Canon GP405, Service Manual

Page: 94 / 682

CHAPTER 4 IMAGE FORMATION SYSTEM
4-8
COPYRIGHT © 1999 CANON INC. CANON 405/335 REV.0 JAN. 1999 PRINTED IN JAPAN (IMPRIME AU JAPON)
B. Shading Correction
The output of the CCD does not necessarily match the light reflected by an original (although
the original may have an even density) for the following reasons:
1. The sensitivity differs from one pixel of the CCD to another.
2. The degree of lens transmission is different between middle and edges.
3 The intensity of the scanning lamp is different between middle and edges.
4.
The scanning lamp deteriorates.
The resulting variations are corrected by means of shading: shading correction executed for
each copying run, and shading auto correction whose target value is set in service mode.
1. Shading Auto Correction
Shading auto correction is executed in service mode (FUNCTION>CCD>MAN-ADJ).
In shading auto correction, the density of the white standard plate and white paper is measured
(by the CCD PC), and the collected data is stored in memory. The data is then subjected to
computation, and the result will be used as the "target value" at time of shading correction.
2. Shading Correction
Shading correction is executed for each scan of an original. The light of the scanning lamp is
directed to the standard white plate (Figure 4-301), and the light reflected by the plate is converted
into digital signals by the CCD PCB. The resulting digital signals are sent to the shading circuit on
the image processor PCB as shading data.
The shading circuit compares the shading data against the target value stored in memory, and
sets the difference as the shading correction value.
The shading correction value is used to correct the variation among the pixels of the CCD
occurring during scanning operation, thereby evening out the image density level.
Figure 4-301 Figure 4-302
Standard white plate
Scanning lamp
No. 1 mirror
No. 2 mirror
No. 3 mirror
Lens
CCD
CCD output
Original density
Standard white plate
Characteristic
before correction
Target value
Measurement data
Characteristic
after correction