Digital Image Processing; Outline; Shading Correction - Canon iR C3200 Series Service Manual

Chapter 7

7.3 Digital Image Processing

7.3.1 Outline

The machine uses the reader controller PCB for digital image processing; the major operations involved are as
follows:

1. Shading Correction

CCD/AP
PCB

7.3.2 Shading Correction

The output of the CCD is not necessarily uniform even when the density of the original is even for the following
factors:
1. variation in sensitivity among pixels of the CCD
2. difference in transmission between the middle and the edge of the lens
3. difference in the intensity of light between the middle and the edges of the scanning lamp
4. deterioration of the scanning lamp
The machine executes shading correction so as to even out the discrepancies in CCD output. Shading correction may
be shading adjustment, in which a target value is determined in service mode, or shading correction, in which a target
value is determined for each scan of the original.
Shading Adjustment
In shading adjustment, the density of the standard white plate is measured, and the result is stored in memory as
density data; the data is then computed to obtain the target value for shading correction.
Shading Correction
Shading correction is executed for each scan of the original. The density of the standard white plate is measured, and
the shading correction circuit compares the result against the target value stored in memory in advance; the result of
comparison is then used as the shading correction value to make up for the discrepancies among CCD pixels
occurring at time of scans, thus ensuring uniform image density.
7-4
Reader controller PCB
J101 J204
Gain
correction data
J102 J205
Digital
image signal
10
EEP-ROM
Target
CPU
value
SRAM
Digital image signal
Shading correction
F-7-4
0006-0674
J203
8
0006-0678