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CHAPTER 2
3. Calibration
When scanning a document of even density, the image signal corresponding to each pixel
may be uneven due to the following reasons:
* Uneven light intensity of the scanning lamp
* Variation in the sensitivity of the light phototransistors
* A slight output from the light phototransistors with no light
Calibration corrects these variations with the following three steps:
Offset Calibration
Coarse-gain Calibration
Programmable Gain Amplifier (PGA).
1) Offset Calibration
Normalize the output data from each pixel at the value of 0 or near 0 by using data aquired
with no light.
2) Coarse-Gain Calibration
Finds the optimum gain setting to have the output voltage from all pixels within in the
adjustment range of Programmable Gain Amplifier.
3) Programmable Gain Amplifier (PGA)
Calculates a gain coefficient to make output constant by correcting the output difference
among pixels.
4. Filter Processing
Changing the resolution and the scaling tends to reduce the image quality. To prevent this,
the filter processing needs to be performed according to the resolution. The filter processing
includes interpolation processing and averaging.
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COPYRIGHT 2003 CANON INC. CANOSCAN 9900F REV.0 FEB. 2003 PRINTED IN JAPAN (IMPRIME AU JAPON)
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