3D Television Technology

How Passive Polarization 3D Works

If you go to the movies regularly, odds are by now you've seen one of the Real-D 3D movies, whether it be an animated film such as How To Train Your Dragon or a blockbuster like Avatar. Why is the 3D movie suddenly popular again; decades after people stopped caring about anaglyph? The answer is passive polarization 3D, used in Real-D supporting cinemas and IMAX.

In order to acheive a 3D effect through stereopsis, it is important that each eye receive a different image to "trick" depth perception. With two polarized lenses, it is possible to separate two images by blocking out one or the other. The image meant for the right eye will be given orthogonal polarization compared to the image meant for the left eye.

Theater systems will either use two projectors with opposing polarizations to show both images, or one projector which quickly switches between two different images and polarizations. The advantage to using a one projector system is the cost. However, since it must show both images, it will only show half as many frames per second. Real-D cinemas use one projector, while IMAX 3D uses two projectors.

A pair of Real-D 3D glasses, which use split circularly polarized light to separate images going to each eye.[1]

There are two options for which type of polarization is used: linear and circular polarization. Some older systems use linear polarization glasses, which has two oppositely polarized lenses for each eye. However, What will happen if you tilt your head? The polarizers's axis would also be rotated, and each eye would see a portion of each image, causing "double vision".

The same problem does not exist with circularly polarized lenses, the kind found in Real-D 3D glasses. The images are projected with opposite circular polarization (one clockwise and one counter-clockwise) from the screen. The glasses are composed of a wave retarder-coating on a linear polarizer. The wave retarder converts the circularly polarized light to linearly polarized light, which is then filtered by the linear polarizer. The linear polarizers in both lenses are aligned in the same direction, but the wave retarder is applied such that one lens gives the clockwise circular polarization the proper polarization, and the other gives the counter-clockwise circular polarization. Videos showing how the lens work can be found here.

Since the wave retarder and polarizer rotate together on the single lens, this scheme has the advantage over linear polarization that tilting the head does not matter. Due to the polarization filter much of the light is lost (at least 50%), producing a dimmer image. Colours however, are very accurate unlike anaglyph systems.

The glasses are passive, in this case meaning they have no moving or electronic parts, and so they can be mass-produced cheaply (although its still in the best interest of the theater to recycle them). The glasses may be inexpensive, but the rest of the system is not. The projectors are high specialized since the project must project the correct polarization and maintain synchronization. Reflections off of a white screen will not properly maintain polarization, so an expensive screen made from silver is required.

Despite these problems, passive polarization is an excellent way to give film the illusion of depth in a large auditorium setting. For home theaters, however, the projection system needed is far from affordable. For home systems, an active shutter method may be more practical.