Slackware, this is 3mm glass and I don't notice any distortion. You can see sample scans at:
http://tenrec.builders/quill/samples/sample-1.jpg
http://tenrec.builders/quill/samples/sample-2.jpg
These are raw images taken off of the cameras.
The angle of the platen (100 degrees) combined with the distance of the light minimizes glare. But you have to scan in the dark, otherwise ambient lights might cause artifacts. It also helps that each LED projects its lights through a pair of lenticular sheets. One horizontal and one vertical. This spreads out the light pretty evenly.
I designed my scanner so I could just put a standard Archivist lighting module on top. That meant that I could take advantage of the existing LED/lenticular sheet/baffle design by Daniel Reetz (see
http://www.diybookscanner.org/archivist/?page_id=236 for some interesting pictures/gifs/videos about lighting).
There is still a little bit of glare that you might notice if the glass is dirty. A fellow going by the username ilsamors on the diybookscanner forum has experimented with specially coated anti-reflective glass to reduce any potential glare even more. That is a potentially interesting (if expensive) avenue to explore.
One big problem with using a glass platen is books with text very close to the gutter. There is a small blind spot where the glass meets. And if there isn't a good margin in the center, that blind spot could cut off text. The other problem is when scanning small normally-closed paperbacks. They have a tendency to snap shut every time you lower the cradle.
-D
