The purpose of 3-d glasses is to completely separate the image for the right eye from the image for the left eye. This separation is necessary for stereo (3-d) sight.

The most common type of 3-d glasses are anaglyph (red/blue) glasses. With this type of glasses, a red filter on the left and a blue filter is on the right. This is the type of 3-d glasses that were used with 3-d movies in the 1950's and are still used with comic books and magazines today. For example, this year's Sports Illustrated Swimsuit Edition has a couple of 3-d layouts (for purely artistic reasons, I assure you.)

Another type of 3-d glasses uses polarized lenses. The two lenses are polarized perpendicular to one another. The old Captain EO movie at Disneyland used this sort of glasses.

The final type of 3-d glasses is used for stereo photography. In stereo photography, prints are developed as a pair of side-by-side images. This type of 3-d glasses (often called "Holmes viewers" after their inventor, Oliver Wendell Holmes) use prisms to keep the eyes' line of sight parallel to one another so they can focus on both images simultaneously.

There is a fourth approach to 3D glasses that works as follows.

Each lens is like a double glazed window filled with liquid crystal that can be changed quickly from transparent to opaque. Alternate frames of the film show slightly different perspectives, so that all the odd numbered frames are suitable for viewing with the left eye and all even numbered frames are for the right eye (or vice versa.)

When viewing the display the left and right lenses are alternately switched on and off so that the eyes only ever get to see the appropriate frames. Due to the phenomenon of persistence of vision, the brain can be convinced that both eyes are receiving an uninterrupted view and so it is tricked into perceiving the alternate two dimensional images as a single three dimensional view.

Because this technique relies on showing alternating perspectives it is most suited to use in film or computer screens as opposed to printed media.

The BBC once distributed still another type of 3D glasses for their 1993 Dr Who special, Dimensions in Time, broadcast in aid of Children in Need.

These glasses had one lens that was essentially transparent, and one that had a strong (but not coloured) filter, like a sunglasses lens.

The effect worked on the principle that the brain takes slightly longer to process dark images than it does light images. Therefore, if one eye is seeing a much darker image, then the total perceived image (including the percieved depth) is distorted because the images from one eye are suffering a slight lag.

This effect would not normally be particularly noticeable (or create any coherent 3D image where there was none before). However, the trick is to keep the camera constantly moving horizontally. Coupled with the slight lag, this simulates the effect of having two cameras, one a little horizontally separated from the other - just like your eyes. Thus, as long as the camera is kept moving, an effective 3D illusion is created.

This method has a couple of advantages. Firstly, you can transmit to any perfectly ordinary television. Secondly, a viewer who does not have the special glasses will be able to watch as normal, albeit without the 3D effect.

The disadvantage is that the camera has to keep moving all the time, which becomes unsettling to the viewer after a while, and is certainly awkward to shoot. During the Dr Who special, there were a lot of short scenes where the camera simply circled the actors, around, and around, and around, and it was quite nauseating.

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