CHAPTER 2
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COPYRIGHT
2002 CANON INC. CANOSCAN 8000F REV.0 OCT. 2002 PRINTED IN JAPAN (IMPRIME AU JAPON)
3. Calibration
When CCD reads a document of even density, the image signal corresponding to each pixel is
not uniform for the following reasons.
* Light intensity of the scanning lamp is not uniform.
* There is variation in the sensitivity of the light phototransistors.
* There is a slight output from the light phototransistors even when no data is input.
These variations are corrected by the calibration in three steps of Offset calibration, Coarse-
gain calibration, and Programmable Gain Amplifier (PGA).
1) Offset Calibration
Takes output data when no data is input and normalizes the output data from each pixel at
or near 0.
2) Coarse-Gain Calibration
Finds the optimum gain setting for placing the output voltage from all pixels in the
adjustment range of Programmable Gain Amplifier.
3) Programmable Gain Amplifier (PGA)
Calculates a gain coefficient for uniforming the output by correcting the output difference
among pixels.
4. Filter Processing
When converting resolution and scaling, the image quality tends to be reduced. To prevent
the image quality reduction, filter processing is performed according to the resolution. Filter
processing includes interpolation processing and averaging.