All types of 3D glasses can be divided into two categories: passive and active. Active glasses interact wirelessly with images on a screen to enhance 3D viewing, whereas passive glasses do not. Passive ones have been around since three-dimensional viewing first arrived in the 1920s, and they are themselves divided into two major subcategories: anaglyphic and polarized glasses.
Practically anyone who has ever seen a 3D movie is familiar with anaglyph glasses, which feature a combination of red and blue lenses. Anaglyphic 3D works by projecting two identical but slightly offset images on a screen, each image tinted with a different color. To the naked eye, an anaglyphic image appears blurry, with reddish and bluish hues. The glasses use color-filtering lenses to target one image to the right eye, and another to the left; the result is that each eye sees a different image, but the mind is tricked into believing it sees only one. The mind compensates for this by focusing in between the two offset images and blending them into one, which creates an illusion of depth.
Passive polarized glasses operate on the same basis as anaglyph glasses, only they filter light waves rather than color. Again, two identical and slightly offset images are superimposed, except in this case each image is polarized to project light differently than the other. With polarized 3D glasses, each eye only processes one image. Again, however, the mind is tricked into blending the two images into one, creating a 3D experience. Unlike anaglyphic 3D, which can be projected from any screen, polarization 3D works best with screens able to relay different light frequencies without sacrificing picture quality.
On a simpler scale, Pulfrich glasses can also create a 3D effect, but only with objects moving across the viewer's plane of vision. These glasses have one completely transparent lens, and another that is heavily tinted. As an object moves across the visionary plane, the image is immediately transmitted to the eye through the transparent lens, but the tinted lens causes a slight delay. This delay causes the brain to add more depth to the image, creating somewhat of a 3D effect.
Since the advent of LCD technology, which is capable of digitally transmitting images at super high-speeds, 3D glasses have made great technological leaps and bounds. Today, active shutter glasses are able to communicate wirelessly with an LCD display, interacting with the action on the screen via infrared signals. This enables the lens on active glasses to shutter back and forth between different light filters, further enhancing the 3D viewing experience.
Another significant upside to active technology is that it is adaptable to 3D TV sets. A 3D-ready television set, a pair of active shutter glasses, and a stereoscopic sync signal connector will allow the LCD display and glasses to communicate with one another. A growing number of television broadcasts are being produced to take advantage of this technology.