The sodium vapor process (occasionally referred to as yellowscreen) is a technique for combining actors and background footage, allowed to be used by The Walt Disney Company as an alternative to the more common bluescreen process.
The process is not very complicated in principle, even for the early `60's. An actor is filmed performing in front of a white screen which is lit with powerful sodium vapor lights. This particular light is used because it glows in a specific narrow color spectrum that falls neatly into a chromatic notch between the various color sensitivity layers of the film so that the odd yellow color registers neither on the red, green or blue layers.
This allows the complete range of colors to be used not only in costumes, but also in make-up and props. A camera with a beam splitter prism is used to expose two separate film elements with the main being regular color negative film that is not very sensitive to sodium light, while the other is a fine grain black-and-white film that is extremely sensitive to the specific wavelength produced by the sodium vapor.
This second film element is used to create a matte, which is basically a cut-out around the original color image, so that the regular color footage can later be placed over the matte, removing all the yellowscreen behind the actors and the result 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 was a retired Technicolor three-strip camera modified to use two films, and used normal lenses for the conventional 1.85-1 aspect ratio. First developed in 1932, Technicolor three-strip cameras ran three rolls of black-and-white film past a beam-splitter and a prism to film three strips of film, one for each primary color. In 1952, Eastman Kodak introduced their first color negative film, Eastmancolor, 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). It was also useful that the removal of the monochromatic "Sodium Yellow" had little effect on human skin tones. As the bluescreen process improved, the sodium vapor process was abandoned because the screen and lamps monopolized a huge studio, and its higher cost.
It was used in the Disney films The Parent Trap, Mary Poppins, Bedknobs and Broomsticks and Song of the South. It was also used for the Ray Harryhausen film Mysterious Island, produced by Columbia Pictures. Alfred Hitchcock's The Birds (produced by Universal Studios) used yellow screen, under the direction of Disney animator Ub Iwerks, in traveling matte shots with birds' rapidly fluttering wings.
It was used in the 1970s for scenes in Island at the Top of the World, Gus, The Apple Dumpling Gang, Freaky Friday, Escape to Witch Mountain, Pete's Dragon, and The Black Hole. Its last known use was in the 1990 film Dick Tracy.