[Phys-l] A geek's observations on "Avatar"

[Leigh Palmer <palmer@sfu.ca>]

I omitted an important point from my posting, for which I apologize, especially to Bob and his daughter. I hope that those of you who see the film at IMAX theaters don't steal the glasses. I don't even know if they are circularly polarized ...

Bob LaMontagne said:

> I'm afraid that collecting "extra" glasses after the show is stealing. My daughter works part time for IMAX and the glasses are an incredible expense for them. They have to be collected by staff after each show (although some people hide them and sneak off with them), sterilize them, and then hand dry them so there will be no streaks or spots. They are of no use as polarizing sun glasses. As you noted, the polarization is not in the vertical plane so they will not serve to reduce glare the way that traditional polarized sunglasses do.

Bob is quite correct, but that comment applies to the IMAX version of "Avatar" only. The display mode used in the IMAX version is Dolby 3D, not RealD 3D, and the glasses are different for the two display modes. The IMAX glasses cost ~ $15 per pair and they are reused after cleaning. (N.b. reused, not recycled.) The RealD 3D glasses (the process described in the technical document I pointed to in my original posting) are reported to cost $0.65. They are dumped into a box for "recycling" after the film's end and presumably are incinerated at high temperature, the method of waste disposal currently in use here in Burnaby, BC. The RealD glasses bear a "7" recycling symbol.

I have not yet seen the IMAX version of "Avatar", but I have seen other IMAX 3D films, and I wasn't paying sufficient attention to the technical aspects (herding children will dull your higher perceptions) to know if the same old process is used for this new film. I suspect it is, since the essence of IMAX (a Canadian success story) is the use of 70 mm film - sideways! The RealD 3D process is much lower resolution than IMAX and does not employ film at all, let alone the immense format of IMAX.

> ... I would highly recommend the movie - great fun and the best 3-D so far. Definitely see it at an IMAX theater if possible.

I agree, and I will take your advice.


Bernard Cleyet said:

> At first I wondered why circular?   Perhaps, so tilting head doesn't affect the 3D and if the screen is tall / wide also?

I think that the absence of crosstalk (ghosting) on tilting the head is the important effect.

> When I Disneylanded, I kept the glasses to use w/ my 3D projector (father's, and grandfather's glass slides from, ca 1903 to 1935).  They are simple linear polarizers.  I'm also surprised broad spectrum circular polarizers can be made cheaply.  Are they not 1/4 wave plates w/ a lin. polarizer?

Yes, that's exactly what they are. quarter-wave retardation plates can be made simply by mechanically stretching plastic sheet. They aren't completely achromatic, of course, but they can be made close if they are stretched only the minimum amount necessary. Stretching more can produce, say, a 3/4-wave retardation plate which is functionally equivalent at one wavelength but which has a smaller spectral accommodation. I'm sure it is all an art worth mastering for commercial reasons.

I used to have a large sheet of circularly polarizing material from which I cut two-inch squares (before we went SI). All my previous experience has been with that antireflection window material, which used to be used in front of Nixie tube displays so you shouldn't see the glass.

> Pleased you're back --

Thanks. I'm glad to be back. I owe it to CPAP.

Leigh