Microform includes both film and fiche. Microfilm comes in roll form. It was developed more than 100 years ago as a part of conventional photographic technology. Early microfilm seems to have been generally stable (see NATURE OF MATERIALS). If microfilm has a quality polyester film base, is exposed and processed to international standards, is housed in stable protective enclosures, and is stored under the appropriate conditions, it will last for a minimum of 500 years. The longevity issue is vitally important, as it delineates the main difference between microfilm and other reformatting technologies such as digital imaging.
Microfiche usually replicates multiple individual pages in single fiche sheets. It can be made up of strips of film cut from rolls and placed in polyester jackets, or made by filming with a "step and repeat" camera, with images imposed directly onto a single sheet of film.
Microfilm is available in 16 or 35 millimeter widths with 35 millimeters the norm for preservation filming. Microform is now available as silver-gelatin, diazo, and vesicular.
Silver-gelatin
Silver-gelatin is the only type recognized for preservation or "archival" purposes. The image is captured by exposing silver compounds to light. Wet-processed silver film is the only type that can be recognized as archival, as dry silver film is not fixed by chemical processing and washing.
Diazo
Diazo refers to the diazonium salts used in the coating layer. The salts are combined with dyes to produce the image. Because diazo is strongly affected by ultraviolet radiation, especially in a reader machine, this type of film can fade.
Vesicular
The word vesicular derives from vesicles, or bubbles that form when an image is developed through heating, which causes nitrogen from diazonium salts to expand. Ultraviolet is then used to decay the salts. Pressure on the film can damage the image-bearing bubbles, and faulty processing can cause the bubbles to burst, resulting in damage to the image.
Other types
Other microform types include color and continuous tone. Color film is notoriously vulnerable to light fading. Although the Image Permanence Institute of the United States has tested one type, Iflachrome, which may have a life of up to 500 years in the absence of light, the testing is still incomplete, and color film cannot be recognized as a preservation medium. Continuous-tone film techniques can produce gray tones not captured by the high-contrast film used for normal filming. Continuous tone can be produced by various methods. These include processing silver-gelatin film by varying exposure time, and processing with a low-contrast developer (see http://www.nedcc.org/plam3/tleaf51.htm for more detailed discussion).
Click on the links below to learn more about the different issues relating to microform:
1. Standards
2. Film Generations
3. Preservation Microfilming Training Materials
4. Storage
5. Microfilm Production
