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 Real 3D Glasses as Linear Polarization Filters ? 
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One Eyed Hopeful

Joined: Wed Apr 28, 2010 12:30 am
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Hello,

I'm in the process of trying to build a linear polarized projection setup and running into a problem with filtering. For testing purposes I am using a pair of Real 3D glasses from the local theater and an older LCD projector. When I hold the Real 3D glasses in front of the projector the image appears bright green. I'm not using a silver screen yet, so I am trying to pin point the problem. I'm assuming it could be 1) the lack of a silver screen to retain the polarization, 2) LCD projector with prexisting polarization, or 3) The Real 3D glasses are circular polarization and I need the IMAX 3D glasses filters instead ( I'm assuming it's #3 ) If anyone has the answer to what is causing the bright green tint for the projector image it would be much appreciated! :woot

Thanks,

-sosloe


Wed Apr 28, 2010 12:51 am
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Petrif-Eyed
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Joined: Sat Apr 07, 2007 4:34 pm
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Location: Sweden
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Most surfaces depolarizes light when it's reflected. I've experienced that some darker/matte surfaces often retains some degree of polarization on the reflected light. Some materials might have this property even on lighter nuances. So i guess the green tint is because your screen keeps a small degree of polarization.
Polarization directly from an lcd-projector is linear but not "sharp". Which means you will not have total extinction trying to use a blocking linear polarizationfilter. The colors from an lcd-projector does not have the same orientation either: Green have one alignment while red+blue shares another (probably the reason for the green tint you experienced).
Your real-d glasses are indeed circular polarized. You can try using those for both projectorfilters (they're most probably a little too tiny) with extra glasses but you'll have to try circular polarization though. The side of the lenses that face your eyes have to face the projector lenses to work properly. Important!: If you get a green or purple tint when attaching the filter to the projectorlense you need to rotate the filter 45°!
Using real-d filters results in circular polarization, however linear is better because of the doubled extinction rate (less ghosting).
And finally you need two similar projectors plus a nondepolarizing screen (silverscreen) to build a passive projection rig (i've-experienced people thinking one is enough). Well, one is enough using either a heavily modified projector or a depthq with a polarizationmodulator. First one is almost impossible to make on hobbybase and second is very expensive.

_________________
Mb: Asus P5W DH Deluxe
Cpu: C2D E6600
Gb: Nvidia 7900GT + 8800GTX
3D:100" passive projector polarized setup + 22" IZ3D
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Wed Apr 28, 2010 10:28 am
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One Eyed Hopeful

Joined: Wed Apr 28, 2010 12:30 am
Posts: 9
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Thanks for the suggestions, when you say rotate the filter 45 degrees do you mean like a wheel turns or as a door swinging open away from the projector? Just to clarify, this green tint is almost certainly caused by the Real 3D glasses as filters as opposed to the LCD projector being the issue?

Thanks! :D

-sosloe


Wed Apr 28, 2010 11:59 am
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Petrif-Eyed
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Joined: Sat Apr 07, 2007 4:34 pm
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Like a wheel turns. 8-)
The green tint is a combination and result of both the internal polarization of the projector and the polarization of the filters. Remember that the "eye-side" of the glassesfilter needs to face the beamerlense to work. If you align the filter straight it will kill the red+blue portion from the projector thus giving a green tint. If you turn the filter 90° it will instead kill the green thus giving a purple tint. Aligning it at 45° damps all color equally and gives a balanced color. This is why 45/135° is the only option for lcd-projectors when using simple filters.

_________________
Mb: Asus P5W DH Deluxe
Cpu: C2D E6600
Gb: Nvidia 7900GT + 8800GTX
3D:100" passive projector polarized setup + 22" IZ3D
Image


Wed Apr 28, 2010 2:40 pm
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One Eyed Hopeful

Joined: Wed Apr 28, 2010 12:30 am
Posts: 9
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I would just like to clarify what the 135 stands for? (I'm assuming it's the tilt of the filter in the glasses of the wearer) As long as I have the RealD filter over the projector lens at the right angle the glasses will be able to be worn as normal correct?

I had some questions regarding the cheapest and best silver screen as well. I have read lots of info on the different materials and paints available but many are harder to find in my area than I expected. I went looking for a silver metallic fabric "Monroe" in Joann's as mentioned in another post but they had nothing at all that matched that description. Just for the heck of it I picked up a can of "Metallic Silver" spraypaint at homedepot for $5 but I have yet to make my sample test screen. Is there a particular ingredient (mica?) or finish (gloss/matte) that I should look for, or is there any major brand spraypaint that should be available at stores in California as opposed to ordering it online from another country? Another concern was what kind of cheap board solution should I paint on to get a nice smooth finish.

Your help is much appreciated!

-sosloe


Wed Apr 28, 2010 3:19 pm
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Petrif-Eyed
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Joined: Sat Apr 07, 2007 4:34 pm
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45/135 glasses: One lense is rotated 45° from vertical plane and 135 is rotated 135° from vertical plane. The lenses are rotated 90° in relativity to each other. A polarizer works equal when rotated 180°.

I only have experience of one silverscreen (the one i have), no experience with sprays but i can give some advices based on intuition. If possible you should use a matte nuance. As what i've read the paint must have a real metalbase. Aluminium seems what works best. So just being "gray or silver" isn't enough.
You should be able to paint on the shiny side of a masonite-board. Chip-board will also work i think but might add a little structure. On the other hand the structure might kill some of the visible pixels from projectors so i'm not sure this is entirely evil. The chip-board surface will also kill some "uneven coating phenomenas" (will cause glares) as well but i'm only guessing. I'm quite sure a dedicated silverscreen will do better than any paint but for a start (and to see that it works) it'll do quite well.

_________________
Mb: Asus P5W DH Deluxe
Cpu: C2D E6600
Gb: Nvidia 7900GT + 8800GTX
3D:100" passive projector polarized setup + 22" IZ3D
Image


Thu Apr 29, 2010 10:24 am
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