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Intro to Federal Census Research

Microfilm Basic Technical Information

Silver Halide Film
  • Silver Halide is the film type used in the camera to make the master film.
  • Silver Halide is the most light sensitive of all the films used, hence it can record a greater variance of density and contrast in the original image.
  • Silver Halide is the only film that is considered archival.
    • Silver-halide print films are of two types: Polarity reversing and direct-duplicating.
    • Silver-halide film should not be stored with other kinds of film.
    • Best film type for census records.
Diazo Film
  • The Diazo process is used exclusively for making copies of the master film. It is not sensitive enough to be used as a camera negative film. The process has been used since the 1920's.
    • Diazo is a polarity maintaining copy. (i.e. negative to negative.)
    • Do not use in strong sunlight. Sunlight contains ultraviolet light and will ruin the film even after processing.
Vesicular Film
  • Vesicular films became popular not only because of the ability to reverse the image but also because it can be handled in daylight and process using a dry thermal process. The image is comparable to silver when projected on a reader screen.
    • Image reversing
    • Can be handled in daylight.
    • Best film type for newspapers.

Manual of Archival Reprography 1989
Reprint Permission granted September 25, 1997 by:
Heritage Microfilm, Incorporated
1450 Boyson Road, Building B
Hiawatha, Iowa 52233

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