European Commission on Preservation and Access, Amsterdam October 1997
2. Requirements for film quality and film organization with reference to the option of film digitization
In both their preparation and execution, filming projects should take into account the possibility of later digitization. In terms of materials and technique, there are only a few requirements that go beyond the normal rules and requirements. However, there are additional points that should certainly be noted in the area of film organization.
For the filming of high-contrast material, such as text, line drawings, and engravings, the current range of pan-chromatic antihalation undercoated (ahu) microfilm on polyester base can be used. In recent years, this has also been improved with a view to optimizing digitization. On the other hand, filming with a half-tone microfilm such as Kodak 2468 or 3468 will be best for material with a greater range of gray tones (continuous tones). This would include books containing photographic reproductions or color material that is to be filmed in black and white. Half-tone microfilm produces a film of positive polarity. A corresponding improvement in the continous-tone quality is achieved by putting ahu microfilm through a special developing process.
As a rule, reproduction quality and, especially, resolution capacity of microfilm systems far exceed those of image digitization, but filming nonetheless requires attention to the correct lighting and exposure, as well as to optimal readability (optical resolution). These depend on the optical characteristics of the camera and correct adjustment of the camera system. It is important to ensure the best possible quality of the master film, taking into account the deterioration between the master and the duplicate (second generation) film, which is produced for working purposes. The guideline here is the Quality Index (qi) 8 (higher quality) in Annex C of the international standard iso 6199. Oriented to the height of the small “e” in printed material (corresponding in manuscripts to double the width of letters such as e, l, g, and f), we arrive at the formula qi = a ? h, where a is the resolution number of the iso resolution test pattern No 2 in line pairs per millimeter (lp/mm) and h the height of the small letter “e”. Microfilm systems that give a value of 120 lp/mm and higher in the middle and at the edge of the image generally meet this standard.
Regarding the kind of microform, 35mm roll film for the master and as the starting point for digitization is best. Its image size guarantees sufficient quality, even with problematic material, up to a size of 60 ? 80 cm. The normal commercial film lengths of 65m or 30.5m are recommended; the longer film is easier to handle because of the shorter preparation time. Far more successful results are obtained from digitization of negative than of positive film. A duplicate film of the lowest possible generation should be used for digitization. As the preservation master exists for preservation purposes and cannot be used directly, a silver halogen duplicate, produced from the preservation master with negative polarity with the help of a same-polarity duplicating film (ddp=Direct Duplicating Print Film), should be digitized. In principle, however, it is also possible to digitize a diazo copy. Filming with the use of blips is always necessary for an efficient working method with microfilm scanners.
It is also possible to digitize microfiche. However, the smaller image field results in a lower reproduction quality in large-scale work. Digitization of microfiche requires many more staff resources and makes more demands on the software, which increases the time and thus the costs compared with work on roll film, which can be made largely automatic. Conversely, when selected extracts, as opposed to a single continuous run, are required, it may be more economical to use microfiche.
A flaw-free film is a prerequisite for the best possible results in digitization. Density of the film, resolution, and background shadow should at least meet the iso standard. Distortions should be avoided, as they can no more be corrected in digitization than can lack of clarity or other shortcomings in the master. Shadows in the book fold should also be avoided, as they can be corrected only to a limited extent, and with additional resources.
Every adjustment of the microfilm scanner requires additional resources and incurs greater costs. Therefore, the material to be filmed should be presented as uniformly as possible. These guidelines deal with the following:
a. Reduction ratio. Ideally, one reduction ratio should be selected for a complete filming project. If this is not possible, no more than one reduction ratio should be used for any single film. If need be, the material to be filmed should be arranged by size. In digitization, the image is scaled up to the size of the original. In most graphic formats, the image header can include details of the selected resolution and of the total number of pixels. If necessary, these can be used in the viewer software for reconstruction and indication of the original size.
b. Positioning of the material. The material should be placed on the filming table in a uniform way. This positioning must not be altered within a film. The optimal positioning is to place the material in the middle of the front edge of the filming table. If this is not possible, the material should be placed in the middle of the table, with pencil or other markings to indicate the correct position.
c. Alignment of the material. The alignment of the material should correspond to the desired appearance on the screen, and should thus be readable, i.e., horizontal. Otherwise, the digitizing service will have to rotate the material, which will add to the cost. Generally, books and documents should be filmed in half steps, in accordance with the image mode 2A of iso 6199. Larger volumes and newspapers should be filmed in full steps, in accordance with the image mode 2B. Changes of the image position and of the film steps within one film should be avoided.
d. Contrast between background and filmed material. The contrast between the background and the material to be filmed should be increased by making the background uniform and dark.
Attention to the above points (b), (c), and (d) is a prerequisite for a largely automatic, and thereby economical, detachment of the background material from the whole digitized image. Elimination of the peripheral zones not only contributes to the optical image, but also reduces the amount of data to be stored.
As with preservation filming, every film should start with an introductory sequence. This should clearly identify the film, including its unique number, relevant information about ownership, content, filming technique (reduction ratio and scale), and a test frame with information about readability and continuous tone reproduction according to din or iso. It may also be appropriate to discuss with the digitizing firm the question of identifying the film in a way that is machine-controllable and facilitates the delivery of individual films or parts of films.
