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

What influencing factors arise when we scan a slide?

The DIASTOR research group (Flueckiger 2016, p. 109) states: “Every scan is a reading under certain conditions”. This comment is based on a three-year analysis of film scanners (2013-2015), but it is also true for the work with image scanners. What can be achieved by scanning is not identical with the original data, it is a (virtual) simulation of the slide’s exterior, based on the “orders” of some binary numbers, which can be re-materialised e.g. with the help of a printer. It is not a faithful reproduction of the object’s material aspects. A scan is just a mosaic composed of regularly taken samples, highly influenced by external conditions.


The Swiss research team has examined a number of parameters. In its article the group states more generally that “economic, technological, institutional, cultural, and aesthetic filters […] effect the results from digitization” (Flueckiger 2016, p. 119). It itemises on a more practical level the factors that influence the process: “Scanning as a technology and practice is connected to the context of the procedure, the physical and mechanical constraints of the scanner itself, the parameters set by an operator based on her [and his] experience, the interface of the scanner, and, last but not least, the quantification that assigns binary numbers to analogue electrical signals.” (Flueckiger 2016, p. 109) In the following, some of the “filters” indicated by Flueckiger will be described to draw attention to some “sensitive points”. The list is not supposed to be exhaustive.


  1. Context of the procedure:

The conditions of the working area where the scanning is done, and the interference of factors from inside and outside the working space can disturb the activity and influence the result.


  1. Physical, mechanical and digital constraints of the scanner:

The performance potential of the scanner itself is important, determined by

  1. the quality of the hardware, e.g. the resolution of the sensor and its pixel structure, the method and velocity of the scanner to read the total surface of the object, the kind of inner light-source and lens used, etc.;
  2. the potential of the software to compress (if wanted) the received binary information;
  3. the pre-programmed defaults: hardware programmed to work automatically with tools such as red-light reading (to detect and eliminate “flaws”), software manipulating the scan automatically (thus uncontrolled) before sending it to the working station;
  4. the colour profile: each device has its particular gamut and needs a “translator” to communicate data more or less adequately to the other apparatus in the workflow.


  1. Parameters set by an operator:
  2. the personal predisposition: the operator’s experience to handle material with deficiencies (e.g. “faded” looking slides, under-/over-exposed images), his/her eye’s sensibility for colours and differences in brightness when correcting colour and balancing light, his/her training by (post production) companies / archive;
  3. the tradition set by certain restoration “schools” (opinions that determine the general professional discussion in the field) and its adoption by the scanner operator;
  4. the training: the way the operator respects recommendations and applies what s/he has learned in teaching courses and practical workshops.


  1. Control monitor of the scanner:

The “control monitor” of a scanner can be integrated (display of the diapositive scanner), or separated as an interface (computer monitor).

  1. the interface‘s capacity to show (in a pre-scan) the slide correctly in its colours, whites and blacks, thus the monitor’s potential to simulate the same qualities which the human eye sees when it looks at the original object;
  2. tools such as histograms to control the effected scan on lightness and colour values and sharpness (to prevent blurring or clipping and allow an immediate retake);
  3. the quality of the software for colour management to ascertain a correct transfer of data from one device of the workflow to the others.


  1. Dependency on hardware with limitations and black box operations:
  2. the quantification: the A/D conversion is programmed by the manufacturer with parameters normally veiled to the user;
  3. the sensor resolution: the ratio between (the theoretically possible) “nominal resolution” and the “effective resolution” (which the scan really offers);
  4. the colour gamut: the part of the RGB colour space which the scanner can actually capture and reproduce.


We would add these other aspects:


  1. Economical and institutional restrictions: limitations inherent to the archive such as budget, staff, space, in historical knowledge, technical skills, aesthetic sensibility, (high or low) interest of the institution’s management in the scanning of the slide collection etc.


  1. Curatorial and ethical concepts, cultural and aesthetical considerations: clear ideas about how to deal with the objects to be scanned, what to achieve by their digital reproduction, to which purpose the scan will serve, the establishing of internal guideline and the following of it etc.


Scanner specialist Helmut Kraus (1989, p. 158) should have the last word. He insists that the scanning result depends on these three factors: “the quality of the document, the quality of the scanner (optic, mechanic, hard- and software) and not the least the accurateness of the scanner-operator. […] lack of care often causes subsequent rectifications […] retouching generally asks for much time and patience.”