Microfilm

Chemicals are still required for some types of film development and duplication, but there are alternatives for those who wish to conduct operations in-house. However, the trend today is toward outsourcing of services and a large number of bureaux in the UK cater for all aspects of filming and duplication - users only handle dry, processed film. Some systems combine both methods of production, with urgent or highly confidential work being handled by a single in-house camera while bulk conversion of paper to film is entrusted to a local bureau.

To understand the technology it is necessary to know that microfilm comes in several formats. Each is designed for a specific purpose and every aspect is fully covered by standards - many of which we helped to prepare when G G Baker Chaired the British and International committees responsible for micrographic standards. The microformats in current use are:-

Roll Film

Roll microfilm is either 16mm or 35mm wide (105mm roll is also used during the production of microfiche, but it is cut into individual A6 size sheets before use).

Unlike cine film, roll microfilm is not perforated and only a small margin at the edges is required to enable it to be held in a camera and subsequently in a microfilm reader or scanner. Almost the entire width is therefore available for recording images which are usually arranged in a single row along the length of the film although two alternative layouts are permitted. Both of the alternative options result in a double row of images. In duplex format the fronts and backs of all documents are recorded side by side across the film width. In duo format the documents run sequentially along the top row and then back down the lower row; this is achieved by filming one track, reversing the film in the camera and then recording the other track.

16mm roll film is ideal for recording most office documentation but a larger image is desirable for recording fine detail of large originals such as maps, plans, newspapers and engineering drawings. 35mm film is generally used for such applications either in roll form or as individual images set into aperture cards which are described later.

Circlips are often used to permit the easy identification, handling and loading of roll film on

reels.They provide dust protection, a flat surface for labelling and facilitate rapid threading of film into a reader or reader-printer. They are almost universally employed with automated image retrieval systems based on roll formats and the design of the circlip has been standardised to permit films to be exchanged between equipment from different sources.

Image retrieval from 16mm roll film frequently relies on counting small marks filmed below the images. Very rapid search times are possible within any specific roll, but the rolls usually have to be loaded or interchanged manually and this increases the total time required to locate a set of images.

Microfiche are sheets of film, They are usually A6 size and contain images set in a grid pattern in regular rows and columns. An eye-legible title along the top of the microfiche permits instant identification without the need for magnification. Standard microfiche are produced to strictly controlled layouts which permit the retrieval of any image via a co-ordinate representing the intersection of the row and column where it is located. At least one index frame is usually included on the microfiche to provide access to its content without the need for an external index. Fiche conforming to ISO and BS Standards thus contain sets of images and integral indexing, enabling them to be duplicated and distributed as required. The images can be recorded from paper originals using special step-and-repeat cameras or they can be produced from digital input via the COM (Computer Output on Microfilm) process.

Somewhat similar in appearance to the microfiche, but differing in its area of application, the microfilm jacket comprises rows of images cut from roll film and held in a carrier which is usually A6 in size. Each jacket consists of two sheets of thin clear plastic cemented or welded together at intervals to produce channels into which strips, or even single frames, of 16 or 35mm roll film can be inserted. Because additional frames can be added at any time the system was particularly suitable for files such as customer records - a separate jacket being allocated to each customer. Combined jackets, holding both 16mm and 35mm widths are also available.

Aperture cards have already been mentioned as a popular microform for handling large originals such as maps and plans. The concept of a single film image set into a “window” cut into a card carrier dates from around 1934, but it came into practical use during the Second World War and was widely adopted for engineering drawing applications after the format was subsequently standardised by the US Department of Defence. The dimensions of current aperture cards are 187.25 X 82.5mm because during the 1970’s Hollerith punched cards of that size were commonly used for data input to computer systems. The mechanical sorters used for computer applications were adapted for sorting the aperture cards. It is still possible to punch the body of the card to record data such as the drawing number and original drawing size and to sort the cards mechanically on special card sorters. Some aperture card printers can interpret data punched or marked on each card and use it to determine the number and size of prints required. Similarly, automated card duplicators can read data on master cards and use it to produce the correct number of copy cards.

From time to time other micrographic formats have been proposed, often involving very high reduction ratios. None has survived long enough to become an adopted standard and potential users are well advised to employ only those products which strictly conform to the BS and ISO standards that have been established to control quality and layout. This ensures the widest possible choice of sources of supply and total compatibility with other systems anywhere in the world.

This site contains specifications for virtually all of the micrographic hardware currently being marketed in the UK. Micrographic equipment is listed under three broad headings:- Production equipment, Quality control & finishing equipment and Readers &reader-printers.

