Canon imageRUNNER ADVANCE C255 Series, Service Manual, Page: 39 / 987

LED (RGB)
Control
Assembly
CPU
AFE
Scanner Unit (CIS) Main Controller PCB
ASIC
Control signal
Image signal
3 Line Sensor
3 Line Sensor LED (RGB)
AFE
Scanner Unit (CIS)
Flash
ROM
DDR
MTF Correction
Shading Correction
Color Displacement
Correction
Control
Assembly
A/D
Conversion
A/D
Conversion
Analog Image
Processing
- Gain Correction
- Offset Correction
Analog Image
Processing
- Gain Correction
- Offset Correction
■ Shading Correction
● Overview
Even if the original density is even, output of the CIS Reading Sensor in the Scanner Unit may not become even. A control that
corrects variations in the output is shading correction.
Main causes of uneven output of the Reading Sensor are shown below.
• Variation in sensitivity of pixel of the Reading Sensor
• Uneven light intensity of the lens
• Uneven light intensity of the LED and light guide plate
• Deterioration of the LED
• Variations in luminance between at the position of the Standard White Plate and at the reading position
To correct unevenness of the Reading Sensor output, shading correction is performed.
In shading correction, there is a type of shading correction that is executed per job.
● Shading Correction (Common to Reader and ADF)
Shading correction is performed for each scanning of original.
With this operation, light of LED Lamp is emitted to the Standard White Plate, and the reflected light is converted into digital data
at the analog image processing part of the Scanner Unit PCB. The amount of digitized reflected light is input to the shading
correction circuit in the Main Controller PCB as the shading coefficient. In the shading correction circuit, the stored target value
and the shading coefficient are compared, and the difference is determined as the shading correction value.
With this shading correction value, variation of pixel of the Reading Sensor of each scan is corrected to make the image density
level even.
2. Technology
28