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CHAPTER 4 IMAGE PROCESSING SYSTEM
B.
Shading correction
Even when reflected light from a document with even density is detected by the CCD, the CCD output will not be consistent for the following reasons. Reason 1: There is variation in the photosensitivity of the individual CCD pixels. Reason 2: The volume of light filtered through the lens center and the peripheral areas is different. Reason 3: The volume of light from the scanning lamp centre and ends is different. Reason 4: The scanning lamp�s quality gradually deteriorates. Shading is performed in order to correct the above. There are two types of shading correction; shading correction for every copy and the automatic shading (the value of which is set in the service mode). Shading target value setting The automatic shading correction is operated and executed by TEST MODE 2:CCD in the service mode. This operation measures and records the density of the white reference plate (behind the copyboard glass). The memorized data are arithmetically processed and used as the �target value� when correcting the shading. Shading correction In this operation, the scanning lamp light irradiates the white reference plate, (refer to Figure 4-302) and the volume of that reflected light is digitized by the analog image processing assembly. The digitized reflected light volume is input as shading data into the image processor PCB�s shading circuit. The memorized target value and the shading data are arithmetically processed. The result is then set as the shading correction value. The shading correction value corrects the unevenness in the CCD�s pixels at the time of document scanning, and is used to create image density uniformity. Shading correction takes place at the following times: � When the power is switched ON. � When the number of pages in a document is between 1 and 19, shading correction occurs before the first page is scanned. � When performing continuous copying (copying 20 pages or more), shading correction takes place before the first page is scanned and then once every 20 pages (after the previous shading correction has taken place, it will take place again after scanning the 20th page). � For Fax transmissions, shading correction takes place before scanning the first page. (Shading correction only takes place once regardless of the number of pages). b. a.
COPYRIGHT © 1999 CANON INC.
CANON GP160 REV.0 FEB. 1999 PRINTED IN JAPAN (IMPRIME AU JAPON)
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