This looks like a calibration issue. Since the pixels on the sensor do not all have the same sensitivity, the scanner should auto-calibrate every time it is powered on. I'm not entirely sure how it does this and the documentation doesn't give any indication. It may scan a test strip internal to the scanner or it may scan the white document mat.
However, your document table (glass) appears to be dirty and this may be the cause of your problem. Another possibility is dirt on the sensor or on the calibration strip (if this is internal to the scanner). Before you try anything else, I would recommend that you do the following:
- Power off the scanner.
- Clean the document table and mat in accordance with the
manufacturer's instructions. Make sure that they are spotlessly
clean but take care not to scratch them.
- Insert the document mat and power on the scanner with the lid closed
and no document inserted.
- Perform some test scans to see if the problem is resolved.
Always switch the scanner off when you have finished using it.