Shading Correction - Canon GP215 Service Manual

3. OPERATIONS AND TIMING

2. Shading Correction

The output of the CCD will not be constant for
the following factors even when the light reflected
by an original with even density is projected to the
CCD.
1 The sensitivity of each pixel of the CCD is
not the same.
2 The degree of transmission of the lens is
different between the center and the sides.
3 The intensity of the scanning lamp is differ-
ent between the center and the sides.
4 The scanning lamp can deteriorate.
Shading correction is performed to correct the
above discrepancies. Shading correction may be
executed for each copy or executed automatically
according to the target value determined in ser-
vice mode.
a. Shading Automatic Correction
Shading automatic correction is executed by
using service mode (*4*; FUNCTION).
In this type of correction, the density of blank
paper and the standard white plate is measured
and stored in memory. The stored data is subject-
ed to computation and used as the "target value"
for shading correction.
Standard white plate
No. 2 mirror
No. 1 mirror
No. 3 mirror
Figure 3-305
3–34
COPYRIGHT © 1996 CANON INC.
Scanning lamp
CCD
Lens
b. Shading Correction
Shading correction is executed for every scan-
ning of an original.
In this type of correction, the light of the scan-
ning lamp is directed to the standard white plate
(Figure 3-305), and the reflected light is turned
into digital signals by the analog image process-
ing block. The reflected light data which has been
turned into digital signals are sent to the shading
circuit of the image processor PCB.
The shading circuit compares the pre-deter-
mined target value and shading data and sets the
difference as the shading correction value.
The shading correction value is used to correct
variations among CCD pixels occurring at time of
scanning and to ensure a specific image density
level.
CCD output
Target value
Measurement
data
CANON GP215/200 REV.0 JULY 1996 PRINTED IN JAPAN (IMPRIME AU JAPON)
Characteristics
after correction
Characterisitics
before correction
Standard white plate
Figure 3-306
Original density