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17 February 2009 @ 06:45 pm
Previews and polarization and 3D Movies  
So first question, does anyone have a good simple explanation for what circular polarization is? How do you make circularly polarized light? How do you make a circularly polarized filter? Linear polarization is simple to understand, both the light and the filters for it, but the circular version is throwing me for a loop. Er, so to speak.

I saw a couple previews before Coraline. It was actually the second time i'd seen one for "9," which wasn't actually 3D. However while watching it this second time it suddenly struck me that it was almost as if Sony had bribed someone to make a Sackboy movie :) It still looks pretty cool though, too bad we have to wait another seven months for it =P

There were two 3D movie previews, "Monsters vs. Aliens" and Pixar's "Up." First i was wondering if perhaps i should keep the 3D glasses around to use for those movies, but then i started wondering if it might be smarter to deconstruct them to make some kind of attachment for my regular glasses, the way some people have clip-on sunglasses for their prescription glasses. I wonder if there's any way i could do that without having it end up looking incredibly ugly? I also wonder if they'll actually let you not purchase the $2.50 glasses if you've saved a set from last time. On the one hand, i bet they'd love to charge you extra regardless, but it seems like there ought to be a class-action suit in there somewhere if they tried it. Certainly if they refused there's no way i'm ever going to recycle their glasses for them. (They have a bin out front where they encourage you to drop your used glasses.)
Leora: ouroborosleora on February 18th, 2009 02:48 am (UTC)
I would seriously object if they made me by the glasses... not that I ever intend to go to a 3D movie. But I'd probably just say: I'm blind. I want to see this movie. Why should I have to buy the glasses?

Of course, I'm not completely blind, so I would actually see some of the movie. Although 3D doesn't work for me last I tried, so I intend to see a 2D version.
Sister Atom Bomb of Courteous Debateakiko on February 18th, 2009 06:27 pm (UTC)
Our theater didn't charge specifically for the glasses but the ticket price was higher.
Steven DeFordwillworker on February 18th, 2009 03:03 am (UTC)
Circularly polarized light is basically light with a linear polarization that rotates over time. So at t=0, it's in the x-hat direction, at t=1 it's rotated around to the y-hat, t=2 is -x-hat, etc. I don't know how one would filter circularly polarized light in any simple way (rotating linear filters is all that comes to mind).

DonAithnendonaithnen on February 18th, 2009 03:15 am (UTC)
Well i'm pretty sure there were no moving parts in the glasses i was wearing at the movie :)

And how does one make the circularly polarized light? You can't just take something that produces linearly polarized light and spin it really fast can you? This sure makes it sound like it's more complicated than just regular polarized light that's spinning around.
the coproduct of doomoonh on February 18th, 2009 04:18 am (UTC)
mantis shrimp can see circularly polarized light.
Celesteashke on February 18th, 2009 05:47 am (UTC)
FYI: If you want to see Corlaine 3D, go see it by Thursday. I know at least around my area, Coraline 3D is leaving theaters by Thursday so that Jonas Brothers 3D can come in next weekend. Just thought you would like to know.
Steuardsteuard on February 18th, 2009 07:28 am (UTC)
Let's see. willworker has already given the basic idea of circularly polarized light: you can think of it as rotating linear polarization. The rate of rotation is given exactly by the frequency of the light.

But there's another way to visualize circular polarization: think of it as overlapping x-hat and y-hat linearly polarized waves that are exactly 90 degrees out of phase. (That is, at t=0 the x-hat wave is a maximum but the y-hat wave is zero. A quarter period later at t=T/4, the x-hat wave is at zero but y-hat is maximum. At t=T/2, the x-hat wave is a minimum (negative) and y-hat is zero again, and so on.) This gives exactly the component form of a "rotating" wave. (And because the rotation is intrinsically tied up with the oscillations of those waves, that's why the frequency determines the rotation rate.)

