CHAPTER 2
2 - 19
COPYRIGHT
2000 CANON INC. CANOSCAN N650U/N656U/N1220U REV.0 JUNE 2000 PRINTED IN JAPAN (IMPRIME AU JAPON)
Input
Buffer RAM I (for writing)
Buffer RAM II (for reading)
Output
Fixed volume has been written
Buffer RAM I (for reading)
Output
Input
Buffer RAM II (for writing)
Sampling point
Moving direction
Scanning unit
Figure 2-19
4. Calibration
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 phototransistors.
3) There is a slight output from the phototransistors even when there is no input.
These variations are corrected by the calibration. 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 calibration converts the image data from 14
bits to 12 bits each color.
5. Gamma correction
Calibrated red, green and blue image data are divided uniformly into 4096 gradations
according to the document density. The contrast and density of this image data are adjusted
by the gamma correction. The gate array writes gamma curve specified by the control program
into the buffer RAM before scanning a document. Image scan starts and the image data input
to the buffer RAM is converted to 8 bits by the gamma curve data and is output.
6. Packing
Processed image data is temporarily stored in the buffer RAM before sent to the host
computer. The buffer RAM is divided into the writing block and reading block. While data is
written into the writing block, the data is read from the reading block. When fixed volume has
been written, the writing switches to the reading.
Figure 2-20
When decreasing the resolution, the scanning unit moves at a fast speed to read less lines.