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Polarized 3D glasses create the illusion of three-dimensional images by restricting the light that reaches each eye, an example of stereoscopy. To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through orthogonal polarizing filters. The viewer wears low-cost eyeglasses which also contain a pair of orthogonal polarizing filters. As each filter passes only that light which is similarly polarized and blocks the orthogonally polarized light, each eye sees only its separately polarized image, producing a three-dimensional effect.
The difficulty arises because light reflected from a motion picture screen tends to lose a bit of its polarization. However, this problem is eliminated if a 'silver' or Aluminized screen is used. This means that a pair of aligned DLP projectors, some polarizing filters, a silver screen, and a computer with a dual-head graphics card can be used to form a relatively low-cost (under US$10 000 in 2003) system for displaying stereoscopic 3d data simultaneously to tens of people wearing polarized glasses. Such a system, called a GeoWall, has been used for several years now in the Earth Sciences thanks to the GeoWall Consortium, with several open source and commercial packages available.
When stereo images are to be presented to a single user, it is practical to construct an image combiner, using partially silvered mirrors and two image screens at right angles to one another. One image is seen directly through the angled mirror whilst the other is seen as a reflection. Polarized filters are attached to the image screens and appropriately angled filters are worn as glasses. A similar technique uses a single screen with an inverted upper image, viewed in a horizontal partial reflector, with an upright image presented below the reflector, again with appropriate polarizers. Polarizing techniques are most simply used with cathode ray technology, as polarizers are used within ordinary LCD screens for control of pixel presentation - this can interfere with these techniques.
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