An actor is filmed performing in front of a white screen and lit with powerful sodium vapor lights. Sodium light is used because it is a narrow spectrum source that falls neatly into a notch between sensitive layers of the color film. This allows the complete range of colors to be used in costumes, make-up, and props. A camera with a beamsplitter prism is used to expose two separate film elements. The first film element is regular color negative film that is not very sensitive to sodium light, while the other is panchromatic fine grain black and white film that is sensitive to the color of sodium vapor lights. This second film element is used to create a matte, so that the regular color footage can later be combined with another shot without the two images showing through each other. Making the matte film at the same time as the live action makes a much better fit in the post production optical printing, rendering the matte "lines" almost invisible.
Disney reportedly made only one sodium vapor camera because only one working prism was ever produced, despite attempts to replicate it. The camera is a retired Technicolor 3 strip camera modified to use two films, and normal lenses for 1:185 aspect ratio. Technicolor 3 strip cameras ran 3 rolls of black and white film past color filters to make color movies before 1952. That was when Eastman Kodak brought color negative film to the market. At the time of its use, the sodium process yielded cleaner results than bluescreen, which was subject to noticeable color spill (a blue tint around the edges of the matte). As the bluescreen process improved, the sodium vapor process was abandoned because the screen and lamps monopolized a huge studio, and its higher cost.
The technique was used in the films Mary Poppins, Song of the South and Alfred Hitchcock's The Birds. It was used in the 70's for scenes in "Island at the Top of The World", "Gus", "The Apple Dumpling Gang", "Freaky Friday", "Escape from Witch Mountain", "Pete's Dragon" and "The Black Hole."