And that's a hint of how you produce circularly polarized light in the first place. There are certain types of crystals where light polarized along one axis (call it x-hat) moves faster than light polarized along the other (y-hat). If you start with light that's linearly polarized at a 45 degree angle compared to the crystal, and if the thickness of the crystal is exactly right, the x-hat component of the light will have gotten exactly a quarter wavelength ahead of the y-hat component when it comes out the other side. (This is called a "quarter wave plate", sensibly enough.)

Let me comment on the new 3D glasses in a separate reply.
DonAithnendonaithnen on February 18th, 2009 07:32 am (UTC)
Ahhhh! Now that explanation makes a lot more sense! Thank you! :)
Steuardsteuard on February 18th, 2009 08:08 am (UTC)
In a modern 3D movie theater, the screen sends out alternating shots of left/right circularly polarized light. (They probably do this by quickly switching between linear polarization at 45 degrees and -45 degrees behind a quarter wave plate.) The nice thing is that if circularly polarized light goes through another quarter wave plate, it gets converted back into linearly polarized light (because the phase difference either cancels out or doubles).

Specifically, the right/left circular polarizations get converted into +45 degree/-45 degree linear polarizations. So the 3D glasses they give you consist of a quarter-wave plate on the outside followed by a linear polarizing filter on the inside. The quarter wave plates on each eye have opposite orientation, so that one of them converts right-circularly-polarized light into the linear polarization that gets through and left-circularly-polarized light into the linear polarization that gets blocked, and the other does the opposite.

There are all sorts of fun things that you can do if you have two pairs of these 3D glasses. For example, if two people wear the glasses and look at each other, their right eyes can see each other and their left eyes can see each other, but no right eye can see a left eye. (For reasons that I only partly understand, the 3D glasses I have work better with this if one person tilts their head at a 90 degree angle; otherwise, I see dark purple instead of black where the light is supposed to be blocked.)

On the other hand, if you line up the glasses back to back (as if they're "wearing each other"), they act exactly like ordinary linear polarizing lenses: if you tilt one pair of glasses by 90 degrees no light gets through both pairs, but if you hold them parallel it looks pretty much normal.

Even with just a single pair of 3D glasses, you can play the usual polarizing games with LCD screens: hold them so the back of the 3D glasses face a flat-panel monitor (as if the monitor is wearing the glasses) and rotate them around. At one particular angle (looks like 45 degrees!), no light gets through; perpendicular to that, almost all the light gets through.

On the other hand, wearing the 3D glasses and looking at an LCD screen is a bit odd: if I tilt my head 45 degrees to the right the screen looks red-ish, while tilting 45 degrees to the left makes it look blue-ish. (This is another manifestation of the same mystery I mentioned before, I think: I'm guessing that unlike the idealized theory, the quarter wave plate actually behaves a little differently for different wavelengths of light.)
DonAithnendonaithnen on February 18th, 2009 04:19 pm (UTC)
Ooohhh! Tricky!

I'd actually noticed some weirdness when looking at other people when we were both wearing the glasses, but i don't remember one eye blacking or purpling out. I _seem_ to remember both eyes seeming weirdly shimmery or something. But it's been awhile, and perhaps we weren't doing it right or something.
Steuardsteuard on February 18th, 2009 04:23 pm (UTC)
If both your eyes are open when you look at the other person you'll see exactly that sort of shimmery effect because only one of your eyes will be able to see each of theirs. Look one eye at a time and you'll see the clear/dark distinction immediately.
Kirinkirinn on February 18th, 2009 06:44 pm (UTC)
This goes a long way towards explaining why modern theater glasses didn't behave quite like I'd have expected when looking at sources of reflected light - I was thinking linear polarization.
Steuardsteuard on February 18th, 2009 07:39 pm (UTC)
Yeah: if you turn them backwards and tilt them at a +45 degree or -45 degree angle, they'll act just like ordinary polarizing sunglasses would. But of course, it's hard to wear them that way. (And they have no UV protection, of course.)

The beauty of using circularly polarized light in the theater, incidentally, is that tilting your head doesn't ruin the 3D effect.
Brian Johnson175560 on February 19th, 2009 01:56 am (UTC)
What a wonderful explanation. Thanks!