Short for "3 dimensional," 3D refers to the way in which humans view the world. Having two eyes spaced roughly 2 inches apart provides us with what is called "binocular vision." We see things from two different perspectives. The two resulting images are fused together in the brain, allowing us to perceive depth—our depth of field is 3 dimensional. There have been many forms of imagery over the years that have used illusion to induce the perception of a "3D effect;" among these, 3D movies have received the most attention among popular culture.
Every now and again a movie is released in 3D format or dually released in both 3D and regular versions. The 3D movie uses a special filming process in combination with tinted glasses worn by the spectator to trick her mind into perceiving true depth—the appearance that the movie is "jumping off the screen."
When making a movie intended for 3D viewing, filmmakers actually shoot the movie with two cameras simultaneously, from two slightly different angles. However, sometimes a modified camera with dual lenses is used.
In the Theater
There are two different setups when it comes to projecting and viewing a 3D movie.
In the first and older system two projectors simultaneously play the two reels shot from different angles. The image coming from one projector is tinted blue or green and the image from the other is tinted red. The audience wears special glasses with tinted lenses—one blue (or green) and one red. Each lens blocks one of the images so that each eye perceives a slightly different picture, or rather two slightly different angles of the same picture. The mind fuses these two images, resulting in the appearance of objects that seem to extend out from the screen. The only problem with this form of 3D imaging is that the movie is limited to the spectrum of only two colors; because of this problem, the red/blue method is reserved primarily for black and white movies and television 3D events.
The second system of viewing movies in 3D involves a process called polarization. Two projectors are used, as in the red/blue method, to project the dual images. However, no tinted lenses are used, allowing the projection of full-color images. Instead, each image is polarized. This means that the waves of light for each image are made to oscillate in different directions. The tinted lenses in the 3D glasses are polarized so that each lens allows through only one orientation of light oscillations. Therefore, as in the red/blue version, each eye perceives only one of the images and the 3D effect is created in the mind of the viewer.
Other 3D Imagery
3D imagery has been used in many forms other than 3D movies, like comic books, hidden object pictures and View-Masters. In View-Masters for instance, a person looks into a binocular-like contraption, with separate viewing holes for each eye. A slightly different picture is projected into each hole, applying the same principle as 3D movies, resulting in a 3D illusion. The same basic principle is applied in all types of 3D imagery; as long as each eye sees the picture from slightly different perspectives, the mind will create a 3D illusion.