Sodium vapor process
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The sodium vapor process (occasionally referred to as yellowscreen) is a
photochemical Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400  nm), visible light (400–7 ...
film technique for combining actors and background footage. It originated in the British film industry in the late 1950s and was used extensively by
Walt Disney Productions The Walt Disney Company, commonly known as Disney (), is an American multinational mass media and entertainment conglomerate headquartered at the Walt Disney Studios complex in Burbank, California. Disney was originally founded on October 1 ...
in the 1960s and 1970s as an alternative to the more common bluescreen process. Wadsworth E. Pohl is credited with the invention or development of both of these processes (Patent US3133814A), and received (with
Ub Iwerks Ubbe Ert Iwwerks (March 24, 1901 – July 7, 1971), known as Ub Iwerks ( ), was an American animator, cartoonist, character designer, inventor, and special effects technician. Born in Kansas City, Missouri, Iwerks grew up with a contentiou ...
and
Petro Vlahos Petro Vlahos ( gr, Πέτρος Βλάχος; August 20, 1916 – February 10, 2013) was an American engineer and inventor, considered to be one of the pioneering scientific and technical innovators of the motion picture and television indus ...
) an Academy Award in 1965 for the sodium vapor process used in the film '' Mary Poppins''.


Description

The process is not very complicated in principle. An actor is filmed performing in front of a white screen that is lit with powerful sodium vapor lights. Such light has a narrow color spectrum that falls neatly into a chromatic notch between the various color sensitivity layers of the film, so the odd yellow color does not register on the red, green, or blue layers. This allows the complete range of colors to be used, not only in costumes, but also in makeup and props. A camera with a
beam-splitter A beam splitter or ''beamsplitter'' is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding wide ...
prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...
is used to expose two separate film elements. The main element is regular color negative film that is not very sensitive to sodium light, and the other 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 Matte may refer to: Art * paint with a non-glossy finish. See diffuse reflection. * a framing element surrounding a painting or watercolor within the outer frame Film * Matte (filmmaking), filmmaking and video production technology * Matte p ...
, as well as a counter-matte, for use during
compositing Compositing is the process or technique of combining visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene. Live action, Live-action shooting for compositing is ...
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 In photography and cinematography, a multiple exposure is the superimposition of two or more exposures to create a single image, and double exposure has a corresponding meaning in respect of two images. The exposure values may or may not be ide ...
). 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 does bluescreen (though both processes degrade the image and introduce 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 as with 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.


History

Disney made one sodium vapor camera. The camera was a retired
Technicolor Technicolor is a series of Color motion picture film, color motion picture processes, the first version dating back to 1916, and followed by improved versions over several decades. Definitive Technicolor movies using three black and white films ...
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 its
color negative In photography, a negative is an image, usually on a strip or sheet of transparent plastic film, in which the lightest areas of the photographed subject appear darkest and the darkest areas appear lightest. This reversed order occurs because t ...
film,
Eastmancolor Eastmancolor is a trade name used by Eastman Kodak for a number of related film and processing technologies associated with color motion picture production and referring to George Eastman, founder of Kodak. Eastmancolor, introduced in 1950, was on ...
, which led to Hollywood's discontinuation of Technicolor cameras in 1954. At the time of its use, the sodium process yielded cleaner results than did 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, its screen and lamps monopolizing huge studios and incurring a higher cost. The first use of the process was in the J. Arthur Rank Organisation's ''Plain Sailing'' in 1956. It was used in Disney's short
Donald and the Wheel ''Donald and the Wheel'' is a 17-minute Donald Duck animated short directed by Hamilton Luske, produced by Walt Disney and released on June 21, 1961. It is an educational-based film, and features a considerable amount of musical vocals. Disney de ...
, and the films '' The Parent Trap'' and '' Mary Poppins''. It was also used for the
Ray Harryhausen Raymond Frederick Harryhausen (June 29, 1920 – May 7, 2013) was an American-British animator and special effects creator who created a form of stop motion model animation known as "Dynamation". His works include the animation for '' Mi ...
film '' Mysterious Island'', produced by
Columbia Pictures Columbia Pictures Industries, Inc. is an American film production studio that is a member of the Sony Pictures Motion Picture Group, a division of Sony Pictures Entertainment, which is one of the Big Five studios and a subsidiary of the mu ...
. Alfred Hitchcock's '' The Birds'' (produced by
Universal Studios Universal Pictures (legally Universal City Studios LLC, also known as Universal Studios, or simply Universal; common metonym: Uni, and formerly named Universal Film Manufacturing Company and Universal-International Pictures Inc.) is an Americ ...
) used yellow screen, under the direction of Disney animator
Ub Iwerks Ubbe Ert Iwwerks (March 24, 1901 – July 7, 1971), known as Ub Iwerks ( ), was an American animator, cartoonist, character designer, inventor, and special effects technician. Born in Kansas City, Missouri, Iwerks grew up with a contentiou ...
, in traveling matte shots with birds' rapidly fluttering wings. The process was used in the 1970s for scenes in '' Island at the Top of the World'', '' Gus'', ''
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'', '' Pete's Dragon'' and ''
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''. Its last known use was in the 1990 film '' Dick Tracy''.


References

{{reflist Cinematic techniques