Canon imageRUNNER ADVANCE 4551i Service Manual page 63

● Gain correction of the Reading Sensor output, Offset correction
The analog video signal output from the Reading Sensor has its amplification ratio aligned with a fixed value (gain correction)
and has its output voltage when there is no incident light aligned with a fixed value (offset correction).
● A/D Conversion for Reading Sensor Output
The corrected analog video signal is converted into the digital signal for each pixel voltage value using an A/D converter.
● Overview of Shading Correction
Even density of an original is even, output of the Reading Sensor may not become even due to the following reasons.
1. Variation in sensitivity of pixels of the Reading Sensor
2. Variation in lens light intensity
3. Difference in the transmission light intensity in the center of the lens and the surrounding area
4. Difference in the light intensity in the center of the LED and the surrounding area
5. LED deterioration
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
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.
Red(R)line
Green(G)line
Blue(B)line
Black/White(B/W)line
Light-receiving section
Reading Sensor
output
Target value
Measurement
value
Enlarged
Output
L L H H L H L H H H L
.........
Characteristics
after correction
Characteristics
before correction
White
Original density
Standard White Plate
50
2. Technology