Canon F-1, Service Manual

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Technology > Controls > Reader > Image processing
Technology > Controls > Reader > Image processing
● Shading processing
Overview
The output of the reading sensor 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 reading sensor.
• 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 reading sensor.
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 scanner unit PCB converts the reflected light into a
digital signal. The result (i.e., a digital signal representing the intensity of the reflected light)
is sent to the shading correction circuit of the reader controller PCB as a shading coefficient.
The shading correction circuit in turn compares the coefficient 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 reading sensor per each scan, thereby keeping the image density
to a specific level at all times.
Reading
sensor output
Original density
Standard white plate
Characteristics before
correction
Target value
Measurement
Characteristics after
correction
White
F-2-63
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 specification.
-0101 Abnormality in the back side shading RAM access, value back side shading is below or
above the specification.