All microform images may be provided as positives or negatives, more often the second.
Three formats are common: microfilm (reels), aperture cards and microfiche (flat sheets). Microcards were similar to microfiche, but printed on paper cardboard rather than photographic films.
Using the Daguerreotype process, John Benjamin Dancer was one of the first to produce micro-photographs, in 1839. He achieved a reduction ratio of 160:1. Dancer perfected his reduction procedures with Frederick Scott Archer’s wet collodion process, developed in 1850-51, but he dismissed his decades-long work on micro-photographs as a personal hobby, and did not document his procedures. That microphotography could be no more than a novelty was an opinion shared by the 1858 Dictionary of Photography, which called the process “somewhat trifling and childish.”
Microphotography was first suggested as a document preservation method in the early 1850s – in 1851 by James Glaisher, an astronomer, and in 1853 by John Herschel. Both men attended the 1851 Great Exhibition in London, where the exhibition on photography greatly influenced Glaisher: he called it "the most remarkable discovery of modern times," and argued in his official report for using microphotography to preserve documents.
The developments in microphotography continued through the next decades, but it was not until the turn of the century that its potential for practical usage was seized by a wider audience. In 1896, Canadian engineer Reginald A. Fessenden suggested microforms as a compact solution to engineers' unwieldy, but frequently consulted materials. He proposed that up to 150,000,000 words could be made to fit in a square inch, and that a one foot cube could contain 1.5 million volumes.
In 1906, Paul Otlet and Robert Goldschmidt proposed the livre microphotographique as a way to alleviate the cost and space limitations imposed by the codex format. Otlet’s overarching goal was to create a World Center Library of Juridical, Social and Cultural Documentation, and he saw microfiche as way to offer a stable and durable format that was inexpensive, easy to use, easy to reproduce, and extremely compact. In 1925, the team spoke of a massive library where each volume existed as master negatives and positives, and where items were printed on demand for interested patrons.
At the annual meeting of 1936, the American Library Association endorsed microforms. Before this official acceptance, microforms were used in related fields: between 1927 and 1935, the Library of Congress microfilmed more than three million pages of books and manuscripts in the British Library; in 1929 the Social Science Research Council and the American Council of Learned Societies joined to create a Joint Committee on Materials Research, which looked closely at microform’s potential to serve small print runs of academic or technical materials; in 1933, Charles C. Peters developed a method to microformat dissertations; in 1934 the United States National Agriculture Library implemented the first microform print-on-demand service, which was quickly followed by a similar commercial concern, Science Service, and in 1938 University Microfilms was established and the Harvard Foreign Newspapers Microform Project was implemented.
In 1940, the format most broadly used today – microfilm – was developed. Formats it bested include the Photoscope, the Film-O-Graph, the Fiske-O-Scope, filmslides.
Early cut sheet microforms and microfilms (to the 1930s) were printed on a nitrate film and pose high risks to their holding institutions, as nitrate film is explosive and flammable. From the late 1930s to the 1980s, microfilms were usually printed on a cellulose acetate base, which is prone to tears, vinegar syndrome and redox blemishes. Vinegar syndrome is the result of chemical decay and produces "buckling and shrinking, embrittlement, and bubbling". Redox blemishes are yellow, orange, red, or spots 15-150 micrometres in diameter created by oxidative attacks on the film, and are largely due to poor storage conditions.
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.
Aperture card 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 finding it difficult to purchase new readers for the old formats.
Microfilm first saw military use during the Franco-Prussian War of 1870–71. During the Siege of Paris, the only way for the provincial government in Tours to communicate with Paris was by pigeon post, and, as the pigeons could not carry paper dispatches, the Tours government turned to microfilm. Using a microphotography unit evacuated from Paris before the siege, clerks in Tours photograped paper dispatches and compressed them to microfilm, which were carried by homing pigeons into Paris and projected by magic lantern while clerks copied the dispatches onto paper.
Libraries began using microfilm in the early 1900s as a preservation strategy for their deteriorating newspaper collections. Books and newspapers that were deemed in danger of decay could be preserved on film and thus access and use could be increased. Microfilming was also a space-saving measure. In his 1945 book, “The Scholar and the Future of the Research Library,” Fremont Rider calculated that research libraries were doubling in space every sixteen years. His suggested solution was microfilming, specifically with his invention, the microcard. Once items were put onto film, they could be removed from circulation and additional shelf space would be made available for rapidly expanding collections. The microcard was superseded by microfiche. By the 1980s, microfilming had become standard policy
Portable readers are plastic devices that fold for carrying, when open they project an image from microfiche on to a reflective screen.
A microfilm printer contains a xerographic copying process, like a photocopier. The image to be printed is projected with synchronised movement on to the drum. These devices offer either small image preview for the operator or full size image preview, when it is called a reader printer. Microform printers usually accept positive or negative films, to give positive images on paper.
Newer readers allow the user to scan a microform image and save it as a digital file.
