We observe strange wave-like patterns in our DIC (brightfield) image of our Perkin-Elmer ultraview confocal microscope. These ripples/waves do not show up in the confocal laser images, but only DIC. Could this be a refraction or rather a vibration problem?

Interestingly the standing waves seemingly move when we change the exposure time in the DIC setting, moving up or down or almost completely vanishing at some specific exposure times (e.g. 11 ms) This to me seems like it could be vibrations. Yet, we are pretty sure that these are not vibrations, since the table is uncoupled (but no air cussion) and touching the mount does not change the wave pattern (I'd assume touching it should change the normal mode of the mount and hence result in a visible change of the ripples/waves).

Any ideas/suggestions?