Three dee!

Oct. 14th, 2009 10:18 pm
mmcirvin: (Default)
[personal profile] mmcirvin
Sam recently took Jorie to her very first movie in a theater, and it was the 3D rerelease of Toy Story. This turned out to be somewhat awkward as there were no 3D glasses small enough to fit well on Jorie. It sounds as if there were very few personnel actually on duty in the theater at the time, so it's unclear to us whether this was a local oversight or if no kid-size glasses of this particular type actually exist. But I actually suspect the latter, based on some brief web searching based on what I figured out later on. Anyway, Jorie seems to have watched much of the movie with the glasses off and just accepted the blur as the way of the world. But she liked the movie anyway and told me all about Buzz Lightyear that evening.

Instead of dumping the glasses in the recycling bin, Sam brought them home, probably figuring Jorie would find them an interesting novelty. Of course I set about reverse-engineering them as soon as I got my hands on them.

Most 3D glasses used in movie theaters use some sort of polarization effect to separate the left- and right-eye views (the lower-quality processes with colored lenses are mostly used for such things as DVD releases, where you can't make a polarization-based system work). Historically, it's been common to use linear polarizers rotated to 45 and 135 degrees for the two eyes. I immediately tried looking at an LCD television with the glasses, since LCD screens emit linearly polarized light.

I expected that with the glasses rotated to some angles, the TV picture would become completely invisible through one lens or the other. What I discovered was unexpected: the picture only disappeared if you looked through the glasses the wrong way around, from front to back. That way, it disappeared when seen through both lenses at the same time (which would make the linear polarizer useless for 3D image separation). But with the glasses on, looking through them back to front, you could always see the TV.

How could the TV be visible looking through a lens in one direction but not the other? Sam suggested it was something like tinted car windows, but that effect just depends on the varying light levels inside and outside of the car. I thought of an example from introductory quantum mechanics instead. If you have polarized photons hitting a polarizing filter such that they are completely blocked, and then you put a different polarizer in between the light source and the blocking polarizer, you can cause some of the otherwise blocked photons to be transmitted, because of the effect of detecting a certain space of polarization eigenstates with the intervening polarizer. (If that's too freaky, you can also analyze the effect classically treating the light as electromagnetic waves. Here's an amusing exchange in which a student runs up against the fact that something that's weird and quantum-paradoxical in one description can be completely mundane in another.)

So the lenses must have had some kind of layered construction with more than one polarizer. The classic classroom demonstration uses crossed linear polarizers, but these clearly weren't just linear polarizers since, looking through the lenses the normal way, you couldn't get the TV picture to black out when they were rotated to any angle.

I surmised that the glasses were using circular polarizers. The two lenses would have filters passing left-handed and right-handed circularly polarized light. Then, behind those filters, there was apparently a second layer with a linear polarizer that was oriented horizontally in both lenses, which explained how the TV looked when I looked through the glasses in reverse. I experimented with looking at one pair of glasses through the other pair, and at looking through them at their reflection in a mirror, and this caused the lenses to look selectively opaque in exactly the way you'd expect from circular polarizers.

It turns out the system being used for Toy Story and other Disney/Pixar releases is called RealD Cinema. It does indeed use circular polarizers. The way it works is actually pretty cool; the projector is a digital LCD video projector swapping left and right images at 144 fps, behind some sort of electro-optical device that switches the circular polarization on the fly. One nice thing about using circular polarizers is that the effect isn't compromised if you tilt your head.

I'm still not sure what the reason is for the linear polarizer layer behind the circular polarizers. My experiments make it clear that it's there, but nothing I've been able to find that describes the system even mentions it. Maybe it's to make sure that the light entering both eyes is polarized the same way, so that some subtle effect going on in the eyeball doesn't make the two images differ in brightness or color balance in such a way that the picture looks funny (Haidinger's brush illustrates that this is not impossible). But that's a guess.

There are people selling kid-sized 3D glasses of many other types, but for this circular-polarizer system the only ones I can see anywhere are the adult-sized chunky ones they were using at that movie theater. Since many of these movies are kid-friendly family pictures, this seems like a serious omission. This is the part where I wring my hands about a young generation's vision being insidiously wrecked by eyestrain, but actually I kind of doubt it--it's just annoying. And, in my experience, most 3D processes give me eyestrain anyway, because of my messed-up stereoscopic vision.
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