Production Equipment is divided into Cameras - planetary type, Cameras - rotary type, Duplicators, and Film Processors.

Planetary cameras

Planetary, or flat-bed cameras are used for applications where the highest possible quality is required. Because both the document and the film are stationary during exposure it is possible to achieve maximum image resolution on the film. Other advantages include the provision of infinitely variable reduction ratios by altering the distance between the camera and the document. The main disadvantage is that the process tends to be slower than rotary filming. Automatic document feeders are available for a few planetary cameras, but documents are normally positioned, filmed, and then removed by hand.

Some retrieval systems require up to three blip sizes beneath the images. Another common requirement is numbering and/or dating each image. Suitable facilities are often available for planetary cameras as optional extras.

Many planetary cameras are designed for engineering drawing applications which may involve the filming of translucent originals. These often require a sub-surface illuminator comprising a glass platen with lights below it.

Library applications involving rare and delicate books frequently demand the use of a book cradle to gently support the open book and bring facing pages into the same focal plane - these may be offered as optional extras.

Some compact planetary cameras are designed exclusively for 16mm roll, in which case there may be provision for two rolls to be exposed simultaneously, thus creating an archival copy and a working copy. Larger roll film planetary cameras are usually able to accept both 35mm and 16mm film. Several models are offered with a range of interchangeable camera heads to permit the production of 16 and 35mm roll, processed aperture cards and also master microfiche by step and repeat filming.
Automation has been applied to lighting and focus adjustment, the processing and titling of aperture cards, frame numbering, the addition of image count blips to roll film and the creation of eye-legible titles on microfiche. Microfiche camera heads must also incorporate a means of altering the image grid format and this may be either mechanical or electronic. All options can not always be combined on one camera.
Planetary cameras can sometimes be linked to a computer system to permit the film address of each image to be input to an index held on the computer. Much of the data can be generated automatically; for example the frame number will increase by one for each exposure. Other data may be manually entered from a keyboard or extracted from bar-code headings on the documents.

Aperture card cameras usually deliver fully developed, fixed and washed aperture cards. Options include image verification on a screen as the card leaves the camera, automated punching of the cards, and the printing of indexing and titling information on each card. Other facilities may include the automatic compilation of job records with details of the size and orientation of each image generated.

Maximum resolution is not usually constant throughout the full range of reductions, but most planetary cameras can record at a minimum of 125 lines per mm. Some cameras feature automatic control of image density so that varied input produces identical images. This greatly simplifies subsequent operations such as duplication and printout.

Rotary Cameras

Rotary filming is considerably faster than planetary filming because it is performed as a continuous operation, most rotary cameras have an automatic document feeder. Pages are filmed as they pass through the camera and the relative speeds of the film and documents are carefully synchronised to maintain good image resolution.

The reduction ratio is usually determined by the camera unit. Interchangeable camera units may be available to provide a range of ratios and also permit each department of an organisation to have its own dedicated camera unit. Widely available options include the ability to film both the front and back of each document, presenting the images side by side across the width of the film in "duplex" format. Alternatively, documents can be filmed along one half of the film width and then back along the other half in "duo" format.

Because the filming process is continuous, there may be no restriction on the length of the input, but its maximum width is limited by the size of the camera throat. The maximum width that can be recorded on film is a little less than the maximum sheet width.
Indexing and retrieval methods for which standard or optional facilities may be offered include one, two or three level blip coding of the images and frame numbering.

Some rotary cameras can stamp and endorse each page as it is filmed to record a unique number, the date of filming or other information for checking, indexing or retrieval. The more sophisticated units can read bar-code patches on the documents and automatically compile an index. It may also be possible for the camera to be programmed for several common job set-ups to save time on repetitive filming operations. Rotary cameras combining high-speed, high-resolution electronic scanning and simultaneous microfilming are available to produce an archival file while inputting documents to electronic image processing systems - these will be found in our Hybrid section.

Film Processing

Most silver halide cameras only expose film. Before it can be read or duplicated, it must be developed, fixed, washed and dried using a film processor. Film processing can sometimes be undertaken in daylight but a darkroom may be required for special applications. Some cameras, COM recorders and CAD plotters incorporate their own processors and deliver film ready for use, some use alternative types of film.

