Shading Correction - Canon Color Image Reader - L1 Service Manual

2
Technology

Shading correction

Overview
The output of the CMOS may not always be uniform because of the following reasons even
when the density of the original is uniform:
•
Variation in the sensitivity among individual pixels of the CMOS.
•
Difference in the level of transmission between the center and the edge of the lens.
•
Difference in the intensity of light between the middle and the edges of the LED lamp unit.
•
Deterioration of the LED lamp unit.
The machine executes shading correction to even out the output of the CMOS.
The machine executes either of the following 2 shading mechanisms: shading correction
that is carried out per every copy and shading adjustment for which the target value is set in
service mode.
Shading adjustment
The machine performs shading adjustment in response to a command made in service mode.
The machine computes the data entered in service mode to produce the target value for use
at time of shading correction.
Shading correction
The machine executes shading correction each time it scans an original.
The machine directs the light from the lamp unit against the standard white plate, and the
analog image processing block on the CMOS PCB converts the refl ected light into a digital
signal. The result (i.e., a digital signal representing the intensity of the refl ected light) is sent
to the shading correction circuit of the reader controller PCB as a shading coeffi cient. The
shading correction circuit in turn compares the coeffi cient with the target value it holds, and
offers the difference as the shading correction value.
The machine uses the shading correction value to correct the variation that may exist among
the individual pixels of the CMOS per each scan, thereby keeping the image density to a
specifi c level at all times.
2
Technology
CMOS line
sensor output
Target value
Measurement
Standard white plate
Error code
E302 (Abnormality in the shading)
-0001 Abnormality in the front side shading RAM access, value front side shading is below or
above the specifi cation.
-0101 Abnormality in the back side shading RAM access, value back side shading is below or
above the specifi cation.
Characteristics after
correction
Characteristics before
correction
White
Original density
2-17
2-17