Buzz
Stereoscopy, the process of creating a perception of depth by presenting different images to each eye, has existed since the birth of photography. Recently, however, the revival of 3D films has been quite significant. Movie theaters showcase 3D films by projecting two images onto a single screen while providing you with special glasses to separate these images. But what is the mechanism behind these glasses?
Polarized Light
Several 3D technologies are in use, but the most common one in cinemas today relies on polarized light. This technology, which dates back to the 1950s 3D film revolution, is still widely used today through linearly polarized stereoscopy. Two distinct images are projected through polarizers set at different angles, typically 45 and 135 degrees relative to the horizon. The glasses you wear filter these images using polarizing films in the lenses, ensuring that each eye sees only one image, blocking the other.
In the early design of this system, two projectors were necessary, and projectionists had to be meticulous in ensuring that the images were properly aligned, synchronized, and equally bright. This issue has been resolved with the advent of digital projectors. However, another significant challenge persists in the linearly polarized system: The glasses must be aligned parallel to the projector screen to avoid images leaking through their respective 'dark' lenses. This means that if you bend down for your popcorn, whisper to a friend, or sit away from the center in the theater, the 3D effect will be affected, and you might even experience a headache.
This issue is addressed with circularly polarized 3D technology, patented in 1989. It's the method used by the RealD system, the most common 3D system in theaters today. In this system, one image is projected with light waves in a left-handed spiral, while the other is projected with light in a right-handed spiral. Each lens has a quarter-wave plate, a passive element that transforms the two counter-spiraling waves into two perpendicular linear waves. The familiar linear polarizers then block one image from reaching your left eye and the other image from reaching your right eye.
How can you tell what type of 3D glasses you're using? Sneak into the bathroom during the movie and look at yourself in the mirror with one eye shut. Circularly polarized light has its handedness reversed when it reflects off a mirror, but the orientation of linearly polarized light remains unchanged. If the lens in front of your open eye is dark, you're wearing circularly polarized glasses. If the lens in front of your closed eye is dark, you're wearing linearly polarized glasses (or possibly active shutter 3D glasses—another topic for a future post).
Andrew Koltonow is a graduate student in Materials Science and Engineering at Northwestern University. He's part of our College Weekend extravaganza.