Conventional silver halide film processing transports the exposed film through liquid developer to convert the silver halide grains that have been exposed to light to metallic silver. This conversion is sensitive to time and temperature. Strict control of both the heat of the chemicals and the speed of the passage of the film through them is necessary for consistent results. A rinse may follow to remove any surplus developer; a stop-bath also helps to avoid staining and streaking of the film and prolongs the working life of the fixing solution. The next stage is fixing in a solution usually composed of sodium or ammonium thiosulphate, which dissolves all undeveloped silver salt. The film is then thoroughly washed to remove residual developer and dried before being wound on to reels, delivered as cut fiche or mounted in aperture cards.

Conventional processing produces a negative image with clear print on a black background. Areas of the film emulsion that have been sensitised by light turn to black silver and the parts which did not receive light (the print on a page for example) are washed away leaving virtually clear film.
COM recording, which uses a CRT or laser to expose the film, creates text, lines and patterns by using light, so conventionally processed silver halide COM film is positive with dark print on a clear background. If negative output is required, a slightly more complex processing sequence can reverse the polarity of the images. The sequence for full reversal processing is first development, first wash, bleach, clear, re-expose to light, second development, fix, wash and dry. It will be seen that a processor with six or preferably seven tanks is desirable for full reversal processing.
Water consumption and the disposal of waste water and chemicals are points to consider when contemplating in-house processing. The alternative is to send films to a local bureau or film laboratory for developing, fixing and possibly duplication.

Silver can be harvested from used processing solutions using electrolysis. Advantages include the prevention of excessive sulphite destruction as well as the intrinsic value of the material recovered. Some bureaux offer facilities for collecting waste chemicals, extracting the silver and safely disposing of the residue.

Film Duplication

Diazo, silver halide and vesicular films are all used for the production of copies from microforms. Each has advantages and limitations, but most document based applications now use diazo material for duplication. The characteristics of the three methods available are outlined below.

Diazo Duplicating

Diazo material is sensitive to ultra-violet light but can be handled in daylight. It is exposed by placing it in close contact with a master film and projecting UV light through the master on to the copy film. Areas masked by dark parts of the master are protected from light while those in contact with clear parts of the master are sensitised.

To develop the diazo film it is passed through ammonia vapour which causes diazonium salts in the coating of the copy film to couple with other chemicals and form organic dyes in the areas unexposed to UV light. Where UV light has reached the copy film the salts are rendered inert and no dye is formed. Diazo is thus a non-reversing medium and a master negative will produce a negative diazo duplicate.

Diazo duplication can be performed in daylight because small quantities of UV light do not affect the film. Unless the duplicator has an internal filtering system, or only moderate volumes of duplicates are produced, it may be necessary to arrange for exhausted ammonia fumes to be vented from the production area.

Diazo duplicators for roll formats contain a UV lamp and a transport mechanism to move the copy and master film in contact through the light beam. The films then separate, the master is re-spooled and the copy film passes into a chamber filled with ammonia gas where the latent images are developed.

Diazo duplicators for flat formats may be fully automated, but many are in two parts. In the first section the master and copy film are held in contact and exposed to UV light. The copy film is then manually removed and placed in a developing section where it is exposed to ammonia. In some cases these two operations are performed on separate machines. In this case the first of the two sections is described as an exposing unit and the second as a developing unit.

Diazo copies are less susceptible than silver halide film to damage by dirt, finger marks or scratching as the image is more integral with the film. They are not used for archival applications requiring the maximum possible storage life, but they are certainly reliable for most commercial applications and if protected from long-term exposure to UV light they are suitable for applications requiring storage for up to 50 years.

Thin base polyester film is often used for 16mm diazo copies. It permits double the length of film to be held in cartridges and cassettes than normal thickness silver halide material. Thin base film is unsuitable for fiche and jacket copies because it lacks the necessary rigidity.
Dry diazo film incorporates a developer in the film coating and only requires heat to release it and develop the image. For fiche and jacket duplication it is also possible to employ DOD paper coated with encapsulated developer which is released by heat. These materials avoid the use of liquid or aqueous ammonia but add to the cost of duplication.

Vesicular Duplicating

Vesicular film is similar to diazo film in many of its characteristics. It too employs diazo compounds, but the light sensitive content is incorporated in a thermoplastic colloid.
When exposed to UV light and then heated to about 130ºC the diazonium salt releases nitrogen gas and forms minute bubbles within the emulsion layer. These bubbles scatter light and appear as dark areas on the enlarged image. Unexposed areas are clear after development, so the film is sign-reversing in that a master negative film will produce a positive vesicular copy and vice-versa.

