My flatbed #scanner, an old Canon CanoScan 8800F, had developed a #banding issue. A much brighter band in the image. I don't use the scanner often, and I have no idea what had caused it.
At first I thought that I need to start correcting that in software, should be easy, just subtract a suitable amount from the band area in each colour channel of the image after scanning. In some suitable software. Probably a single trivial ImageMagick command-line.
But I did some googling, and found out that problems like this can have an almost mechanical trivial root cause. Before each scan, the scanner calibrates its lights+sensors against a strip of uniform whiteness under the scanning surface of the scanner. If there is a darker area there, the scanner will make the corresponding band brighter.
Four screws to open and some some wiggling to get the top glass assembly loose (no cables luckily), and yep, a darker blotch on the calibration strip.
Still no idea what has caused that stain. Possibly I have cleaned the glass with some unsuitable fluid that has leaked into the calibration strip and discoloured it. Or the wife (who is the main user of the scanner) has. Lesson learnt, in that case...
The calibrating strip is under the glass and I didn't want to pry the glass loose. So I simply cut a strip of bright white 160 g/m² paper and taped it on top of the calibration strip area. Problem solved.
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