Example of 200 DPI scan of a ruler with a Mustek ScanExpress A3 Scanner that has a CIS sensor.
The left end is touching the glass, the right end is on top of a 2 inch high wood block.

You will notice that where the wood block is touching the glass, it is sharp, the ruler above the block is blurred. The left end of the ruler touching the glass is sharp. The image gets unsharp much quicker where the ruler is not touching the glass.