motion-picture process that gives a three-dimensional quality to film images. It is based on the fact that humans perceive depth by viewing with both eyes. In the 3-D process, two cameras or a twin-lensed camera are used for filming, one representing the left eye and the other the right. The two lenses are spaced about 2 ¹/₂ inches (6.3 cm) apart, the same as the separation between a person's eyes. The resulting images are simultaneously projected onto the screen by two synchronized projectors. The viewer must wear differently tinted or polarized glasses so that the left- and right-eye images are visible only to the eye for which they are intended. The viewer actually sees the images separately but perceives them in three dimensions because, for all practical purposes, the two slightly different images are fused together instantly by his mind.

Studios and independent producers experimented with 3-D throughout the 1920s and '30s. Many of the technical problems were later solved by the Natural Vision process, which used striated polarized lenses (with similarly striated viewing glasses for the audience) that made it possible to film in natural colour and correctly applied the convergence principle of the human eye in the filming. The first 3-D film in Natural Vision was Bwana Devil (1952), which was followed by several hastily shot action films. It is generally believed that the popularity of 3-D in the United States subsided after about a year because of the low quality of the films presented. Filmmakers in Italy, Germany, The Netherlands, Great Britain, and other countries experimented with 3-D at about the same time as did those in the United States, but its popularity in Europe soon faded when the illusion of depth was no longer a novelty. The process experienced a minor revival beginning in the 1970s.