My wife and I just saw "Up." We sat in the back of the theater, and at the beginning of the show, I stood up to look at the 3D projector. To my amazement, I saw what looked like a huge single Barco projector with a single lens. There was what seemed like a rectangle polarizer mounted in front of the lens. I took off my 3D glasses and peeked through it with each eye, rotating the glasses to see if the projection would go dark. I was surprised that nothing happened.

A decade ago, I remember seeing two projectors side by side, each outfitted with polarizers with their "grain" mounted perpendicular to each other. The 3D glasses also had their "grain" mounted in the frame such that it would cancel out the light from the other projection.

My question is... how are they projecting 3D today using only a single lens??

I just found some articles.
Real D Cinema - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Real_D_Cinema)
YouTube - Lenny Lipton discusses the REAL D digital 3D system (http://www.youtube.com/watch?v=9B04tsJAZeg)
RealD LP makes 3D projection a touch more portable (http://www.engadget.com/2009/05/05/reald-lp-makes-3d-projection-a-touch-more-portable/)

...To my amazement, I saw what looked like a huge single Barco projector with a single lens. There was what seemed like a rectangle polarizer mounted in front of the lens.
That's right. In RealD theatres the circular polarizer is electronically activated to change from clockwise to counter-clockwise and back again 144 times per second, therefore flashing each eye for each frame 3 times. It is so fast that you can't perceive the flicker.

That's right. In RealD theatres the circular polarizer is electronically activated to change from clockwise to counter-clockwise and back again 144 times per second, therefore flashing each eye for each frame 3 times.

So this is all done electronically inside the projector? The polarizer was mounted in front of the lens, and there were no electronics visible on or around the polarizer mount.

This is amazing. Unlike the older projection systems, the 3D effect remained even if I tilted my head left and right.

You can tilt your head because circular polarization is used (instead of linear polarization.)

The polarizer is electronically controlled so there must be at least two wires going to it. The projector doesn't have much to do with it since it is the same projector used when they show a 2D film in that same theatre. The polarizer is simply swung out of the way for 2D projection.

The one caveat is that the screen must be silver to retain the polarization.

When I saw Coraline I had the feeling that RealD didn't handle motion well. It seemed to stutter and I felt like the image flattened a bit too. I assumed it was because of the way the RealD projector flickers as opposed to a side by side.

At 144Hz you shouldn't perceive any flickering from the polarizer. I think the flickering you were seeing is simply a side effect of stop-frame animation having zero motion blur during pans. I (and my kids) absolutely love watching The Nightmare Before Christmas but I find the same thing happens. Ditto for the Aardman films.

You can tilt your head because circular polarization is used (instead of linear polarization.)

Nope, circular polarization is just a term to say that after the polarizer there is a 2nd filter to "depolarize" the light. So in term of polarization , linear and circular are the same (obviously).

you can tilt your head simply because the 3d effect is obtained by "switching" the eyes
with the glass instead separating the left/right picture with a polarizing filter.
So the pictures on screen is not polarized , left and right are simply sent at different time and the glasses are blocking the left and right eye at the proper time.
No polarization is required here, except that the LCD shutter used to switch pictures on the projector and on your nose could introduce some polarization. This polarization is probably lost when light hit the screen, since to keep light polarized, you need a special screen.


In a polarized projection, requiring 2 projectors, one is polarized at different angle from the other, and both pictures are displayed at the same time. This require to separate the L/R picture with another set of polarizer on the glasses. this is another 3D technoly that is simpler (passive glasses) but require more equipement (2 projectors) and special screens.

Giroud.

The ReadD system we're referring to (the largest amount of installations in North America) uses passive circular polarized glasses (clockwise vs counter-clockwise) with a single projector. It is the projector that cycles the left/right images through an attached electronically controlled circular polarizer that shows each frame 3 times per eye (2 x 3 x 24 = 144Hz). A silver screen is used to retain the polarization of the projected images.

The issue of tilting the head originated in passive glasses systems that used linear polarization (x-axis vs y-axis) where the viewer had to keep his/her head level or the polarizer for each eye would be out of phase and not block the other eye completely. Linear polarizers are actually better than circular polarizers (less cross-talk) but you can't expect a theatre audience to watch a 90-120 minute film with their head perfectly level. That's why RealD chose circular polarization instead.

I don't think any commercial 3D projection system in the U.S. uses active glasses anymore. The glasses are too expensive, they require batteries, and need to be maintained. I remember active glasses years ago (late 80's - early 90's) at an IMAX presentation but even they switched to a passive system.

you are right , i ignored that light can be polarized circularly like electro-magnetic field.