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How RealD 3-D Works


Circular Polarization

Linear polarization has gone by the wayside. If you've been to a 3-D movie in the past few years and slipped on those fashion-challenged glasses, you've worn lenses featuring circular polarization. Just as with glasses that use linear polarization, those with circular polarization are made to filter out specific wavelengths of light.

The big difference is that the polarized light waves don't travel in a straight line. Instead, they move forward in a spiral. To create that spiral effect, a projection system like the one in RealD might use two projectors with different polarizing filters: one to create a clockwise spiral; the other, a counter-clockwise spiral. Each spiral works with only one lens on your glasses, so each eye sees just one set of images. But these dual systems tend to be too unwieldy or pricey for many theaters.

Instead, RealD uses a sophisticated one-projector scheme. The projector sends its images through a polarizing beam splitter, which divides the light into two beams. Both beams of light bounce from a mirror toward an achromatic polarization rotator, which rotates these beams of light to specific angles for each lens on your 3-D glasses.

Images then pass through the ZScreen, which is a liquid-crystal screen placed in front of the projector lens. The ZScreen acts as a fast-switching polarizing filter (also called a push-pull modulator). Each time it switches, it alternates between images meant for your left and right eyes. It syncs precisely with the movie projector thanks to the help of an electronic controller.

It does this task at 144 frames per second, or 72 per second for each eye. Or put differently, it means that each of the 24 frames per second of the movie is displayed for your eye three times before the next frame appears. That way your eyes and brain can't detect sickening flickering.

And because the resulting light waves move in spirals, they can hit your glasses at different angles and still make fantastic images. That means you have more leverage to tilt and cock your head while you watch.

The system isn't perfect. At certain angles, viewers may still see ghosting, which is a type of image leakage. In ghosting, one eye may see the tiniest glimpse of images meant for the other eye. When this happens, it can disrupt the fun and may make you feel a little woozy.

Another challenge for RealD with ZScreen is brightness. As the projected light moves through all of those filters, there's tremendous light loss, meaning the movie you see looks darker than it should. To counteract this phenomenon, some theaters use silver screens.

Silver screens have actual silver dust embedded in them. The silver is so reflective that it reduces light loss and maintains more brightness than a modern white, matte screen.

Silver screens are actually a throwback to the olden days of movie theaters. Early projection systems had the same handicap as 2005-era RealD systems — they were just too dim and required very reflective screens to maintain sufficient brightness for audiences. Silver screens might be a primitive technology, but they are more expensive than basic white screens, and as such, they're another drawback for owners looking to invest in 3-D.