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

What is digitisation?

What is digitisation? Digitisation is the reproduction of a continuous analogue signal which a human being can see (light), hear (sound) or feel (electricity) into the two signs (“bi-nary”) of the digital “language”: zero and one.

 

The optical, auditive and electrical signals have a commonality: they are made of waves as light, sound and electricity travel through space and time in wave-form; they swing in longer or shorter curves (called “amplitude”) and sometimes move fast, sometimes slow (called “frequency”). A digitisation device takes advantage of these characteristics: it “checks” the quality of the continuously passing undulation and, in a certain rhythm, takes “samples”. These “cut-outs” are called “discrete” (= separated, on its own) as they no longer represent a continuous phenomenon in time (like a light-wave) but “cut-outs”, they depict equidistant “moments in time” of the analogue signal. During the digitising process the device “maps” the object by “segmenting” its surface into horizontal and vertical points. These have precise “coordinates” revealing the qualities of the signal at the moment when the original signal has been intercepted. The process is called “sampling”, the rhythm with which it takes the “excerpts” is called “sampling frequency”. It can refer to time (so many samples per given time unit) or geography (so many punctual checks over a given surface). In a process called “quantification” (= attributing qualitative and quantitative values to something) each of these taken analogue samples is measured (“evaluated”), and a value is assigns. This is called “sampling depth” or “bit depth” because the assignment is based on a reference scale. These values can then be transformed into long lines of binary numbers which are stored in files, each containing a little section of the original information. In principle digitisation means to turn the flow of analogue information into mathematical codes by breaking up its continuity, evaluating the resulting pieces and keeping them reproducible by attributing specific coordinates to each piece of captured information; this is necessary as contemporary viewing systems work with a raster image.

 

This is a rough explanation. What follows offers deeper insight into the technical process which is normally hidden. This is necessary to understand what happens when we scan slides (or photograph them, the used technique is identical or similar). As the digital “recording” of an artefact is never “innocent” it is important to know how a scanner or digital still camera works, what the potential of this technique is and which consequences it can have for the integrity of the created scan. In the following section the focus is on the image scanner.