Canon Color imageRUNNER C5180 Series, Service Manual

Page: 211 / 851

Chapter 7
7-1
7.1 Digital Image Processing
7.1.1 Overview
Digital image processing is performed at the reader controller PCB.
Major functions are as follows:
(1) Shading correction
F-7-1
7.1.2 Shading Correction
Despite of the even original density, the CCD output will not always be even because of the following reasons:
(1) Variation in sensitivity of pixels of CCD.
(2) Difference of transmission light intensity between the center area and the peripheral area of the lens.
(3) Difference of light intensity between the center area and the peripheral area of the scanning lamp.
(4) Deterioration of the scanning lamp.
Shading correction is performed to correct unevenness of the CCD output.
Shading correction is performed by determining the target value at service mode and also by every scanning of
the original.
1. Target value Correction
This correction is performed by measuring each level (R, G, B) with the standard white plate to store these
density data. The memorized data is processed by computation to use as a target value at shading correction.
2. Shading Correction
This correction is performed at every scanning of the original. By measuring the density of the standard white
plate to compare the measured value with the target value that is pre-stored at the shading correction circuit, set
the difference as a shading correction value. Using this shading correction value, it corrects the variation in each
element of the CCD at original scanning to even the density level.
F-7-2
Gain correction data
EEP-ROM
SRAM
CPU
Shading correction
Digital image signal
CCD/AP
PCB
Reader controller PCB
10
Digital image signal
8
Target
value
J101
J102
J204
J205
J203
CCD output
Original density
Standard
white plate
Characteristics before correction Target value
Measured value
Characteristics after correction
White