Microfilm

Microfilm is an analog storage medium for books, periodicals and engineering drawings. Its most standard form is a roll of black and white 35mm photographic film. Another form, more common for engineering drawings, is a Hollerith punch card that mounts a single exposure. Most microfilm media have a digital indexing system exposed on the edge of each image, but this data is not required to use the microfilm, but rather to support automated retrieval systems.

Microfilm is not the most compact analog microform in wide use. Microfiche is more compact.

The medium has numerous advantages. First, it is compact, with far smaller storage costs than paper documents. Generally, a year of a periodical takes 10% of the space and 3% of the weight. Second, it is lower cost than a standard subscription. Most microfilm services get a bulk discount on reproduction rights, and have lower reproduction costs than a comparable amount of printed paper. Third, it is a stable archival form. Most library microfilms use polyester with silver-halide dyes in hard gelatin, with an estimated life of 500 years in air-conditioning. In tropical climates with high humidities, fungus eats the gelatin used to hold silver-halide. In the tropics, diazo-based systems with lower archival lives (20 years) are preferable, because they have polyester or epoxy surfaces. Fourth, since it is analog (an actual image of the orginal data), it is easy to view. Unlike digital media, the data format is instantly comprehensible to persons literate in the language; the only additional equipment that is needed is a simple magnifying glass. This reduces the possibility of obsolescence.

Systems that mount microfilm images in punch cards have been widely used for archival storage of engineering information. For example, when airlines demand archival engineering drawings to support purchased equipment (in case the vendor goes out of business), (as of 1999) they normally specified punch-card-mounted microfilm with an industry-standard indexing system punched into the card. This permits automated reproduction, as well as permitting mechanical card-sorting equipment to sort and select microfilm drawings. Hollerith-mounted microfilm is roughly 3% of the size and space of conventional paper or vellum engineering drawings. Some military contracts around 1980 began to specify digital storage of engineering and maintenance data because the expenses were even lower than microfilm, but these programs are now fnding it difficult to purchase new readers for the old formats.

The principal disadvantage of microfilm is that the image is too small to read. Libraries use special readers that project full-size images on a ground-glass screen. Another disadvantage, is that a conventional photocopier cannot reproduce the images. Libraries using microfilm often have a few viewers that can produce a photocopy of an image, for a nominal fee. The final disadvantage is that microfilm can only be reproduced a limited number of times, while digital media regenerate and often include error detection and correction schemes.

Links

• The Library of Congress Photoduplication Service (http://www.loc.gov/preserv/pds/)
• The Northeast Document Conservation Center (http://www.nedcc.org/plam3/tleaf51.htm)