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-   -   What ever happened to polarized screens? (http://www.avsforum.com/forum/23-screens/19916-what-ever-happened-polarized-screens.html)

work permit 08-21-2001 08:27 AM

I seem to remember a while back that polarized screens were used in the manner that greyhawk screens are used today. What happened to the concept?

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Alex

[This message has been edited by work permit (edited 08-21-2001).]

KFung 08-21-2001 09:34 PM

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.

Regards,

Kam Fung

work permit 08-22-2001 08:19 AM


Quote:
What I didn't consider before was that the light reflected off the screen surface was not polarized
A silver screen would maintain polarization, but I suppose that introduces its own problems.

Quote:
Finding a seamless polarizing filter with sufficient size for a home theatre screen is hard
I could have sworn Da-Lite had a polarized screen with the polarizer built into its surface. Seems to have dropped off the face of the earth.


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Alex

KFung 08-22-2001 09:53 PM

It could be, I think they probably found that the market wasn't large enough for it and axed it.

Regards,

Kam Fung

gn2 08-23-2001 06:58 PM

DaLite makes a grey lenticulated screen that enhances black to the point where you can have a watchable bright image with the lights on and still have black blacks. I couldn't wash the blacks out no matter how bright I made the picture. The catch is, because the screen is lenticulated fabric, it can't be seamed and is only available in 70"by 70". Maybe they'd do a custom that was 70" wide and long enough to mount sideways for a 16X9 aspect ration screen that was 70" tall. I can't remember the name of the screen but the woman at DaLite knew the type when I described the properties. It has a gain of 1.8 so it not only enhances blacks, but is super bright.

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Thanks, Paul.

kelliot 08-23-2001 10:46 PM

The lenticular fabric is also susceptible to moire effects. It has quite a bit (too much) texture for my tastes.

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Ken Elliott


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