In ordinary film projects, certain elements of the organization of filming are often”wrongly”ignored. They are more important in the case of film digitization. They include take-counters, subdivision of films by indication sheets, placement of blips, and documentation on the filming procedure. Structuring a film by legible indications on the filmed material, running take numbers, blips, and appropriate indications on a take frame (see Figure 1), together with a consistent documentation of this structuring, makes indexing for retrieval and further processing much easier and reduces costs. Besides the single blip, which in conjunction with a take-counter usually suffices to identify individual frames, it is also possible to use group or sequence blips. This is particularly important in relation to data organization, i.e., accuracy of access and avoidance of superfluous page turning on the screen. Its high value justifies the extra effort required during the stages of preparation and filming.
The resources committed to structuring the information depend on the nature of the filmed material and how it will be used. It makes no sense to put a 300-page book on the screen with no markers to facilitate retrieval. In all cases, material to be filmed requires more extensive indexing that can also improve access to the original microfilm. The time required for this, and, therefore, the implications for personnel and costs, must not be underestimated.
Reproduction quality depends essentially on the installation of suitable filming equipment. The requirements that have been described are met by most modern planetary cameras, which guarantee resolution of at least 120 lp/mm over the entire screen. The equipment should also include the following: automatic focusing, lighting that adjusts automatically to the material being filmed, a camera head that can turn, adjustable lamps (for lighting book folds), image field projection, adjustable image masking, automatic lighting of blips, and take counters. For filming books and archival material, the camera should produce optimal results with reduction ratios of between 8 and 24. For conservation reasons, the planetary camera should also have a device for protecting bindings and book covers, such as a two-part book-cradle with a sufficiently open glass plate with adjustable pressure. It should be possible to film heavier and oversize volumes without damaging them.
Since second-generation films (duplicating masters) are normally used in digitization, the film should be silver-halogenide duplicate film of the same polarity (ddp film). Duplicating should be undertaken with high-quality duplicating equipment (working under vacuum on parallel-running films) to minimize the loss of resolution.
In general, for filming with a view to subsequent digitization, the choice of system and the procedures usually will be dictated by the same criteria as apply in the case of good-quality microfilming. However, more attention must be paid to making the film form as unified as possible, and to the organization of the filming, the structure of the film, and its documentation.
It is possible to digitize existing films and film copies. In such cases, it is essential to work with films of the lowest possible generation. It is advisable in every case to thoroughly analyze the films in terms of material, state of preservation, reduction factor, reproduction quality, filming technique, nature of the material, and organization of the filming. This analysis is best undertaken in cooperation with an experienced service provider. Before a contract is awarded, digitization tests should be carried out with standard test material. It is only on such a basis that a firm can arrive at a realistic price, which will include the possibility of improvement through treatment of individual parts of the film and image enhancement. The intended use, in the context of cost, will determine agreement on the quality standard required. Any damage to the film, such as scratches, dirt, or fraying, will also influence the quality of digitization.
The starting point for digitization of color film should be a high-resolution, permanent-color bleach-fixing-process microfilm on a polyester film base. This should yield a high-resolution, reproduction quality microfilm that matches the quality of black-and-white microfilm.
In the past, duplicates of color microfilm have not proved entirely satisfactory. Exceptionally, therefore, and applying all the measures of film conservation, preservation masters are digitized. It is, for that reason, an advantage to be able, as is possible with some cameras, to produce two preservation masters in the same working run.
Because of its cost, the practice until now, almost without exception, has been to digitize color film using a proprietary system developed for the amateur market. The cheapest version limits the area of the image that can be digitized to 24 ? 36 mm. However, a “full-step” color microfilm image occupies 32 ? 45 mm. Film made using the maximum size of the full-step, which is best for reproduction quality and for further processing when dealing with larger or more difficult material, is not possible with a normal photographic cd. With half-step filming and smaller image areas, it is necessary to establish in advance whether the picture format can be carried by the system, as reels can only be wound in one direction. Transfer of uncut microfilm is certainly possible. However, as spool devices are not part of the film scanner, the film can be damaged. Under this system, the film material is digitized with differing resolutions and transferred in compressed form onto photographic cd. The lowest resolution of the five resolution steps is 128 lines ? 192 pixels, the highest 2048 lines ? 3072 pixels.
The photographic cd system was developed above all for the large amateur photographic market and is therefore widely and cheaply accessible for the digitization of color film. It has only limited use, however, in producing color microfilm, particularly with regard to format. Failure to use the full-step format of unperforated 35 mm microfilm normally leads to loss of quality, especially in the case of color microfilm. But loss of quality does not have to be accepted. Color film scanners have been introduced in the reprographic field that can work with film up to a format of 6 ? 9 cm and with filmstrips. They are able to digitize full-step 35 mm color microfilm and have a resolution of up to 2000 dots per inch (dpi). The output format is not limited to a photographic cd; it may be produced in any of several other formats. However, the current state of technology and the comparatively low demand for digitization of color microfilm make this a fairly expensive process. Still, in view of developments we can expect in the future, it would be a mistake to sacrifice reproduction quality and standard compatibility or tested systems of working for the sake of a currently useable, producer-independent system, even if this were economically advantageous.