A master file is the starting point for all further processing of digital copies. It is used to generate JPEG files, for instance. The most important parameters for the production of the master file are explained in more detail in the following section:
Resolution refers to the number of pixels per unit of length. Its unit of mass is dpi (dots per inch) or ppi (pixels per inch). The higher the resolution, the more detailed the digital copy and the greater the volume of data.
- 300 dpi for greyscale and colour originals
- 400 dpi for special manuscripts, prints or maps with delicate content
- 600 dpi for bitonal scans (black-and-white originals)
- Higher resolutions: only make sense for special applications, such as studying paper structures or image digitization (3,000 to 4,000 dpi for negatives and slides)
The colour reproduction of an image can differ greatly from one image and reproduction device to the next, such as scanner, image monitors and printers. The Colour Management System (pdf, 1.28 MB) (external link) guarantees an identical colour reproduction, regardless of the imager and output device used. Calibrating the devices with a standardised colour profile can generally even out any colour differences. As a rule, the ISO-certified ICC Profile (external link) by the International Color Consortium is used. The colour profile deployed is stored together with the digital copy.
For originals where colour is a key criterion for research questions, the inclusion of an additional standardised colour scale (or colour checker) along with the original is recommended.
Practical example –colour management e-rara.ch
In the project e-rara.ch (external link) every fourth page is scanned together with a colour scale:
However, this scan is technically disabled and not visible in the digital work online.
Colour intensity refers to the amount of the colours that can be represented. It is indicated as the number of available bits per pixel. A high colour intensity facilitates fine colour shades, but requires more memory and incurs higher storage costs.
Major differences: a scan in colour requires three times as much memory as the original in 8-bit greyscales and many times the bitonal version.
The intended use of the digital copy is important for the colour intensity required:
- For manuscripts and old prints (until approx. 1750), colour digitization is recommended (German Research Foundation)
- For later prints or books with (non-colour) illustrations, a greyscale scan is usually sufficient
- For the mass digitization of simple, illustration-free prints from the 19th and 20th centuries, black-and-white – so-called bitonal – digitization is usually suitable