Types of Microforms
The term “Microform” covers all forms of micro-images – these may be transparent or opaque, and they may be in the form of film rolls of various widths and lengths, in the form of film strips, or in the form of sheets or cards of various sizes. All microforms are reduced photographic images of documents that must be magnified on a reader or viewer in order to be read. [1]
The microforms in use today are basically four in number: (1) microfilm, (2) microfiche, (3) microcard, and (4,) microprint.
(I) Microfilm: Microfilm may be defined either as (1) a transparent flexible material for the photo-graphic reproduction of documents in reduced size, or as (2) a series of photographic reproductions on a medium. The microfilm in use today is of three types:
(1) The silver-halide emulsion type (2) The diazo or ozalid type (3) the Kalvar type.
Emulsion film consists of a thin strip or roll of cellulose acetate, known as the Base, containing a coating of light-sensitive emulsion on one side. Usually the emulsion side of microfilm has little or no gloss while the non-emulsion side is very glossy. The only permanent record films, i.e. those that have archival permanence, are the silver-halide emulsion type.[2]
Ozalid or Diazo type film consists of a thin strip of cellulose acetate that has a light-sensitive diazo dye either incorporated into the film base or coated on the base. When this film is exposed to ultraviolet and blue light and developed by an ammonia vapor or an alkaline solution an image is formed. With this process a negative image will produce a negative image.
Kalvar film consists of a thin strip of polyester film that contains a diazonium emulsion. This film when exposed to ultraviolet light and then processed by heat, forms an image. With Kalvar film a negative image will also produce a negative image.
Kalvar and Ozalid films are, at present, not suitable for use in a microfilm camera but only for preparing film duplicates. Neither one has been approved for archival permanence. Both Ozalid and Kalvar film are generally more suitable for the reproduction of film copies from high contrast materials such as newspapers or line drawings than they are for the reproduction of archival materials.
(II) Microfiche: A microfiche consists of a number of rows of reduced images of documents produced on transparent sheets. There are a number of ways in which microfiche may be made. They may be produced by inserting strips of microfilm in very thin acetate jackets, by taping strips of microfilm on clear acetate sheets, by placing strips of microfilm in a special holder, or by using a special camera known as a step and repeat camera which records images in a series of rows on a sheet of film or on a series of sheets of film.
Microfiche is produced in a wide variety of sizes. The most common of these are (1) 3 x 5 inches (75 x 125mm.); (2) 31/2 x 4-3/4 inches (90 x 120 mm.) – a size quite common in Europe; and (3) 4 x 6 inches (105 x 148 mm.).
(III) Microcard: A microcard consists of a number of rows of reduced images of documents reproduced on an opaque card. It is produced by photographic processes.
(IV) Microprint:
A microprint is similar to a microcard except it is produced by a printing
process. [3]
[1] Albert H. Leisinger Jr, Microphotography for Archives, International Council on Archives, Washington, D.C., 1968, p. 1.
[2] Ibid; p. 1.
[3] Ibid; p. 4.
