2Tricky:
In fact the printed imperfections can be around 6mm (@ 30 fpm = 0.039 approx. seconds).
There is a huge difference between 6mm and 10 microns and I still haven't got my head around what resolution and frequency of measurement you're trying to achieve. Is 6mm the discrepancy between the distance moved on the surface of two cylinders? From your last post I get the impression that this is more like jitter / backlash / play rather than slip (sustained gradual difference in position).
If that's correct, then over what sort of timescale does this jitter occur and how small a jitter are you aiming to measure?
I'm starting to imagine a solution based on a couple of optical mice pointed at a test pattern attached to the surface of each roller, but I'm still not sure I have understood the problem correctly, and every time you mention 10 microns it worries me.
Once you have got a number that shows there is jitter, what are you going to do with it? I guess you want to know what the source is so that you can eliminate it. In that case maybe you should be looking for backlash in the mechanical drive system and measuring eccentricity and radial play of the rotation parts. Is the drive mechanism loaded? If you want to define position/speed accurately you will probably find you need to apply a small load to take out the backlash - for example a light brake on each output drum to take out angular backlash, and a lateral load on each bearing to take out radial play. (If you've got belts/chains/gears etc involved, these will already be applying lateral loads so that may be taken care of).