Chromogenic Slide Film

Chromogenic processes use three layers of gelatin silver halide emulsion to produce a full-color image. Each of the three layers is sensitized to a different color of light: one to red, one to blue, and one to green. Compounds known as dye couplers present in the emulsion layers (or, as was the case with Kodachrome, introduced during processing) react with the silver in the exposure and other chemicals during processing to form ‘dye clouds.’ The red-sensitive layer produces cyan dye, the blue-sensitive layer produces yellow dye, and the green-sensitive layer produces magenta dye. The silver is then bleached out to leave only the color dyes. If the film is going to be used to produce paper prints, the colors produce a color negative, in which all the colors are complementary colors to those in the actual image, which is then used to produce a color positive print.

Cross-section view of a chromogenic slide showing the emulsion layers. [1]

In the case of color slide film or color transparency film, the development process transforms the negative image produced by the exposure to a positive color image, which then remains on the transparent plastic base support which held the unexposed emulsion. For slides, the base support is then placed in a cardboard, plastic, or metal mount to protect the slide and allow for easier handling. This allows for the image on the slide to be projected by a projector, which enlarges it and allows it to be viewed easily by many people at once. While 35mm slides were the most widely used size of color slide film, it was produced in a variety of sizes.

A typical example of a 35mm slide. [1]

The use of dye couplers and multiple layers of emulsion to produce a color photograph was first developed by Rudolf Fischer, a German chemist, in the 1910s, but he was unable to perfect the process. The first successful chromogenic transparency film was developed by Leopold Mannes and Leo Godowsky, two professional musicians who began experimenting with color photography in their spare time before being hired by Kodak. In 1935,  the first film using the chromogenic process developed by Mannes and Godowsky was introduced under the name Kodachrome in a 16mm motion picture format; 35mm still slide film was introduced the following year. The first chromogenic film that incorporated the dye couplers directly into the emulsion was released in 1936 by the German company Agfa.

Chromogenic slide film was very popular in the 20^th century with both professional and amateur photographers. It was widely used by professionals, aside from some pictorial advantages such as wider tonal and color range and more visible detail, because it was easy to make color separations for print reproduction in magazines and newspapers directly from slides. It was widely used by amateurs who enjoyed displaying their photographs in slideshows for friends and family. Even as the popularity of slideshows waned among amateurs, professionals continued to commonly use slide film until digital photography became the preferred form for print reproduction.

Since chromogenic slide film was widely used by professionals, it is not often associated with the work of any specific photographers. One widely known example of a photograph taken using chromogenic slide film is ‘Afghan Girl,’ a photo taken by Stephen McCurry using Kodachrome color slide film which appeared on the cover of National Geographic in 1984.

Afghan Girl by Stephen McCurry. 1984. [2]

Chromogenic slide film can be identified by noting that that the color image is on a transparent support base. These very often take the form of a 35mm slide in a cardboard, plastic, or metal mount. If the image is examined under a microscope, the dye clouds produced during processing are visible. The dating of a chromogenic color slide is often made easy by the presence of a processing date, usually month and year, on the slide mount. In the absence of such a date, knowing the manufacturer of the film or the specific product name can help narrow the time period. In the case of Kodachrome slides, a protective lacquer was applied to the slides between 1939 and 1970. If the lacquer is present, iridescent colors will be visible on the emulsion side of the slide.

The main preservation concern with chromogenic slide film is dye fading. Chemicals left in the film after processing can cause one or more of the colors to fade over time. Color stability can be improved by storage in the dark at a low temperature and humidity. Higher temperature and humidity can also cause damage to the plastic support base of the slide.

[1] Images taken from graphicsatlas.org.
[2] Image taken from https://en.wikipedia.org/wiki/File:Sharbat_Gula.jpg.

Bibliography

Baldwin, G., & Jürgens, M.C. (2009). Looking at photographs: A guide to technical terms. Los Angeles, CA: Getty Publications.

Hirsch, R. (2015). Exploring Color Photography: From Film to Pixels (6th ed.). Burlington, MA: Focal Press.
Image Permanence Institute. (2015). Graphics Atlas website. www.graphicsatlas.org.

Lavédrine, B. (2009). Photographs of the Past: Process and Preservation. Los Angeles, CA: Getty Publications.

Weaver, G., & Long, Z. (2009). Chromogenic characterization: A study of Kodak color prints, 1942-2008. Topics in Photographic Preservation, 13, pp. 67-82.