It seems like an excellent idea, doesn't it? When you consider that both LCD's and D-ILA both output polarized light, you realize that you can get the benefits of a grey screen without losing any light!
I've explored this a little and I've found lots of little things that can go wrong.
- Unfortunately (and rather counter-intuitive), a lot of projectors output the light for different colours with different polarizations. So at the optimum polarization for one colour you lose some of the other two, this gives you an off colour image. http://www.avsforum.com/ubb/frown.gif
Fortunately, I can tell you that the D-ILA outputs all its colours with the same polarization. Rejoice!
- What I didn't consider before was that the light reflected off the screen surface was not polarized. So all the light from the projector passes through the filter on the way in, it is then depolarized at the screen and you lose ~50% on the way out! Gain coatings help preserve polarization, but it may actually make the gain curve worse because it passes more of the directly reflected light. However, you get twice the benefit of a grey screen because ambient light is attenuated twice when going through the polarizing film (because it is unpolarized going in and going out).
- Finding a seamless polarizing filter with sufficient size for a home theatre screen is hard! If you are careful it should be possible to hide the seam quite well if you are not sitting too close. I wouldn't care to guess what the cost of such a large filter would be though.
- I think it might work in very narrow applications, one in which you have a lot of lumens with ambient light. The other is with very high gain screens (which preserve polarization). <cough> Torus <cough> In all cases, you need a D-ILA or an LCD that outputs its colours with the same polarization.