When there is a curve in the artwork, most notably the crease on the spine of a booklet, the way that the light interacts with the curve can alter the scan.
If the scanning element moves in parallel (along) with the crease then the light will bounce evenly and predictably on the curve and the image will look as expected.
But if the scanner sensor moves perpendicular to (across) the crease, then the light will bounce unpredictably as the sensor moves across the curve. This will cause an obvious haze/glow/cloud along the curve.
The best practice is to always scan a booklet so that the crease is the same direction as the movement of the scanner. On a regular sized 8.5" x 11" scanner this unfortunately means that the booklet will have to be scanned twice. Because the scanner is not wide enough in the 8.5" direction to fit the whole booklet. Thus it is best to scan both sides and then stitch the images together. Usually you can combine the images right down the crease since there is already uneven lines in that area and will mask any image edges. This is a major contributor to buying an A3 size scanner, as a standard booklet will easily fit in the 11.75" width of an A3 scanner. Thus requiring only 1 scan and less time post processing.
Here is an illustration of the movement.
This is parallel movement. The scanner moves in the same direction as the crease. Image quality is consistent.
- scan_no_reflection_parallel_action.jpg (132.79 KiB) Viewed 7334 times
- scan_no_reflection_perpendicular_action.jpg (161.06 KiB) Viewed 7334 times
Parallel Scanning (no reflection)