3D Glasses - Active and Passive, Polarized, Shutter and Color Anaglyph 3D Glasses
Current 3D technology relies on the viewer wearing a set of glasses designed to deliver a separate image to each eye. The illusion of depth is created by sending a slightly different image to each eye. Human beings rely on stereoscopic vision for depth perception. Our two eyes see everything at a slightly different angle due to the space between them and our brains process and combine these two images into one complete picture. We can very reliably process depth out to about 20 feet because of this. If you'd like to see how important the stereoscopy is to depth perception have someone throw you a baseball, then cover one eye and have them throw again and see how much more difficult it is to catch the ball. While our brain can give us some idea of depth based on the the changing focus of the lens in one eye it is not nearly as accurate as when the brain is able to process two images.
3D Glasses can be divided into two main categories, active 3D glasses and passive 3D glasses. Active glasses are powered, either by batteries or by power adapter. Passive 3D glasses use a number of different methods to deliver a different image to each eye.
Active 3D Glasses
Shutter glasses are the most commonly used Active 3D glasses, the lenses are actually small LCD screens. When voltage is applied, the lens goes dark, the "shutters" close. This behavior is synced with the screen displaying the 3D content. As each different image is flashed the eye shutters alternate in time. Each eye sees a slightly different angle of the same image creating the 3D effect. On an LCD or LED television this method of 3D effectively cuts the refresh rate in half and has been known to cause headaches in some people. 3D capable DLP televisions also use this method of displaying 3D content.
Display glasses are another active method of creating 3D. These glasses are actually two small screens in the glasses that display separate images to each eye. An entirely solo experience, these glasses do not requie a television at all because the screens are in the glasses themselves.
Passive 3D Glasses
The old style colored 3D glasses use a method called Complementary Color Anaglyphs.These work by using a filter to block certain colors from each eye. The most commonly seen glasses utilize a red and cyan lens to pass red to one eye and blue and green to the others. At first this could only work with a black and white image since the filters blocked colors but modern methods allow for color viewing although the colors will be muted. Other methods of this technique use red and green filters or blue and amber filters. Although improvements have been made with this method of creating a 3D effect it is still considered inferior to polarization methods due to color loss.
Polarization is another method of displaying 3D content via passive glasses. It works by using lenses that block certain wavelength of visible light. Linear polarized glasses use vertical polarization on one lens and horizontal polarization on the other. In a movie theater two images shot at slightly different angles are shown on the screen with two projectors. Each image is polarized to match one glasses lens and each image is then shown to a separate eye. This produces a 3D effect as long as the users head is kept straight. Tilting the head will break the 3D effect.
Circularly polarized lenses are polarized clockwise for one eye and counter-clockwise for the other. This method of polarization will maintain the 3D effect if the head is tilted and it also only requires a single projector. A circular polarizer is placed in front of the projector and it quickly alternates between the two polarizations to create the 3D effect.
In Dolby 3D Theaters a technology called Infitec is used. Infitec stands for interference filter technology. This method uses colors filters like the previously mentioned glasses but use a much more advanced method. Each color is split into two channels (Red is R1 and R2, Blue is B1 and B2 and Green is G1 and G2). Each lens lets one channel through and full color 3D is produced with only a very slight color difference sent to each eye.