Below you see two screenshots taken from the camera page of eyevec-control. In both cases we’re wearing spectacles. The head is tilted just enough to make the eye-tracker see a reflection of the illuminator on the spectacle glasses.

This is the result of "cleaning" the spectacles by wiping them with a clean handkerchief. The result is that the grease build-up on the glasses is not really removed, but rather spread out:

Screenshot of spectacles wiped with handkerchief as seen by eye-tracker

This is the result of cleaning the spectacles using dish soap:

Screenshot of spectacles cleaned using dish soap as seen by eye-tracker