The colorimetric description of production tolerances in newspaper printing

Summary

To ensure that the digital data intended to be used in newspaper printing can be correctly prepared, it is essential that the colorimetric properties of the printing process are known. Only then is it also possible to produce a binding colour proof that will sufficiently simulate the appearance of the production run. ISO/DIS 12647-3:2004 [3] lists the reference values for a standardised newspaper print. Among other things, it describes the shade of the newsprint to be used as well as the target values and tolerances for the colour gamuts of the primary colours cyan, magenta, yellow and black on the production paper (Tables 1 and 2). For the specified tolerances, a distinction is made between the so-called deviation tolerance and the fluctuation tolerance [2]. The deviation tolerances describe the permissible deviation of the “ok to print” copy at the press from the target values of the standardisation, or a binding colour proof respectively. As opposed to this, the fluctuation tolerances indicate the degree to which the colour gamuts of the print run may vary from the value of the “ok to print” copy. The ISO/DIS 12647-3:2004 [3] standard also contains reference values for the two-colour overprinting of cyan and yellow, cyan and magenta, magenta and yellow. Because the colour coordinates of the secondary colours depend largely on the printing conditions in each case, the corresponding tolerances were only listed in an informative annex. Target values exist also for dot gain in the primary colours in the mid-tone range (Table 3). A distinction is made here also between deviation tolerance and fluctuation tolerance. In addition, there is another restriction that the tonal value differences of the chromatic coulours cyan, magenta and yellow (spread) must be less than 6 percent, as otherwise the result would be a noticeable colour hue shift in the print. A revision of the ISO 12647-3 standard was begun during the realisation of this research project and was still at the discussion stage at the time this Special Report was being prepared. In the interim, the revised version has reached the official status of DIS (Draft International Standard). It has been published and is available for downloading, e.g. from the web site of the Beuth publishing company (www. beuth.de) [3]. It was a matter not only of changing the colour gamuts, but also and especially specifying the relevant colour tolerances (Table 2). It was also proposed to set dot gain at 26 percent and 30 percent. The 26 percent dot gain applies on an international scale whereas the 30 percent dot gain is confined exclusively to North America. The present research report investigates the effects of the production tolerances on the colours printed in the newspaper and their appearence. Several printing tests were carried out for this purpose, with the application parameters systematically varied within the tolerances. Moreover, also presented in this report will be various mathematical models for determing the colour coordinates of the single-colour tone colour patches. Because it is important for the judgement of the colorimetric tolerances that the measured colour difference should correspond to that as perceived by the human eye, an investigation of the uniformity of colour difference formulae was carried out. As part of this, the conventional CIELAB colour difference formula [4] was compared to the new CIEDE2000 colour difference formula [5].