Canon imageRUNNER 1750 Series Service Manual page 47

2
Technical Explanation > Original Exposure and Feed System > Controls > Image Processing
● Original Reading Sensor Drive
The original reading sensor is a 1-line linear image sensor consisting of 5147 photocells.
After completion of photoelectric conversion in the lightreceiving block, the signals are output
to the reader controller PCB in parallel for each channel (total eleven channels) of the original
reading sensor array.
Original reading Original reading
sensor 11 senor 10
Light receiving block
L L
● Original Reading Sensor Output Gain Correction and Offset
Correction
The analog video signals output from the original reading sensor are corrected so that they
will have a specific gain level (gain correction), and the output voltages generated in the
absense of incident light are also corrected so that they will have a specific offset level (offset
correction).
● Original Reading Sensor Output A/D Conversion
After completion of the gain correction and offset correction, the analog video signals are
converted to digital signals corresponding to individual pixel voltage levels by the A/D
converter.
2 Technical Explanation > Original Exposure and Feed System > Controls > Image Processing
Original reading
sensor 1
output
H H L H L H H H L .........
F-2-31
● Outline of Shading Correction
The original reading sensor outputs are necessary even for the following reasons even when
the density of the original is uniform:
(1) Variation in sensitivity among original reading sensor pixels
(2) Variation in light intensity of rod lens array
The machine performs shading correction to even out the original reading sensor output.
There are two types of shading correction: shading adjustment performed in the service mode
and shading correction performed for each job.
● Shading Adjustment
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.
● Shading Correction
The machine performs shading correction for each scan. It measures the density of the
standard white plate, and compares the measured value with the target value stored in the
shading correction circuit to use the difference between the two as the shading correction
value. The machine uses this shading correction value to correct the variation among the
original reading sensor pixels when scanning the originals, thus evening out the image
density level.
Original reading
sensor output
Target value
Measurement value
Standard
white plate
2-19
Characteristics after correction
Characteristics before correction
Original density
White
F-2-32
2-19