02-06-2015, 02:49 PM
Quote:In a normal OLPF the first bifringent material increases the optical path length (OPL) some - maybe .5mm or something. Unfortunately bifringent materials had 'bad' polarization properties (i.e they happen to alter polarization instead of "purely" doing one thing... linked motions are no fun). A wave plate then corrects the polarization state twice as much as necessary. Finally a second bifringent material does the horizontal (vs vertical) separation and voila, OLPF.
Is that the case? Couldn't they just replace it with some "ordinary" optical glass to keep the same filter characteristic other than low pass effect?
In a cancellation one the back bifringent plate is just 180deg rotated so it cancels the first, as they are equivalent parts. This maintains the longer-than-previous OPL but probably not perfectly. So long as it is an integer number of waves away from the old one (+/- some margin, maybe up to 25-30%) results will be fine.