Stereoscopy—the illusion of depth created by showing a separate image to each eye—is at least as old as photography itself. In the last few years, however, stereoscopic 3D movies have come back in a big way. Theaters show you 3D movies by projecting two images on one screen and giving you specialized glasses that separate the images. But how do those glasses work?

Polar Opposites

There are several competing 3D technologies, but the most prevalent one in theaters today is based on polarized light. The technology that fueled the 1950s 3D boom, which is still in use today, is linearly polarized stereoscopy. Two images are projected through polarizers of two different orientations, typically 45 and 135 degrees relative to the horizon. The projected images are then filtered using polarizer films in the lenses of your glasses en route to your eyes. In this way, one image is excluded from your left eye while the other image is excluded from your right.

In the original formulation of this system, two projectors were used, and projectionists needed to take great care to make sure the two images were well aligned, perfectly synchronized, and equally bright. This problem has been eliminated with the rise of digital projectors. One of the other major problems, though, is inherent to the linearly polarized system: It requires the glasses to be parallel to the projector screen in order to prevent the images from leaking through their respective 'dark' lenses. This means that if you bend down to grab the popcorn, or turn to whisper to your friend, or if you sit off to the side of the theater instead of in the center, then the 3D effect will be compromised and you may get a bit of a headache.

The latter problem is corrected with circularly polarized 3D, patented in 1989. This is the method used by the RealD system, the most widely used system in theaters today. Here, one of the images is projected using light waves that trace out a left-handed spiral, and the other using light that traces out a right-handed spiral. Each lens contains a quarter wave plate, which is a passive device that transforms the two counterspiraling waves into two perpendicular linear waves. Then the familiar linear polarizers cut out one image from your left eye and the other image from your right.

So how do you know what kind of 3D glasses you're wearing? Slip into the bathroom during the movie and look in the mirror with one eye closed. The handedness of circularly polarized light is reversed when it reflects off a mirror, but the orientation of linearly polarized light is preserved. So, if the lens in front of your open eye is blacked out, then you are wearing circularly polarized glasses. If the lens in front of your closed eye is blacked out, then you are wearing linearly polarized glasses (or possibly active shutter 3D glasses—a topic for another post).

Andrew Koltonow is a graduate student in Materials Science and Engineering at Northwestern University. He's part of our College Weekend extravaganza.