Canon iR C6800 Series Service Manual page 199

<Timing of Detection>
<1> at the end of a job
<2>between sheets (for every sheet read)
*Executed only if dust has been detected at all points, i.e., A, B, C, D, and E in the previous job. If so, the machine
uses the point with least dust as the reading position and executes dust correction before starting reading operation.
<Particulars of Control>
at the end of a job (makes a change to the point of stream reading dust reading position)
The machine checks the light reflected by the platen roller at the reading position to see if there is dust, moving to
the next point if it finds dust (from A to B, C, D, and then to E; 5 points max.). It uses the first point that is free of
dust as the reading point for the next job.
At the start of a job
Start key
ON
1st
SCAN
Dust detection*
0.5mm
B
At time of a job
*1
2nd
SCAN
Dust detection Dust detection
F-5-27
0.5mm
0.5mm
0.5mm
C
A D E
*1
*1
*2
F-5-28
pattern 1
pattern 2
pattern 3
pattern 4
pattern 5
Chapter 5
5-17