> I was in Las Vegas USA many years ago, and watched a 'special venue' 3D movie
> there on a large domed screen. (Some form of IMAX, probably.) I *think* the 3D
> glasses I wore were shutter glasses of some sort (but I could be wrong; I don't
> remember exactly.) The 3D effect was flawless-- and I was amazed that I could
> turn at any angle to look at the imagery, even directly overhead. I assume the
> 3D glasses were circularly-polarized-- my first experience with that technology.
>
> But when I went to see AVATAR in 3D-Imax at my local big cinema, I remember
> tilting my head to see what would happen-- and instead saw two overlapped images
> in each eye, with no 3D :-(  The glasses they gave out (on loan!) were simple
> polarizers, nothing active. So no circular polarization there, AKAIK.

Both circular and linear polarised systems are passive, the filters in 
both eyes are simple polariser sheets that are very cheap, small and 
require no power source or electronics. The display equipment though 
needs to be more complex as it needs to transmit two images with two 
different polarisations. Normal TVs and monitors cannot do this.

Active, or shutter, as the name implies, works by quickly blacking out 
one eye, then the other, in sync with each frame being shown. This 
allows it to work with a normal TV or monitor (plus a device to 
sync/transmit to the glasses). Note that these systems probably do 
"black out" each eye using effectively a 1-pixel LCD in each eye (and 
hence work due to polarisation), but they don't rely on any special 
polarisation from the display device itself.

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