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EDITED:
Does not work, see post 5. You can't make mistakes if you don't try. At least I tried something.
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I'm building a passive projection system with two dlp projectors.
I have a tip to improve ghosting reduction at the cost of some reduction in brigtness. Use two filters for each projector.
Let's say you need to place the filter for the left eye at 45 degrees. Use two filters and place them in front of each other at angles of 23 degrees and 67 degrees. Play with the angle between the two filters until you are satisfied with the brightness and the ghosting reduction.
I got filters from Edmund optics (NT45-668). Even when crossed, they don't filter all visible light. To experiment, I crossed two filters and placed them in front of a projector projecting on the wall. I still see some light on the wall. That means that even with a perfect polarization preserving screen, it won't be perfectly without ghosting. I plan on testing other filters later.
I got a tip from experience to give to people experimenting with filters in front of a projector. Don't try to use cheap theme park styles glasses as a filter in front of a projector. It will only take about 30 seconds to 2 minutes before you burn the filter in the glasses, rendering them unusable.