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# What is three-dimensional (3-D) vision?

When a three-dimensional movie is filmed, two cameras film the movie from slightly different positions. When the film is projected on the screen, each projector uses a separate polarizing filter. The left projector might use a horizontally polarized filter, while the right projector uses a vertically polarized filter. The viewer wears polarized glasses. The glasses allow the left eye to see only the image produced from the horizontally polarized image of the left projector, while the right eye sees the image produced by the vertically polarized right projector. This arrangement simulates the different perspectives that each eye sees when looking at a real-life 3-D scene, allowing the brain to interpret the difference as depth (the third dimension).

The newest methods of producing three-dimensional views use digital methods rather than film. In the method best suited to 3-D television the images from the left and right camera lenses alternate at a rapid rate. The viewer wears glasses in which each lens can be switched from transparent to opaque on command from an infrared signal sent from the television set. Thus each eye sees only the frames captured by the appropriate camera lens. Another method that is more suited to movies, again uses digital images that alternate between those captured by the right camera lens with those captured by the left. A device placed in front of the projector lens switches the polarization of the light coming from the projector so that left images are polarized one way, right images are polarized the other way. The movie viewer wears polarized glasses so that each eye sees only the appropriate frames.

Seeing in three dimensions, which is how a person with normal eyes sees, means that in addition to perceiving the dimensions of height and width (such as seen on a piece of paper, a poster, or a TV or movie screen), one can see the third dimension of depth. We see real objects in 3-D because we have two eyes that see slightly different perspectives of the same view. The combination of these views, when interpreted by our brain, gives us the ability to perceive depth, the third dimension.

If you close one eye, your ability to perceive depth is eliminated. With only one eye, the world won’t look very different to you, but you’ll experience difficulty in judging distances.

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