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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
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Microform
