Canon iR3245, Service Manual

Page: 175 / 746

Chapter 5
5-19
5.3.6.4 A/D Conversion of CCD Output
0019-4664
iR3225 / iR3225N / iR3235 / iR3235N / iR3245 / iR3245N
The corrected analog video signal is converted into a digital signal that is suited to the voltage level of individual pixels by the A/D converter.
5.3.6.5 Shading Correction (overview)
0019-4665
iR3225 / iR3225N / iR3235 / iR3235N / iR3245 / iR3245N
The output of the CCD is not necessarily even for the following factors even when the density of the original is uniform:
1) Variation in the sensitivity of the CCD among pixels
2) Variation in the intensity of the rod lens array
3) Variation in the intensity of light that goes through the center and surroundings of lens
4) Variation in the intensity of light at the center and surroundings of LED
5) LED deterioration
The machine executes shading correction to even out the output of the CCD.
Shading correction may be the shading adjustment executed at power-on or the shading correction executed for each job.
5.3.6.6 Shading Adjustment
0019-4666
iR3225 / iR3225N / iR3235 / iR3235N / iR3245 / iR3245N
In this adjustment, the machine measures the density of the standard white plate, and stores the results in memory as density data. It then performs computations
on the shading data, and uses the result as the target value for shading correction.
5.3.6.7 Shading Correction
0019-4667
iR3225 / iR3225N / iR3235 / iR3235N / iR3245 / iR3245N
The machine executes the shading correction for every scan made.
In this correction, the analog image process unit on CCD PCB digitalizes the LED light reflected by the standard white board. After the reflected light is digitalized,
it is held in the shading correction circuit on reader controller PCB as a shading coefficient.
Shading correction circuit compares the stored target value with the shading coefficient. The difference between the two will be held as the shading correction value
for use in correcting variation among CCD pixels when scanning the original, thus evening out the density levels of the image.
F-5-29
CCD output
Original density
Standard white plate
Features before correction
Target value
Measurement
value
Features after correction
White