The advantages of vesicular duplication include the absence of liquid chemicals, daylight handling and its sign-reversing characteristics which enable COM film to be conventionally processed to produce a positive master and then reversed to negative distribution copies. Vesicular film has reasonable resistance to scratching but is inferior to diazo in these respects and if positive duplicates are produced from negative camera film they tend to show finger marks and dust. While 3rd generation duplicate copies can usually be produced from diazo films, it is difficult to produce subsequent copies from a vesicular film, although it produces good paper printout.

Vesicular film duplicators are similar to diazo units and some machines can accept both materials. The difference is that vesicular film only requires heat to develop the image after exposure to UV light, so the developing section of the duplicator does not need to contain ammonia.

Experience confirms that vesicular copies are adequate for most commercial applications although they are not normally recommended for applications requiring more than 25 year storage life.

Silver Halide Duplicating

Silver halide copies are normally sign-reversed so that a negative master produces a positive copy, but direct-duplicating silver halide film is also available which does not reverse the image polarity. Alternatively, it is possible to use reversal processing to produce negative copies from negative masters.

Silver halide duplicating film, processed in accordance with BS/ISO requirements, is fully archival with a storage life in excess of 100 years - much longer than the paper on which most of the original documents will have been printed. It has a far wider tonal range than diazo or vesicular material and is therefore more suitable when the master film contains images with illustrations requiring this characteristic. Silver duplication is widely used in libraries and archives where maximum storage life is an essential feature.

Silver film duplicators usually only expose the copy film in close contact with the master. Subsequent image development is in the same type of film processor as is used for original camera film. The duplicator must either be located in a dark-room or fitted with light-tight cassettes to hold the duplicating film as it is very sensitive to daylight both before and after exposure and prior to fixing.

Film duplicators fall into two categories. Roll to roll duplicators accept a roll of 16, 35 or 105mm master film and a roll of copy film of similar width. Sheet to sheet, or card to card duplicators produce fiche, jacket or aperture card copies.

To automate the copying process, some duplicators designed for fiche and jacket duplication contain a roll of 105mm duplicating film and cut it into separate A6 copies. Aperture card duplicators usually hold a stack of master cards and a stack of diazo copy cards, ready mounted with unexposed diazo film; a method of reproducing printed or punched data is also usually incorporated to generate exact copies of the master cards.

With hand-fed units the speed of duplication largely depends on the skill of the operator. Some units can expose several fiche or jackets together to speed up the process, development can be carried out in one section while exposure is taking place in the other.

Quality control and finishing operations require cartridge loaders, densitometers, inspection stations, film cleaners, jacket fillers and splicers.

The quality of the films being produced or duplicated must be monitored and equipment is available to assist and speed the process. If old films are to be duplicated it is sensible to put them through a film cleaner prior to duplication to prevent dust marks on the copy film.

Inspection tables for use with roll formats often incorporate cartridge loaders and space may also be provided for a film splicer. A magnifier or full microfilm reader may be incorporated to assist verification and a powered take-up spool with foot control is a common feature for roll applications.

In order to meet the conditions set out in BSI/ISO Standards for archival permanence, silver halide film must be processed under strict quality control. The aim when exposing, processing and duplicating microforms is to achieve consistent results so that each image is easy to read and good quality prints can be produced without adjusting the printer. If all images are of near identical density good results can also be obtained from image scanning with no need for test runs.

The density of silver halide images is measured with a densitometer which meters the amount of light transmitted through the material. Clear areas are never perfectly transparent and some light can always pass through the dark areas. A typical standard might call for a background density between 0.9 to 1.1 with negative film and a base fog density in the clear areas below 0.12. For most applications it is best to aim for a background density of around 1.0. To simplify testing it is normal to film test patches at the start of the film to provide large areas from which readings can easily be taken. These can be incorporated within a test target which also includes five or more resolution test charts.

Several variables can affect the density of microfilm images, these include the lighting on the camera, the temperature and strength of the developing solution and the length of time that the film is in contact with the developer. Camera lamps produce less light as they age and they should be regularly replaced; processing chemicals require replenishment to ensure consistent results.
A reliable bureau will process film to within very strict limits and this greatly assists quality control, since any variation will almost certainly be due to the filming operation. The most effective test is to expose a series of frames altering the camera lighting by one step for each frame. After the film has been processed the best frame can be identified and the setting used to produce it can be adopted for all film of the same type and make. The test must be repeated periodically to ensure that ageing of the camera lamps is not causing a variation.

