correc-
tion takes place before the first page is scanned and then once every 20 pages
(after the previous shading correction has taken place, it will take place again after
scanning the 20
• For Fax transmissions, shading correction takes place before scanning the first
page. (Shading correction only takes place once regardless of the number of
pages).
COPYRIGHT © 1999 CANON INC.
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CANON GP160 REV.0 FEB. 1999 PRINTED IN JAPAN (IMPRIME AU JAPON)
CHAPTER 4 IMAGE PROCESSING SYSTEM
4-5