Canon iR2318 series, Руководство по обслуживанию, Страница: 124 / 308

Chapter 4
4-18
The reader controller PCB is used to process images on a line-by-line basis.
Its major functions are as follows:
1)Analog image processing
- CMOS sensor drive
- CMOS sensor output gain correction and offset correction
- CMOS sensor output A/D conversion
2)Digital image processing
- Shading correction
F-4-27
4.3.6.2 CMOS Sensor Drive
0020-4241
The CMOS sensor used in this machine is a 1-line linear image sensor con-
sisting of 7488 photocells. After completion of photoelectric conversion in
the light-receiving block, the signals are output to the AP circuit in the reader
controller PCB in parallel for each channel (total four channels) of the
CMOS sensor array.
F-4-28
4.3.6.3 CMOS Sensor Output Gain Correction and Offset
Correction
0020-4242
The analog video signals output from the CMOS sensor are corrected so that
they will have a specific gain level (gain correction), and the output voltages
generated in the absence of incident light are also corrected so that they it
will have a specific offset level (offset correction).
4.3.6.4 CMOS Sensor Output A/D Conversion
0020-4243
After completion of the above corrections, the analog video signals are con-
verted to digital signals corresponding to individual pixel voltage levels by
the A/D converter.
4.3.6.5 Shading Correction (Outline)
0020-4244
The CMOS sensor outputs are necessary even for the following reasons even
when the density of the original is uniform:
(1) Variation in sensitivity among CMOS sensor pixels
(2) Variation in light intensity of rod lens array
The machine performs shading correction to even out the CMOS sensor out-
put.
There are two types of shading correction: shading adjustment performed in
the Service mode and shading correction performed for each job.
4.3.6.6 Shading Adjustment
0020-4245
The machine measures the density of the standard white plate, and stores the
measured density data. It then processes the stored data to use it as the target
value for shading correction.
4.3.6.7 Shading Correction
0020-4246
The machine performs shading correction for each scan. It measures the den-
sity of the standard white plate, and compares the measured value with the
target value stored in the shading correction circuit to use the difference be-
tween the two as the shading correction value. The machine uses this shading
correction value to correct the variation among CMOS sensor pixels when
scanning the original, thus evening out the image density level.
F-4-29
Target
value
ensor (1 line)
Analog image signal
image
ssing
rrection
orrection
A/D
conversion
CMOS sensor
drive control
CMOS senso
control signa
Gain
correction
data
Digital
image
signal
IC1
10
Digital imag
signal
8
Reader controller PCB
act image sensor (CIS)
RAM
Shading correction
IC2
Serial
communication
I/F
CMOS sensor CMOS sensor 4 CMOS sensor 3
H L L H H L H L H H L
output
Light receiving block
CMOS
sensor
output
Original density
Standard white plate
Characteristics
before correction
Target
value
Measurement
value
Characteristics
after correction
White