If processing is conducted in-house, special pre-exposed “test wedges” are available from all film suppliers. These are simply short lengths of film which has been very precisely exposed. Any variation in the result of processing a test wedge must, therefore, be due to the processing operation as it can not be caused by exposure.

Resolution test targets permit the examination of processed film to see how well closely spaced lines are defined. Good quality test charts are essential and the standard for them is strict. They contain a series of L shaped nested patterns in which the line thickness and the space between the lines is equal, the size of the patterns reduces progressively and the resolution of the image is defined as the number of the finest pattern in which separate black and white lines can be distinguished, multiplied by the reduction ratio used when the chart was filmed. Test charts are particularly useful for checking duplicate film as they permit measurement of any quality loss in the duplication process which should not exceed one pattern.

Resolution test targets are examined with a microscope which should have a maximum magnification of about 100X. The actual magnification employed should be between 0.5 and 1.0 times the number of line pairs per mm expected on the smallest resolved pattern. The minimum acceptable resolution will depend on the reduction ration employed so that with documents filmed on fiche at 24:1 the 5.0 pattern should be resolved on the master film and the 4.5 pattern on a copy film, while drawings filmed at 15:1 require minimum resolutions of 7.1 for the master and 6.3 for a duplicate copy.

The life of silver halide film is determined by the conditions in which it is stored and the amount of thiosulphate remaining in the emulsion after it has been processed. All reputable bureaux regularly test for residual thiosulphate and many will accept films produced externally for testing. The procedure is rather too complex to be undertaken by the normal in-house microfilm unit.

Duplicate film must be examined to ensure that it is of adequate quality. Silver copy film is subjected to the same tests as camera film. The density of diazo film can be measured using suitable filters, but readings are less precise because different densitometers can produce different results. Nevertheless, for internal quality control where all tests are made on the same instrument, diazo film density can be monitored with the aid of a densitometer and a microscope can be employed to measure the resolution of duplicated test patterns.

Similarly, the characteristics of vesicular film give rise to variations between different instruments, but densitometers are available which permit satisfactory monitoring of vesicular film density for internal quality control purposes.

All film production operations should be conducted in a clean environment. If film that has been used in reading equipment requires duplication it should first be passed through a film cleaner to remove any dust it may have accumulated. Staff should wear clean cotton gloves when handling film to avoid fingerprints and particular care must be taken to ensure that processed film is not handled with gloves that are dampened by contact with chemicals.

The majority of modern microfilm applications rely on a bureau for filming, processing and production. Quality control then becomes the responsibility of the bureau, but it is sensible to insist that regular tests are conducted and to ensure that test charts and density patches are included at the start of each roll so that quality can be monitored in-house.

Modern micrographic materials have considerable tolerance so that it is not difficult to produce consistent results, but tests are desirable before any substantial quantity of unusual input is filmed, particularly if it involves coloured paper or faint carbon copies.

Those responsible for the in-house production of microforms should closely adhere to British or ISO micrographic standards, most of which are now identical.

Very few new jacket applications are reported because electronic document management systems are superior for systems requiring constant addition to individual records. There are, however, large applications still in use and this creates a small demand for replacement jacket fillers. As these machines have few moving parts this market is largely being filled by reconditioned units. Specifications for popular machines are provided under Jacket loaders.

Film splicers are needed to join separate lengths of roll film, the two popular widths being 16mm and 35mm. A splice may take the form of an adhesive patch or a weld. Adhesive splices. which may be supplied as separate tabs or in roll format, can be used on any type of film. Ultrasonic welding has the advantage of not increasing the film thickness, It is excellent on polyester film but it is not suitable for some acetate film bases.

Microfilm Readers optically enlarge the microfilm image for viewing only. Input may be limited to one microformat but many models can accept multiple formats by using adaptors to hold the film. Motorised film transport facilities normally relate only to roll formats.

As a general rule, readers with square or portrait shaped screens are suitable for camera-filmed documents - if they are fitted with an image rotation facility they can display images in comic or cine-mode. Those with wide screens are used for COM film and (for large screen models) the simultaneous display of two portrait pages.

Microfilm Reader-Printers - Optical Type

Optical reader-printers enlarge the microfilm image on to a viewing screen and can produce a print directly without being connected to a computer system. They are only suitable for small to medium volumes of printout. Factors to consider include the choice of lenses, film carrier options and image retrieval methods available. Features typically include the ability to rotate the image, alter image polarity, mask any part of the image, correct skewed images and superimpose repetitive data on prints.

Those seeking printout from microforms should also examine the data sheets for Film scanners on this site.

Introduction