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

Additional scanning tools

A ruler, a grey scale and a colour checker are often described as valuable instruments when scanning slides. Nevertheless, the use of these control targets is not considered an absolute obligation by the authors of the study Benchmarking Art Images Interchange Cycles. Final Report 2011: tests among American museums have shown that control tools can help to improve the quality of the set-up, detect changes (e.g. in colour, brightness) when files are used by different people, facilitate trouble-shooting if problems occur in a workflow, but an incorrect use of the test instruments and metamerism can also produce misleading results (Frey 2011, p. 98, 102, 105, 113, 120). Some tests have shown that it is more important to use colour management (e.g. ICC colour management) among all participants and to circulate information between all those involved in the workflow: “When a target is included, its purpose and proper use must be clearly communicated to all who may subsequently use the file.” (Frey 2011, p. 105) Everybody concerned should be trained how to handle the control instruments and be informed why they are used. In an earlier reflection on this topic, scanning expert Franziska Frey (2000) was more sceptical as to the use of targets as they increase the file size, but the decreasing cost of storage space has made this a lesser concern.


  1. The grey scale

When scanning, some archives add a so-called “grey scale” showing a “series of grey tones extending in regular steps of increasing depth of tone from white (or clear) to black (or opaque)” (Focus 1969, p. 722). According to John Hedgecoe (2004, p. 229) each grey is 50% lighter or darker than the one it follows. While a grey scale in a scanner has 256 different light values (8-bit) of which most are nuances of grey, those for printing have considerably less.


A grey tone can also be useful to check whether all monitors of the workflow have the same gamma-value. Gamma (from the Greek letter γ) is a sensitometric quantity. It signals whether the contrast in a picture is low (many grey tones, thus a soft progression in density) or high (only few steps between black and white, thus a hard, steep increase between the monitor’s blackest black (black point) and whitest white (white point)). The Benchmarking Art Images Interchange Cycles. Final Report 2011 shows the consequences of differently set monitors: in its test, persons were faced with grey scales in “just-noticeable-differences within each set of black, gray and whitish colors”. The result was: “About half of the observers were unable to distinguish all seven shades in the JND-grayscale, indicating a too-low or too-high contrast resulting from an improper setting of gamma on the display.” (Frey 2011, p. 80) The report states that the difference in identified shades indicated a difference “in the gamma configuration (and thus the contrast) of the display”.


  1. The colour checker

A colour checker serves, as the name says, to check the colour quality of the reproduction. It contains primary (RGB), secondary (CMYK) and tertiary printing colours (three colours mixed) to assure the printer company of the congruence with the original colours. As there is no specific checker for slides, it could be interesting to use test targets especially produced for paintings. An overview of targets used by the graphic arts industries can be found in Frey (2011, p. 129-131).


Hints by a scanner operator:Scanner expert Joseph Stark (2015, p. 255) describes his experience with so-called “IT8-targets” serving to calibrate old, partly faded diapositive and negative film material: for (almost) each historical film stock by Kodak, Fuji, Agfa, Ilford, Orwo etc. a reference image with correct original colours (“target”) was established, taking into account their specific colour spectrum and the sensibility of each type and each produced unit of emulsion. The targets served as reference for colour calibration when setting the parameters during the pre-scan and in post production. It has to be added that Stark is not convinced of their usefulness.


  1. Additional software and pre-programming

Scanners need additional software to program them, sometimes to improve their potential and to compensate for “flaws” (e.g. colour temperature of the light source), mostly to ensure good results (calibration) and to work with the scans later (post production). The manual of the acquired scanner generally indicates the most suitable parameters for the equipment and the material used. Special magazines or websites should be consulted to know what is currently on the market and which potential it has. Also charts to test the real “effective resolution” of the scanner are useful.


Scanners allow users to “interfere” with the scanning operation by e.g. controlling the colour and light values with histograms, choosing the colour depth and whether light should be registered gradual in equal steps (linear) or with more nuances in the darker parts of the image (non-linear) as this suits the human eye more, etc. If the archive has experienced staff it should experiment with the scanner’s potential instead of accepting the “default position” pre-installed by the companies. Flatbed scanners allow to tune the brightness of the image and vary its contrast as well as its colour saturation. This could be of interest for “small scale scanning” when there is enough time for each slide to be tested how to get the best out of it.


Pre-programming can be necessary in “mass reproduction”. As series of slides can be harmonious in materiality (colour, contrast, brightness etc.), it helps to keep the same setting for all of them. Of course, for each slide colour and light could be adjusted (called “grading”), but this is time-consuming and produces variations with untrained operators. (To pay attention to calibration is required to grant homogeneous high quality throughout the process. On calibration see also “Calibration of displays” in the technical section “Technical processes?”.)


It is necessary to control the capturing process as much as the manufacturers of hardware and software allow. Sometimes companies seem to consider that the original setting of a device are business properties and not all should be divulged, as the Swiss DIASTOR team found out when they analysed the reproduction potential of movie scanners: certain companies don’t inform sufficiently about what is pre-programmed in the software. Sometimes the researchers didn’t receive information at all from the producers, sometimes the answers were not satisfying. In an article the group speaks of “black box operations” (Flueckiger et.al. 2016, p. 109) when they couldn’t identify factors that influenced the scanning result. This is also the case for manufacturers of image scanners and still-cameras.


When using software to enhance certain aspects of the original and thus manipulate the reproduction already during the scanning process (by “pre-scanning” the slide with a “scan pilot” and manually “correcting” brightness, colour balance, sharpness etc. beforehand), the archive should take into consideration to follow deontological guidelines (e.g. by the International Council of Museums (ICOM)) to avoid an “anything goes”-attitude. And it should be aware that such automatic interventions change parameters which can have consequences later on.