Today I found out about another shortcoming. The waste heat of the scanner sled does not get transported away properly, which leads to permanent traces on the scanner glas. Fortunately the effect is not yet visible in everyday scans, but this situation might get worse.
To visualize the effect, scans were taken with open cover in a dark room. I combined the results from different scans in multiple layers (GIMP) with different opacities to diminish the influence of little dust particles. It would have been ideal to use this, which I could not get to work unfortunately.
To highlight the structures in the picture, Gimp's
Retinex filter was used, which performs quite well on this task. The result can be seen in the picture below.
The scanner's sled moves to a rest position away from the edge of the scanner, if not used for a while. This is where the sled cools down and the traces emerge.
SEL: These traces are from the PCB, which is attached to the sled.
SPL: Below this dot lies a small cut-out in the sled. It transports heat from the power lines of the lamp to the glas.
SLI: This horizontal stripes lies above the lamp, that illuminates what is to be scanned.
SSCR: Some scratches produced by the sled. Maybe there was something to heavy on the glas?
F: Probably finger prints that arose while cleaning the edges ;)
SCR: other scratches?
The picture below shows the same kind of image, this time produced with a Canon CanoScan LiDE 35. Even though this scanner not being the most recent model, the qualitative difference is clearly visible:
I wonder if it might be possible to augment this technique to identify which scanner model a document has been scanned with? Comments and suggestions are welcome as usually.
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