xhonzi on avs:
I think it's actually based on wavelength. http://en.wikipedia.org/wiki/Dichroic_prism
As far as I understand it and in simplied terms, a trichroic prism is a squarish prism with two types of dichroic mirrored surfaces intersecting at 45 degrees and 135 degrees. One reflects red 90 degrees and the other reflects blue -90 degrees. It doesn't filter/reflect green. Therefore the green goes straight to the lens while the blue and red are horizontally flipped after the panel and before the lens. But then it's all flipped again at the lens.
Here- I drew a picture. It's probably wrong, so hopefully someone can correct my understanding:
I'm still not sure where the green is getting its polarity rotated 90 degrees off of red and blue. Unless there's a retarder (or something working as a retarder) in there somewhere we don't know about. Or it's possible (but unlikely methinks) that the green panel is vertically polarized while the red and blue panels are horizontal.
:Originally Posted by BlackShark
If these don't create the different primary colours polarisation angles, there should be some different type of prism, or there should be some kind of device or filter that destroys the polarisation after the prism.
Are you saying there "should be", as in "there isn't but there should be"?
Or are you saying that there "must be", as in "you don't know where, but something is causing the effect"?
Originally Posted by BlackShark
I read about special polarising filters made by Advisol designed specifically for use with LCD projectors which are supposed to provide much greater light transmission than traditional polarising filters. They're called "SP" filters or "SPAR" (SP + anti reflection coating) I don't know how they work though
Interesting! Too bad they're so expensive... But it does seem to do what I wanted: retard the green so it matches the red & blue. Minimal light loss! I like it!