How to digitise slides. Recommendations and working lists for the reproduction of a very special artefact

Recommendations and general remarks by the working group of “A Million Pictures”, the persons responsible for “Lucerna” and the Trier “eLaterna Archive”, as well as practitioners who scan and photograph slides

In the sections on scanning and photographing slides we tried to be as impartial as possible when describing the conditions and necessities of these activities and how they influence the digitisation process, as well as its results. Nevertheless, due to our experience in handling slides and our passion for this medium, we want to express our particular concerns by indicating what we consider necessary for a faithful data transfer from analogue to digital. Most remarks concern both digitisation methods. We consider the following points a draft for an outline that should guarantee professional standards in digitising slides.


1.1. Completeness

When a digitisation is done, it is important to capture the entire artefact, not just the image. The wooden frame, the mechanisms, the bindings and masks, all hold important (historic) information about the slide, and all should be made visible and permanently legible. If a slide is digitised in its entirety, no second scan has to be done later, which could be the case when just the slide’s image was taken.


1.2. “Over-Digitisation”

For the digitisation of slides, a small space around its edges should be left. This allows the user to see the frame form as well as possible damages (e.g. loose edge paper, fraying, fissures etc.), details that should be preserved to allow widest possible research.


1.3. Positioning

In case the slide is reproduced as a “flat object” and not as a three-dimensional one:

Before putting the slide on the glass pane of a scanner, it is necessary to check that the side keeping the image is turned to the light source (thus away from the scanning operator). When photographing, the image should point at the camera lens. Simple glass slides without protection cover show a slight relief which can be felt with the hand, and be revealed by holding the object aslope to the light. This relief signals the side wearing the content. Slides with two glasses (the second protecting the image on the first) are more difficult to check and therefore need more preparation time.


1.4. Backside and front

No side should be neglected, both sides are to be reproduced. If historical marks are written, stamped or impressed on the edges, they should not be skipped. While capturing, this may be time-consuming, but it avoids rescanning. And it definitely takes less time than, at a later moment, to get the slide out of and back into storage and to reprogram the scanner. Moving fragile slides around is also risky, so scanning all at once helps to preserve the slide in the long run.


1.5. Ruler and colour chart

As the future user will not see the object, s/he needs to understand its dimensions. A ruler reproduced next to the item makes it apprehensible. An added colour chart endows the user with a parameter that illustrates the physical condition of the artefact (e.g. whether the colours are pastel or intense, saturated or unsaturated, bright and brilliant or dark and sombre, the dyes look “faded” etc.)


Nevertheless, colour charts are present-day tools and do not cover necessarily the dye range of the past. “Well processed photographic silver images are quite stable because the image is buffered from the external environment by the glass and tape binding. Although the actual dyes used for an individual slide may be from almost any source, they have generally proven to be stable, especially compared with early colour photographic processes.” This statement is taken from a research report by the Australian archive NFSA in 1993. It shows that “commonly used hand-colouring dyes” stay quite stable under an accelerated aging test (Ishikawa, Weinert 2010), that they keep their original hue for a long time. This makes them highly interesting for a 1:1 reproduction. A modern colour chart can help to become aware of, and to better understand the crucial differences between dyes of the past and contemporary ones.


1.6. Homogeneity

Once a decision is taken on how to digitise slides, the parameters should be tested. If the test results are satisfying they should be kept as long as it is practical and useful. This will guarantee that the growing digital collection keeps a certain homogeneity, which makes its future handling easier and the assessment of its potential more reliable. Image specialist Donald D’Amato (2000) suggests to control the image quality with test patterns during the whole digitising project: “If test patterns are periodically interspersed and substandard image quality is detected, all images generated after the last above-standard test pattern must be considered substandard. Thus, the frequency with which test patterns are interspersed must be balanced against the cost of rescanning image batches that might have substandard quality.”


1.7. No automatic software for the adjustment of the scan

We think that the software of the scanner and the still-camera should not interfere while the scan is made, thus no sharpening the image, adjusting its colour balance, removing (“repairing”) fine scratches in the original slide, cropping the edges of a frame or modifying otherwise the captured information. This can produce artefacts and will reduce the potential to treat and rework the scan in post production. The “raw scan” should contain all the information that was captured, without any alterations.


1.8. Quality control

Every digitising process risks digital (artefacts) and human errors (for instance incorrect colour balance), therefore the control of the results is obligatory, whether the files are produced inside the institution or by a professional service. Parameters defining “quality” have to be determined beforehand; they have to be respected during the whole workflow until the project is completed. The controlling tools, the checking rhythm, the precise control moment during the operation etc. has also to be agreed on as well as to deal with the consequences if the results are not satisfactory.


1.9 Documentation

Especially in smaller institutions, time is generally scarce. However, documenting the work can be extremely helpful in many ways, as museum expert Linda Serenson Colet (2000) stresses: “One should also document the procedure for handling materials (i.e., the procedure for when the work enters and leaves the digital studio during which it is captured, and the digital file prepared for access and archival purposes). Analyzing this workflow will help identify bottlenecks. The workflow will continually be revised as the team improves or changes methods in the digital studio. It is also important to document what digital and computer equipment is being employed and the particular settings used. This will be helpful when one has to identify problems with the equipment or go through the process of migrating digital files. Knowing how the digital images were created will provide good information when change is required.”  (