Shading Correction - Canon ImageRunner iR8500 Series Service Manual

Chapter 6
As needed, the machine rotates the image, thereby reducing waste of paper. The time it saves from not having to detect the orientation of all originals also
helps increase its productivity.
All image data is
collected, and the text
area is identified.

6.3.4 Shading Correction

/ iR8070
a. Outline
The output of the CCD will not necessarily be uniform because of the following factors even if the density of the original in question is perfectly uniform:
(1) The level of sensitivity of a CCD pixel differs from that of another.
(2) The level of penetration of light differs between the center and the periphery of a lens.
(3) The intensity of the scanning lamp differs between the middle and the ends of the lamp.
(4) The scanning lamp is subject to deterioration.
Shading correction is executed to correct discrepancies in the output of the CCD, and it may be of either of the following two: shading adjustment used to
determine a target level in service mode and shading correction executed when scanning each original.
To make up for the fluctuations in the intensity of light occurring at short intervals, edge
area gain correction is also executed.
b. Shading Adjustment
In this adjustment, the density of white paper and that of the standard white plate are measured, and the results are stored in memory.
The data is computed for use as the target level during shading correction. The adjustment is designed for service mode and is used upon installation of
the machine, after replacement of the scanning lamp, or when correcting changes in the intensity of the scanning lamp occurring over time.
c. Shading Correction
This correction is executed each time an original is scanned. The density of the standard white plate is measured and the result is compared against the
target value stored in the shading correction circuit. The difference is used as the shading correction value, which will be used to correct the variation in
CCD pixels, thereby ensuring a specific level of image density.
6-6
Reader controller PCB
Image data
8
Text area
extraction block
2
Text data
storage block
To increase processing speed, the
image data is processed into binary
data and stored.
0˚
The result of detection is sent to
the reader controller
[1] 0˚
[2] 90˚
[3] 180˚
[4] 270˚
[5] not know (output as is)
The stored text data is
Text direction
matched against the
identification block
fonts in the dictionary
to find out the direction
of characters.
Dictionary
storage block
Flash ROM
Language dictionaries
are stored.
F-6-10
0008-9646