Mustek 600CP/1200CP/1200CP+ Calibration strip problems

If your scanned images contain vertical stripes that are brighter than their surroundings, such as in the figures shown below, it is possible that the calibration strip of the scanner is dirty or stained.

The calibration strip is a white strip inside the case of the scanner, right before the start of the scan area, ie., it isn't visible from the outside. It is used to calibrate the pixels of the scanner head. If that strip isn't equally white everywhere, the calibration of certain pixels can go wrong, typically resulting in the vertical stripes shown above.

How can you verify that your calibration strip is dirty without opening the case? Well, the SANE mustek_pp driver for CIS scanners has a hidden option that can be used to make a scan of the calibration area. To enable it, you have to add the following line to your mustek_pp.conf file, right after the declaration of the scanner:

option calibration_mode

With that option, the driver will not send the scanner head to the start of the scan area after the calibration, but back to the home position, such that it starts scanning even in front of the calibration strip. If you make a scan of about 2 cm, you should get a picture like this:

The gray area on top is the inside of the case in front of the calibration strip. The white area in the middle is the calibration strip, and the black area at the bottom is the start of the scan area (the bottom of the cover is black). The calibration strip should be bright white with no visible defects.

If, however, you see some stains on the strip, with brighter vertical stripes running through them, your calibration strip is dirty. The picture below shows a close-up of a stain on the calibration strip, provided to me by Hendrik Lipka (the close-up, not the stain :-).

How can you fix this?

A first option is to customize the driver such that it tries to avoid the dirty areas during the calibration. For this, you need to be somewhat familiar with building the driver from source code, and have a bit of programming experience. If you feel up to it, just contact me (eddy_de_greef (at) tiscali (dot) be), and I'll guide you through it. It is not guaranteed to succeed, though. If the calibration strip contains too many dirty spots, your only option is the next one.

The better solution, obviously, is to clean the calibration strip. Do this at your own risk, though, because it is not without danger. First, you need to open the scanner case without damaging the fragile mechanics and electronics inside. To make matters even worse, the calibration strip is located in between the case and the glass plate, and the glass plate is glued to the case. So if the dirt is on the strip itself, you have to remove the glass plate. There is no guarantee that you can remove the plate without breaking it!!! In Hendrik Lipka's case, the glue wasn't that strong, so the glass plate could be pulled loose relatively easily.

If you succeed, it is up to you to decide what to do next. If the dirt cannot be removed, you may have to replace the calibration strip or cover it with something else, such as white tape, for instance. In Hendrik's case, cleaning the strip solved his calibration problems. Note that, if you use tape to cover the strip, you should put it in between the glass and the strip, not on top of the glass. Otherwise the movement of the scanner head may be obstructed and your scanner could be damaged badly.

If you need more information, you can always contact me (eddy_de_greef (at) tiscali (dot) be) or Hendrik Lipka (hendrik (dot) lipka (at) gmx (dot) de).

Last update: June 2, 2004