Microfilm
From Encyclopedia Jr, free information reference for Kids
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[edit] Definition
Microfilm is the technology of small images of documents.
[edit] Uses
Microfilm use is in decline. It enables :
- carriage of a lightweight image
- storage with negligible physical decay, in little space and with easy access
- security against accidental loss
- recording of images in time
- low cost distribution of any number of copies of any set of images
This article considers :
- microforms, that is the forms that microfilm may take for use
- image creation : the various cameras, computer output devices, and film processing
- format conversion : producing aperture cards and microfiche from roll films
- duplication
- use : reading and printing.
- some examples of microfilm use
[edit] Microforms
Microforms are the processed films that carry images of documents to users for transmission, storage, reading and printing. How these are produced is described later in this article.
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.
The microforms available are described now.
[edit] 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, described below.
[edit] Roll Film
Roll film may be either 16 or 35 mm wide to motion picture standard. It is normally unperforated. 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
- 800 images of broadsheet newspaper pages.
One roll of unperforated 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
- about 10000 small documents, perhaps cheques or betting slips, with both sides of the originals set side by side on the film
Roll microfilms are stored on open reels or put into cassettes.
[edit] Aperture cards
Aperture cards are Hollerith cards with a hole cut in. 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.
[edit] Microfiche
A single microfiche is called a fiche.
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
[edit] Definitions
Copy is the work to be photographed.
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.
COM is computer output microfilm.
[edit] Film for Image Capture
Normally microfilming uses high resolution panchromatic high resolution monochrome stock.
Roll film is provided 16, 35 and 105 mm wide in lengths of 30 metres ( 100 feet ) and longer, normaly unperforated. Roll film is developed, fixed and washed by continuous processors.
Sheet film is supplied 105 mm x 148 mm , that is 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.
Positive color film giving good reproduction and high resolution is also provided. This is not considered in detail here.
[edit] Flat Film Camera
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 a exposure advances the film into a box and cuts the frame off the roll for processing as a single film.
This type of camera is generally operated by a photographer.
Lights are adjustable by the operator.
[edit] Planetary Roll Film Cameras
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 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.
[edit] 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.
[edit] Microfiche Cameras
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.
[edit] 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
[edit] Materials
Three sensitised films are available for microfilm duplication, described now.
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 image positive copy. The black image film can be used for further copying.
Vesicular film is sensitised with a diazo 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 is destroyed quickly which releases 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] Processes
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.
[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] Readers and Printers
Desk top 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, as an office copier. 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 give positive images on paper, many on the touch of a button accept positive or negative films.
[edit] Uses
Systems that mount microfilm images in
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