In photography, a transparency is a still, positive image created on a transparent base using photochemical means. The terms slide and reversal film are also used. Contrast with negative and print.
The earliest practical color photography was the Autochrome process - this was an additive screenplate method which produced a colour slide, but was rather dim, and with the colour resultion limited by the fineness of the screen-plate..
The earliest practical method using a 'subtractive' method was the Kodachrome process, which produced much brighter color transparencies. Originally used mainly for news reportage, it gradually gained wider popularity. As a medium for serious amateur photographers, it gained popularity as an alternative to black and white print film starting in about 1945. At this time, color print film had many shortcomings including high cost of film and processing and short print life. Amateurs who could afford slide film and projection equipment used it extensively until about 1970, when color print film began to displace it.
Through about 1995, color transparencies were the only photographic medium used for serious publishing, and were widely used in commercial and advertising photography, reportage, sports, stock, and nature photography. Digital media have since gradually replaced transparencies in many of these applications. The use of slides for artists submitting to juried shows or applying for solo exhibitions, applying to art schools or for residencies (or the like), however, is still nearly universal for a number of reasons, among which is the actual or perceived lack of colour fidelity in digital media.
Slides are still generally preferred by professionals and many amateurs when working with traditional film. Slides are often sharper and have better colour reproduction. Generally slides have a longer life span than colour prints. Kodachrome is well known for its archival qualities. Color does not fade in Kodachrome for a long time. Theoretically they should last about 200 years compared to perhaps 50-70 years for negative colour-film processes (e.g. Kodacolor or Afgacolor) and 20-30 years for colour prints. The Kodachrome process uses toxic and difficult to control chemicals in the development process and so remains in use in only a few locations worldwide.
Direct positive slide film is less forgiving of exposure errors than than the negative - print - and development process chain. With negatives, the overall value may be sensed after processing and the exposure of the positive image controlled to compensate. The simplest point and shoot and disposable cameras do not even control exposure, a demonstration of the wide exposure lattitude of the processes. It is also more cumbersome to display if only a few images are to be shown, although small battery powered direct viewers are available and suitable for use by one or two viewers.
A slide is a special type of transparency intended to be projected onto a screen using a slide projector. This allows the photograph to be viewed by a room-full of people at the same time. Slides were at one time an important media for presentations, but LCD projectors which are now widely available have largely replaced traditional slide projectors for this purpose.
The most common form of modern slide is the 35mm slide, essentially a positive-image printing onto the standard 35mm film used in the movie industry, then placed inside a cardboard or plastic shell. Older projectors used a sliding mechanism to manually pull the transparency out of the side of the machine, where it could be replaced by the next image, and it is from this that we get the name "slide". Modern projectors typically use a carousel that holds a large number of slides, and viewed by a mechanism that automatically pulls a single slide out of the carousel and places it in front of the lamp.
Transparency film, in sizes ranging from 35mm roll film up to 8x10" sheet film, are produced by Kodak, Agfa, Konica, Scotch, and Fujifilm. Essentially all film sold today uses either the E-6 process or the K-22 process, with the overwhelming majority using the E-6 process.
Black and white transparencies can be made with many types of black and white film using modified processing. While once popular for presentation of lecture materials using 4" by 5" glass mount slides, positive black and white projection is now rarely done except in motion picture use. Even that is quite rare and is used largely to reproduce a film noir appearance.
Photographic Specialties is no longer producing slides and transparencies. I have been self-employed for 35 years, but looking forward to retirement since my wife died five years ago. With the decline of conventional photography and the recent recession, the availability of film and chemicals as well as the volume of work has declined to such an extent that it is no longer possible to have fresh chemicals to develop film. I hope that if you still need conventional slides and transparencies, you will be able to find someone who can produce them for you. Thank you for your orders over the years.