Microform - Duplication

Duplication

All regular microfilm copying involves contact exposure under pressure. Then the film is processed to provide a permanent image. Hand copying of a single fiche or aperture card involves exposure over a light box and then individually processing the film. Roll films are contact exposed via motor, either round a glass cylinder or through a vacuum, under a controlled light source. Processing may be in the same machine or separately.

Silver halide film is a slow version of camera film with a robust top coat. It is suitable for prints or for use as an intermediate from which further prints may be produced. The result is a negative copy. Preservation standards require a master negative, a duplicate negative, and a service copy (positive). Master negatives are kept in deep storage, and duplicate negatives are used to create service copies, which are the copies available to researchers. This multi-generational structure ensures the preservation of the master negative.

Diazo-sensitised film for dye coupling in ammonia gives blue or black dye positive copies. The black image film can be used for further copying.

Vesicular film is sensitised with a diazo dye, which after exposure is developed by heat. Where light has come to the film remains clear, in the areas under the dark image the diazo compound is destroyed quickly, releasing millions of minute bubbles of nitrogen into the film. This produces an image that diffuses light. It produces a good black appearance in a reader, but it cannot be used for further copying.

Modern microfilming standards require that a master set of films be produced and set aside for safe storage, used only to make service copies. When service copies get lost or damaged, another set can be produced from the masters, thus reducing the image degradation that results from making copies of copies.