Canon CANOSCAN FB330 series Service Manual page 62

CHAPTER 2
B. Image Processing Functions
1. Contact image sensor
The CPU outputs clock signals based on the command from the host computer to the
contact image sensor. The contact image sensor lights the LED in order of red, green and blue
according to the clock signals, then outputs 1 line of image signal proportional to the light
intensity received by the phototransistor.
2. A/D conversion
Analog image signals output from the contact image sensor are converted into the digital
image data of 12 bits each by A/D converter in the analog IC in order of red image signal, green
image signal, and blue image signal.
3. Shading correction
When the contact image sensor reads a document of an even density, the image signal
corresponding to each pixel is not uniform for the following reasons.
1) Light intensity of LED is not uniform.
2) There is variation in the sensitivity of the light phototransistors.
3) There is a slight output from the light phototransistors even when there is no input.
These variations are corrected by the shading correction. Calibration data is used as
standard density data when scanning a document. Scanned image data is compared to the
standard density data for the image data correction. The shading correction converts the
image data from 12 bits to 10 bits each color.
4. Gamma correction
Shading-corrected red, green and blue image data are divided uniformly into 1024
gradations according to the document density. The contrast and density of this image data are
adjusted by the gamma correction. The CPU writes gamma curve specified by the host
computer into the buffer RAM before scanning a document. Image scan starts and the image
data input to the buffer RAM is converted by the gamma curve data and is output.
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COPYRIGHT © 1999 CANON INC. CANOSCAN FB330/FB630 SERIES REV.0 OCT. 1999 PRINTED IN JAPAN (IMPRIME AU JAPON)