In photography, the terms reversal film, slide film, positive film, and transparency film describe a type of photographic film that produces a positive image on a transparent film base. In place of negatives and printed photographs, reversal film produces diapositive photographs, known as diafilm and dia images. A diapositive slide is a mounted frame of transparency film intended for large-format projection of the image onto a screen by a slide projector. Such large-format projection permits many spectators to view the diapositive image at one time. The usual transparency-film format is the 35 mm slide, wherein one frame of film is mounted into a 2 in. ×2 in. square made either of cardboard or of a plastic material. Reversal film is produced in format sizes that range from 35 mm to roll film to 8 in. ×10 in. sheet film. Moreover, specialized photographic processing laboratories produce diapositive slides from digital-camera images in formats such as the JPEG, from computer-generated presentation graphics, and from images of physical materials such as fingerprints, microscopic evidence, paper documents, astronomic images, etc. Reversal film also is used as motion picture film in the 16 mm, Super 8 and 8 mm cine film formats, and produces a positive-image film that was originated in the cinematographic camera. In terms of film-production costs, such a cinematographic application of reversal film is more cost-effective than filming with negative film that then must be developed into a positive-image film for projection in a cinema.
History
Additive method The earliest practical and commercially successful color photography reversal process was the Lumière Autochrome, introduced in 1907. It was an additive method, using a panchromatic emulsion coated on a thin glass plate previously coated with a layer of dyed potato starch grains. Autochrome plates were discontinued in the 1930s, after the introduction of Lumière Filmcolor in sheet film and Lumicolor in roll film sizes. Also using the additive principle and reversal processing were the Agfa color screen plates and films and Dufaycolor film, all of which were discontinued by 1961.
Subtractive methods
Leopold Godowsky, Jr. and Leopold Mannes, working with the Eastman Kodak Company, developed Kodachrome, the first commercially successful color film to use the subtractive method. Kodachrome was introduced in 1935 as 16mm motion picture film, and in 1936 as 35mm film for still cameras. Kodachrome films contain no color dye couplers; these are added during processing. The final development process published by Kodak as K-14 involves multiple re-exposure steps to sensitize the unsensitized grains. In late 1936, Agfacolor Neu was launched, Agfa having overcome earlier difficulties with color sensitivity problems. This film had the dye couplers incorporated into the emulsion, making processing simpler than for Kodachrome. Early color negative film had many shortcomings, including the high cost of the film, processing and printing, the mediocre color quality, rapid fading and discoloration of highlights of some types of print that became noticeable after several years. Amateurs who owned projection equipment used reversal films extensively because the cost of projection equipment and slide film was offset by not having to pay for prints. Eventually, print quality improved and prices decreased, and, by the 1970s, color negative film and color prints had largely displaced slides as the primary method of amateur photography. Until about 1995, color transparency was preferred for publication because of the films' higher contrast and resolution, and was widely used in commercial and advertising photography, reportage, sports, stock and nature photography. Digital media gradually replaced transparency film.
Film types In motion picture film (like Super 8, Regular 8, and 16mm), one can find both color reversal film and black and white reversal. In still film (like 35mm, 120, 110, 620, etc.), the most common reversal film is color reversal; black and white reversal film also exists for still film but it is less common.
Color reversal film All color reversal film sold today is developed with the E-6 process. The non-substantive Kodachrome films, the last of which was discontinued in 2009, were processed with the K-14 process. Polaroid produced an instant slide film called Polachrome. It was packaged in cassettes like normal 35mm film. A separate processing unit was used to develop it after exposure.
Black and white reversal film
Black-and-white transparencies can be made directly with some modern black-and-white films, which normally yield negatives. The negative image is developed but not fixed. The negative image is removed by bleaching with a solution of potassium permanganate or potassium dichromate in dilute sulfuric acid, which is removed by washing and a clearing bath containing sodium metabisulfite or potassium metabisulfite. The remaining silver halide salts are re-exposed to light, developed and fixed, and the film is washed and dried. Black-and-white transparencies were once popular for presentation of lecture materials using 3¼"×4" (3¼" square in the UK) glass-mounted slides. Such positive black-and-white projection is now rarely done, except in motion pictures. Even where black-and-white positives are currently used, the process to create them typically uses an internegative with standard processing instead of a chemical reversal process. Black-and-white reversal films are less common than color reversal films.
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