The digital reproduction of a slide is limited by constraints such as the scanner’s efficiency in dealing with the material’s specific features. These can be e.g. the particular aesthetics of a (hand) painted / printed / photographed / “copied” slide, the quality of the dyes / printing inks / gelatine image, their translucency / opacity (density) and their saturation grades, the physical state of the slide etc. which can be described as “material-scanner interaction” (Flueckiger et.al. 2016, p. 110).

Also, the scanner’s technical parameters restrict an accurate depiction of the object: scanners have, as already stated, a gamut, a specific colour space which is adapted to today’s needs and which is selected by the producer of the scanner. The DIASTOR team checked this point with modern scanners for historic colour film prints, among them some from the early period (before 1914). As the magic lantern is considered a so-called predecessor of early cinema, the colourists that hand-painted early film copies often came out of the lantern tradition: “many of the techniques used for [film] colouring were initiated by lantern slide makers” (Read 2009, p. 11). Therefore, image scanners, but also cameras have the same problem: their selected colour gamut is not identical with the dye variety used in the 17^th and later centuries. As the antique object’s colour range is different, the data may not be reproduced adequately with a scanner (or a camera) made sensitive to modern colours. All that is non-standard, risks to fall out of the device’s gamut and will be falsely reproduced as it is “outside the scanner’s spectral sensibility” (Flueckiger et.al. 2016, p. 115). What the English film specialist Paul Read (2009, p. 27) says about early films, which were tinted and toned with dyes commonly used in the 19^th century, is also valid for painted slides: “The results are rarely satisfactory, as the best that can be achieved is a near colour match to the original. […] many dyes are highly saturated and some are outside range of the recording system.” He is not satisfied with what colour and light correction in post production delivered. DIASTOR has mixed feelings: it is enthusiastic that “the post production environment offers such vast possibilities that it is more or less possible to approximate the results in colour grading”, but sees that this is not the perfect solution: “For a film archive, however, it is not the ultimate visual representation that is important. The initial digital representation needs to be taken into account as well […].” (Flueckiger et.al. 2016, p. 115)

A last remark on scanners’ capacities to be faithful to the original. Colour expert Charles Poynton (1997, p. 7) states: “Scanners are most often used to scan images such as color photographs and color offset prints that are already “records” of three components of color information. The usual task of a scanner is not spectral analysis but extraction of the values of the three components that have already been recorded.” Printers work with a specific colour profile where each nuance is defined by its coordinates.  If Poynton’s statement on scanners is correct, this device is strong on reproducing values that match the generally used colour profiles; if a slide’s colours “[…] are not already a record of three components, chances are your scanner will not report very accurate RGB values.” This confirms the observation that scanners have difficulties with colours “out of gamut”. On the other hand, as the graphics on a toy lantern slide were printed before their transfer onto the glass plate (the process was called decalcomania, the printing technique is chromo-lithography)  (http://www.magiclanternsociety.org/about-magic-lanterns/lantern-slides/) their range of colours is limited and presumably there is little chance that colours are not covered by the colour profile of the scanner.

Due to technical restrictions, it is an illusion to believe in a digital simulation of the slide that has stored all data exactly as on the object scanned. Nevertheless, this should not be an excuse to leave aside all efforts to get the best that can be achieved. One possible way to counterbalance this effect, proposed for film scanning by the DIASTOR team (Flueckiger et.al. 2016, p. 111), could be to combine retrieved historic information on slides and to ask questions such as: what were the components of dyes as well as of glass around 1830 and 1890 (both had specific optical qualities in various periods) and how were they applied? Which company bought dyes from which manufacturer (or dealer), and where did these companies acquire their source material? Which products were sold on the (inter-)national market that could have served for the making of slide series, and who had access to them? etc. Another method could be to measure slides and note with their characteristic wavelengths with the help of X-Ray Fluorescence (like in art restoration). This is already partly done for early film material by Ulrich Ruedel (2017) in Germany. For both approaches it is necessary to retrieve original handbooks and procedure descriptions from the past that indicate methods and tools.

Following these parameters can help to evaluate whether a scanned image is close (correct) or distant (erroneous) to the known practices of an (inter-)nationally selling company or a locally operating colourer. Analyses of 19^th– and 20^th-century organic and synthetic dyes (and the way they decay), which were also used to colourise photographic slides, have been done by film restorers such as Paul Read in the United Kingdom and Luciano Berriatúa (1998) in Spain. The latter established a “library” of standard dyes with their characteristic wavelengths. Research on specific light refraction / absorbency of 18^th– to 20^th-century glass plates would be necessary as well. The collaboration with specialists of painted glass windows and glass sheet photography could deliver the desired key measures.

All efforts to reproduce the historical-particularity of slides can be ruined by the dilemma of economical necessities.  The industry proposes for “small budgets” lossy compression with JPEG and other encoding formula. The human eye is taken as parameter to excuse the elimination of “redundancies”. Nevertheless, the original object contained all the “unnecessary” details, and the scanner captured them, they are part of the object’s materiality. This evokes ethical questions: keeping all possible information from the original, and thus renounce to encode the data for storage, but sooner or later to experience problems such a full digital storage space, costs for more hardware and tapes, raising prices for electricity etc. But the ability to migrate all the stored information once newer generations of software will make it necessary. Or be satisfied with what the pioneers of colour photography in the 1860s did who kept “all that was needed […] to stimulate the eye in a similar manner” to the original (Read 2009, p. 11) and compress the scan but be faced later – once the slide is no longer in good shape or may even be gone – with artefacts due to migration of manipulated data, no longer “supported” algorithms which makes files unreadable, with decreasing chances to transcode files to future file formats etc.

The “human factor”, as the DIASTOR group calls, it is another influential aspect. The skills of the operator, her/his historical knowledge about slides, understanding of the scanner’s technical parameters and skill in programming the device are crucial for the result. If two or more operators are using the scanner it is important that all follow the rules that the archive has committed to. Whether this be settings according to the material’s characteristics or targets to be reached by digital reproduction, only respect for settlements will prevent heterogeneous quality and reduced usability of the scans. Therefore, it is important to train the staff regularly with regard to scanning procedures to keep the quality continuously at a high level.

One last point: only the external material aspects of a slide can be reproduced (to a certain extent) by a scanning device. If one wants to represent how a slide appears on the screen, the use a still-camera is necessary. As to the effects of a panorama or moving slides, a video camera with settings for trick photography is obligatory. Both approaches are filmic reproductions, not scans, which have their own limitations.

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