How RealD 3-D Works

If you imagine movie projection equipment as a room full of dusty film reels, you'd be taken aback by RealD. It's a system that's digital from the ground up and optimized for movies shot in digital formats instead of archaic film.

RealD 3-D setups generally consist of a computer used to process the 3-D data, a digital projector, a proprietary ZScreen polarization switch, the projection screen and, of course, the 3-D glasses that you wear. The system can be adapted to various digital cinema projectors.

Of course, the 3-D system really all begins with the movie set. You have two eyes that let you see the world in stereo. Because your eyes are about two inches apart, their perspectives are just a little different. That's why directors use dual-mounted cameras to film 3-D scenes. One camera captures images ultimately intended for your left eye; the other, for your right.

After the movie is sent to your local theater, projectionists load the data into a server. This computer sends the data in two parallel streams to the projector, which displays them, quickly alternating between images meant for your left and right eyes. Those images then strike the movie screen and bounce back to your glasses.

The glasses you wear are polarized filters, and the lens for each eye is calibrated to let in light waves that strike the lens at a specific angle. All others bounce away or are absorbed. That means directors can purposely design 3-D effects for every aspect of their movies and project them in a way that makes sense to your eyes.

The 3-D systems from years ago didn't work very well. To understand exactly why, you have to know how light works.

Light is a type of wave, and those waves move up and down and side to side and basically all over the place. Light is a bit chaotic. A polarizing filter blocks many light waves but allows those moving in a direction parallel to the filter to pass through. In other words, a polarizing filter cuts down on visual noise.

This is readily apparent if you take a pair of polarized sunglasses and rotate them while looking at a bright surface with a lot of shiny, reflected light. At the right angle, the glasses will block much of the glare and make the scene easier on your eyes.

Engineers can make eyeglasses with lenses polarized differently for each eye. That is, one lens blocks certain light waves and the other blocks a different set of light waves. That's ultimately how 3-D theater technology works. And we've come a long way since the early days.

Old-school 3-D projection technology was a headache, literally, for both theater owners and for many viewers. For starters, these antiquated systems required not one, but two projectors, which cast two different linearly polarized images onto the screen from 45 degrees left and right of center.

Viewers wore linearly polarized glasses, which had to rest at a precise angle (in other words, straight on and stationary with respect to the screen) in order to present crisp images. Thanks to the glasses, your right eye saw only images from the right projector. Your left eye saw images only from the left projector.

It sounds workable in theory, but if you tilted your head, you'd see colors bleeding into each other and stomach-churning, distorted images that gave many movie-goers headaches. Other problems arose if the angle of the projectors was off even slightly, the images weren't perfectly synchronized or if they weren't exactly the same brightness. It was an altogether frustrating experience for projectionists and audiences.

Fortunately, systems featuring linear polarization are no more.

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.

The initial RealD 3-D system that's equipped with first-generation ZScreen technology is also the most widespread 3-D projection system in the world, available in many thousands of theaters. This version does have some significant limitations, particularly in terms of the size of the screen on which it can project, specifically screens that measure 45 feet (13 meters) or less.

In 2008, RealD introduced a second-generation system called 3-D XL. True to its moniker, it works on extra-large screens up to 80 feet (24 meters) wide. The XL is designed to provide more than enough brightness for large screens, making for a bigger and ideally more immersive 3-D experience. Whether your theater uses the first RealD system or the XL, both work specifically with the company's 3-D glasses.

The glasses that you wear during a RealD feature film aren't your everyday polarized eyewear. If you tried to slip on your normal sunglasses during one of these flicks you'd miss out on the special effects (and likely upchuck your popcorn, too). The glasses also have some subtle refinements, such as low-glare frames to reduce scattered light that would distract you from the movie.

Each time you buy a ticket to a RealD movie, you receive a brand-spanking-new pair of 3-D glasses. The idea behind this is to provide the best possible clarity for your movie experience, but also to reduce the heebie jeebies you might feel about wearing glasses that other people have already used.

After each show, most theaters collect the glasses for recycling. Some people do keep their glasses for reuse, and others like them just as souvenirs.

Some industry insiders feel like those glasses should just gather dust for a while as the 3-D film industry recalibrates itself. Ticket sales for 3-D movies are sliding downward, and many pundits blame poor sales on overuse of the effect, in addition to poorly produced conversions of 2-D to 3-D, which often results in ugly, almost unwatchable films.

Movies that make full use of the potential of 3-D, such as 2009's "Avatar," were first imagined as multidimensional experiences. "Avatar" was directed and shot with 3-D in mind, and the superior results translated into massive ticket sales.

Whether 3-D will be around for many years is up for debate. What's certain is that companies like RealD will continue to innovate and refine the technology behind 3-D in the hopes that it will capture audience imaginations like no other cinema experience in history.