3D specs, peculiar effect thereof.

Explanation please.

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3D specs, peculiar effect thereof.

#1  Postby j.mills » Jul 29, 2011 8:23 pm

If you go to see a 3D movie and they're using the RealD 3D projection system, they give you (sell you) a pair of specs in which each lens is polarised to accept light at 90 degrees to that accepted by the other lens. Naturally you retain these and play with them. I found they behave very oddly when you look in a mirror.

At first all seems fine: in your reflection you can see the specs, and behind the specs you can see your eyes, only a little dimmed. But if you close one eye - say, the right - the opposite lens suddenly appears black: the only eye you can now see is the one that's closed! Swap eyes and the effect reverses: you can only ever see the closed eye, not the one that's actually doing the seeing!

:scratch:


I tried to get my head around this, thinking about the light passing through the lenses twice and such, and wondered if the polarity of light flips 90 degrees on being reflected. But a clear explanation from somebody with a solid understanding of the matter would be much appreciated. I've attempted to capture the flavour of the problem with the photo below: you can see that the lens that the camera isn't pointing through is clear, while the one that it is pointing through is almost black.

Polarised specs.JPG
Polarised specs.JPG (45.91 KiB) Viewed 1272 times
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Re: 3D specs, peculiar effect thereof.

#2  Postby byofrcs » Jul 30, 2011 4:02 am

When you close one eye then the open eye is the only one providing data to the brain. The light that travels through both lenses will lose both polarisations and so will be blocked. The light that has only gone through one lens will just be polarised to one plane. Ergo the light on the closed eye lens will be blocked i.e. appear black as it has passed through both lenses.

With both eyes open the brain gets signals from both eyes and it has this pretty nice trick of mixing the images to create one image. So where it has no signal it is happy to fill it in. It thus layers the image of the eye that is working over the missing light so it appears normal to you.
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Re: 3D specs, peculiar effect thereof.

#3  Postby Varangian » Jul 30, 2011 11:00 am

Strange phenomenon indeed! One would expect that the other lens would show up dark, as its polarization is 90 degrees to the other one. Reflected light gets turned 90 degrees, which is "cross-grains" to the polarization of the first lens, which subsequently shows up dark.
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Re: 3D specs, peculiar effect thereof.

#4  Postby byofrcs » Jul 30, 2011 11:05 am

Hold on, scrub that, I'll have to draw a diagram with the polarisations.
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Re: 3D specs, peculiar effect thereof.

#5  Postby byofrcs » Jul 30, 2011 11:26 am

The handness of polarised light is reversed by the mirror so the light from behind the closed eye is e.g. L but bounces off the mirror and goes to R and so passes through the open-eye lens (which passes R) whereas the light from behind the open eye i.e. the open eye, its lashes, crowsfeet etc, goes through the open-eye lens and is now R (because that is what it passes), bounces off the mirror and turns into L which is then blocked by the R lens over the open-eye.
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Re: 3D specs, peculiar effect thereof.

#6  Postby j.mills » Jul 30, 2011 12:29 pm

Gotcha! That's kind of what I thought, but I wasn't sure if the mirror flipped polarity. It's a good trick anyway. Startle your children with it! :grin:
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Re: 3D specs, peculiar effect thereof.

#7  Postby twistor59 » Jul 30, 2011 12:54 pm

If I ever knew that's what happened on reflection, I forgot !...

http://en.wikipedia.org/wiki/Ellipsometry

Ellipsometry measures the change of polarization upon reflection or transmission. Typically, ellipsometry is done only in the reflection setup. The exact nature of the polarization change is determined by the sample's properties (thickness, complex refractive index or dielectric function tensor).
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Re: 3D specs, peculiar effect thereof.

#8  Postby ramseyoptom » Jul 30, 2011 8:12 pm

According to Wikipedia http://en.wikipedia.org/wiki/RealD_Cinema RealD 3D screens use circularly polarised light ( and hence lenses) to achieve the effect. And rather than dealing with the plane of polarisation the lenses work by dealing with the handness of polarisation. Interestingly the RealD 3D web site did not in the quick look I had at it give the method of polarisation. (The lenses are available to prescription from all good optometrists - shameless plug :grin: )

I am so used to dealing with plane polarisation (cutting down glare from water surfaces etc) that at the moment I can't quite get my head around circular polarisation.
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Re: 3D specs, peculiar effect thereof.

#9  Postby Teague » Sep 06, 2011 9:37 am

Is there ever a way they can make a 3d screen and scrap the glasses? Presumably they need to over come the perspective as the 3d effect is render to the user much the same way that everyone that views a rainbow is always at it's center?
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Re: 3D specs, peculiar effect thereof.

#10  Postby byofrcs » Sep 06, 2011 9:56 am

Yes, you can already buy these at the hardware store. Look in the section that sells "Windows" :naughty2:
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