Microform
From Wikipedia, the free encyclopedia
Microforms are processed films that carry images of documents to users for transmission, storage, reading and printing. Microform images are commonly about 25 times reduced from the original document size. For special purposes greater optical reductions may be used.
All microform images may be provided as positives or negatives. For use in readers and printers negative images are preferred, that is with a dark background; the low light available to be scattered gives cleaner images.
Two formats are common: microfilm (reels) and microfiche (flat sheets).
Contents |
[edit] Uses
Systems that mount microfilm images in punch cards have been widely used for archival storage of engineering information.
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.
Hollerith-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.
[edit] Advantages
The medium has numerous advantages:
- It is compact, with far smaller storage costs than paper documents. Generally, a book or a year of a periodical fits on one fiche and takes 0.05% of the space and weight of the paper work.
- It is cheaper than a paper copy. Most microfiche services get a bulk discount on reproduction rights, and have lower reproduction costs than a comparable amount of printed paper (US$5 per fiche in 2003).
- It is a stable archival form. Most library microfiche use polyester with silver halide dyes in hard gelatin, with an estimated life of 500 years in air-conditioning(citation needed). Unfortunately, in tropical climates with high humidity, fungus eats the gelatin used to bind the silver halide. Thus, diazo-based systems with lower archival lives (20 years) which have polyester or epoxy surfaces are used.
- Since it is analog (an actual image of the original data), it is easy to view. Unlike digital media, the data format is instantly comprehensible to persons literate in the language; the only additional equipment that is needed is a simple magnifying glass. This reduces the possibility of obsolescence.
[edit] Disadvantages
- The principal disadvantage of microfiche is that the image is too small to read with the naked eye. Libraries must use special readers that project full-size images on a ground-glass screen.
- A significant disadvantage is that when stored in the highest-density drawers, it is easy to misfile a fiche, which is thereafter unavailable. Some libraries therefore keep the microfiche cabinet in a restricted area, and retrieve fiches on demand. Some fiche services use lower-density drawers with labelled pockets for each card.
- Another disadvantage is that a conventional photocopier cannot reproduce the images. Libraries using microfiche often have a few viewers that can produce a photocopy of an image, for a nominal fee.
- The final disadvantage (endemic to all analog media) is that microfiche can only be reproduced a limited number of generations, while data stored on digital media does not degenerate and control software often include error detection and correction schemes.
[edit] Readers and printers
Desktop readers are boxes with a translucent screen at the front on to which is projected an image from a microform. They have suitable fittings for whatever microform is in use. They may offer a choice of magnifications. They often have powered movement of roll film. When coding blips are recorded on the film a reader is used that can read the blips to find any required image.
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.
[edit] Media
- Flat film
105 x 148 mm flat film is used for micro images of very large engineering drawings. These may carry a title photographed or written along one edge. Typical reduction is about 20, representing a drawing that is 2.00 x 2.80 metres, that is 79 x 110 inches. These films are stored as microfiche.
- Roll film
16 mm or 35 mm film to motion picture standard is used. It is normally unperforated. It is Roll microfilms are stored on open reels or put into cassettes.
The standard length for using roll film is 30.48 m (100 ft). One roll of 35 mm film may carry 600 images of large engineering drawings or 800 images of broadsheet newspaper pages. 16 mm film may carry 2400 images of letter sized images as a single stream of micro images along the film set so that lines of text are parallel to the sides of the film or 10000 small documents, perhaps cheques or betting slips, with both sides of the originals set side by side on the film.
- Aperture cards
Aperture cards are Hollerith cards into which a hole has been cut. A 35 mm microfilm frame is mounted in the hole. The film is secured by an adhesive tape which slightly reduces the size of the frame compared to roll film.
They are used for engineering drawings, for all engineering disciplines. There are libraries of these containing over 3 million cards. Aperture cards may be stored in drawers or in freestanding rotary units.
- Microfiche
A microfiche is a flat film 105 x 148 mm in size, that is ISO A5. It carries a matrix of micro images. All microfiche are read with text parallel to the long side of the fiche. Frames may landscape or portrait. Along the top of the fiche a title may be recorded for direct reading. Often this occupies a row that could have been used for images.
The most commonly used format is a portrait image of about 10 x 14 mm . These are filled with office papers or pages of magazines. Office size size papers or magazine pages require a reduction of 24 or 25. Microfiche are stored in open top envelopes which are put in drawers or boxes as file cards, or fitted into pockets in purpose made books.
[edit] Image creation
To create microform media, a planetary camera is mounted with vertical axis above a copy that is stationary during exposure. A flow camera moves copy smoothly through the camera to expose film which moves with the reduced image. Alternatively, it may be produced by computers, i.e. COM (computer output microfilm).
[edit] Film
Normally microfilming uses high resolution panchromatic high resolution monochrome stock. Positive color film giving good reproduction and high resolution can also be used. Roll film is provided 16, 35 and 105 mm wide in lengths of 30 metres (100 feet) and longer, and is usually unperforated. Roll film is developed, fixed and washed by continuous processors.
Sheet film is supplied 105 mm by 148 mm, i.e. ISO A5. 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.
[edit] Cameras
- Flat film
The simplest microfilm camera that is still in use is a rail mounted structure at the top of which is a bellows camera for 105 x 148 mm film. A frame or copy board holds the original drawing vertical. The camera has a horizontal axis which passes through the centre of the copy. The structure may be moved horizontally on rails.
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 application 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.
[edit] Duplication
All regular microfilm copying involves contact exposure under pressure. Then the film is processed to provide a permanent image. Hand copying of single fiche or aperture cards uses exposure over a light box and individually processing of the film. Roll films are contact exposed round a glass cylinder containing a lamp. 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.
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.
[edit] Format conversion
These conversions may be applied to camera output or to release copies. Single microfiche are cut from rolls of 105 mm film. A bench top device is available that enables an operator to cut exposed frames of roll film and fit these into ready made aperture cards.
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.
[edit] Digital conversion
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 only handle 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, such as tiff or jpeg. Normally, bitonal tiff's are used where file size must be kept as small as possible, and grayscale jpeg's where image quality and readability is paramount.
However, 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. For other types, the data must be entered in manually - a very time consuming process.
[edit] See also
- Historical documents
- Microfilmer
- Preservation issues
- ProQuest
