Sodium Vapor Process

The sodium vapor process (occasionally referred to as yellowscreen) is a photochemical film technique for combining actors and background footage. It originated in the British film industry in the late 1950s, and was used extensively by The Walt Disney Company in the 1960s and '70s as an alternative to the more common bluescreen process. Petro Vlahos is credited with the invention or development of both of these processes, and received (with Wadsworth E. Pohl and Ub Iwerks) an Academy Award in 1965 for its use in Mary Poppins.

The process is not very complicated in principle. 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, as well as a counter-matte, for use during compositing on an optical printer. These complementary mattes allow the various image elements to be cleanly isolated, so that as they are re-exposed onto a single, fresh piece of negative, one at a time and in jigsaw fashion, the various images do not show through one another, as they would using simple double-exposure. Acquiring the matte film element (as a first-generation 'original') at the same time as the live action makes a much better fit during optical printing, because it requires fewer separate, duplicate film generations than bluescreen (each of which degrades the image and introduces more "error" to the resulting matte) in the process of achieving sufficiently dense mattes. This increased accuracy ultimately renders the matte "lines" almost invisible, though like bluescreen, its use may be signaled by hard separation or mismatched coloration and contrast between elements, or in this case, a telltale white/yellow fringe.

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). The increased accuracy allowed for the compositing of materials with finer detail, such as hair or Mary Poppins' veiled hat. It was also useful that the "Sodium Yellow" light (and its removal via the matte) had a negligible 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.

Its first use was in the J. Arthur Rank Organisation's Plain Sailing in 1956. It was used in the Disney films The Parent Trap, Mary Poppins, and Bedknobs and Broomsticks. 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.