Service manual

CHAPTER 2

2 - 14



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.