Sheet film is supplied in ISO A6 size. This is either processed by hand or using a dental X-ray processor. Camera film is supplied ready mounted in aperture cards. Aperture cards are developed, fixed and washed immediately after exposure by equipment fitted to the camera.
In a darkroom a single film may be inserted into a dark slide or the camera may be fitted with a roll film holder which after an exposure advances the film into a box and cuts the frame off the roll for processing as a single film.Roll film For engineering drawings a freestanding open steel structure is often provided. A camera may be moved vertically on a track. Drawings are placed on a large table for filming, with centres under the lens. Fixed lights illuminate the copy. These cameras are often over 3 metres (10 feet) high. These cameras accept roll film stock of 35 or 16 mm.
For office documents a similar design may be used but bench standing. This is a smaller version of the camera described above. These are provided either with the choice of 16 or 35 mm film or accepting 16 mm film only. Non adjustable versions of the office camera are provided. These have a rigid frame or an enveloping box that holds a camera at a fixed position over a copy board. If this is to work at more than one reduction ratio there are a choice of lenses.
Some cameras expose a pattern of light, referred to as blips, to digitally identify each adjacent frame. This pattern is copied whenever the film is copied for searching.Flow roll film cameras A camera is built into a box. In some versions this is for bench top use, other versions are portable. The operator maintains a stack of material to be filmed in a tray, the camera automatically takes one document after another for advancement through the machine. The camera lens sees the documents as they pass a slot. Film behind the lens advances exactly with the image.
Special purpose flow cameras film both sides of documents, putting both images side by side on 16 mm film. These cameras are used to record cheques and betting slips.Microfiche camera All microfiche cameras are planetary with a step and repeat mechanism to advance the film after each exposure. The simpler versions use a dark slide loaded by the operator in a dark room; after exposure the film is individually processed, which may be by hand or using a dental X-ray processor. Cameras for high output are loaded with a roll of 105 mm film. The exposed film is developed as a roll; this is sometimes cut to individual fiche after processing or kept in roll form for duplication.Computer Output Microfilm
Equipment is available that accepts a data stream from a mainframe computer. This exposes film to produce images as if the stream had been sent to a line printer and the listing had been microfilmed. Because of the source one run may represent many thousands of pages.
Within the equipment character images are made by a light source, this is the negative of text on paper. COM is sometimes processed normally. Other applications require that image appears as a conventional negative; the film is then reversal processed. This outputs either 16 mm film or fiche pages on a 105 mm roll.
Because listing characters are a simple design, a reduction ratio of 50 gives good quality and puts about 300 pages on a microfiche. A microfilm plotter, sometimes called an aperture card plotter, accepts a stream that might be sent to a computer pen plotter. It produces corresponding frames of microfilm. These produce microfilm as 35 or 16 mm film or aperture cards.
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.
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.
Transparent jackets are made A5 size each with 6 pockets into which strips of 16 mm film may be inserted, so creating microfiche. Equipment allows an operator to insert strips from a roll of film. This is particularly useful as frames may be added to a fiche at any time. The pockets are made using a thin film so that duplicates may be made from the assembled fiche.
Another type of conversion is microform to digital. This is done using an optical scanner that projects the film onto a CCD array and captures it in a raw digital format. Since the different types of microform are dissimilar in shape and size, the scanners are usually able to handle only one type. However, there are some that have the possibility of swapping modules for the different microform types. Software (normally on the scanner itself) is then used to convert the raw capture into a standard image format for archival.
The physical condition of microfilm greatly impacts the quality of the digitized copy. Microfilm with a cellulose acetate base (popular through the 1970s) is frequently subject to vinegar syndrome, redox blemishes, and tears, and even preservation standard silver halide film on a polyester base can be subject to silvering and degradation of the emulsion -- all issues which affect the quality of the scanned image.
Digitizing microfilm can be inexpensive when automated scanners are employed. The Utah Digital Newspapers Program has found that, with automated equipment, scanning can be performed at $0.15 per page.
For the resulting files to be useful, they must be organized in some way. This can be accomplished in a variety of different ways, dependent on the source media and the desired usage. In this regard, aperture cards with Hollerith information are probably the easiest since image data can be extracted from the card itself if the scanner supports it. Some types of microfilm will contain a counter next to the images, these can be referenced to an already existing database. Other microfilm reels will have a 'blip' system: small marks next to the images of varying lengths used to indicate document hierarchy (longest: root, long: branch, short: leaf). If the scanner is able to capture and process these then the image files can be arranged in the same manner. Optical character recognition is also frequently employed to provide automated full-text searchable files. Common issues that affect the accuracy of OCR applied to scanned images of microfilm include unusual fonts, faded printing, shaded backgrounds, fragmented letters, skewed text, curved lines and bleed through on the originals. For film types with no distinguishing marks, or when OCR is impossible (handwriting, layout issues, degraded text), the data must be entered in manually - a very time consuming process.