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Measurement backings

Background:

Problem description: Target values for process control in offset printing

The printer working at the production machine is supplied with a colour-accurate test print as a reference for the production run. Colour accuracy means that the colorimetric specifications with regard to the media standard for test printing lie within the tolerances and the test print is therefore not noticeably embellished in favour of an intuitive visual impression (e.g. too colourful). The printing conditions to be simulated (e.g. FOGRA51 for commercial sheet-fed offset printing on paper with optical brighteners, FOGRA40 for web offset printing or PSRLWC for gravure printing) are based exclusively on colour measurements with white backing (wb).

However, the typical practical conditions in a print shop, as well as the transparency of reverse-printed or thin papers, do not permit a white backing for process control in offset printing. In the early days of conventional proofing on production paper, the prints could simply be placed on the black backing (bb), and the printer was able to derive the colour or density values as a target for process control.

But since current proof and printing papers differ greatly, especially in their opacity, this approach no longer makes sense in the current processes in prepress and printing. A thick proofing paper, which was used, for example, for the simulation of web offset printing, will have considerably smaller colour differences than the thin, translucent production paper when changing from a white to a black backing. Consequently, the spectral measurement of a proof print on a black backing does not provide the correct target values for the production run.

Instead, the printer needs target values that are matched to its current production paper and thus guarantee the same colour impression between proof and print (on white backing). In other words, starting from the colour-accurate test print, colour values on a black measuring base are required, which lead to a print result that is as similar as possible to the test print on a white measuring base.

Manual remedies have so far failed

In a pragmatic solution approach, the differences between white and black backing for primary and secondary colors are specified in the ProcessStandard Offset for the various paper types. The printer must therefore measure the proof print colorimetrically and apply the differences to these values with the help of a computer program in order to obtain target values for the print.

These difference values are based on averaging of colout measurements of typical printed sheets on white and black backing. Unfortunately, the differences between the production papers, even within a paper type, are far too great for sufficient accuracy to be achieved. The inadequacies of the process as well as the cumbersome and complex application in everyday production, which is characterized by time pressure, led to the fact that this method hardly found its way into practice.

Definition:

White backingBlack backing
Opaque (transparency > 99 %)Opaque
Diffuse-reflectingDiffuse-reflecting
CIELAB C* ≤ 3 (should ≤ 2.4)Not spectrally selective
No fluorescence (∆B ≤ 1)Visual density: 1.6 ≥ VD ≥ 1.3
Spectral reflectance factors:
0.74 ≥ R(380nm) ≥ 0.12
0.80 ≥ R(390nm) ≥ 0.19
0.84 ≥ R(400nm) ≥ 0.37
0.87 ≥ R(410nm) ≥ 0.61
0.88 ≥ R(420nm) ≥ 0.75
0.89 ≥ R(430nm) ≥ 0.78
0.90 ≥ R(440nm, 450nm) ≥ 0.79
0.91 ≥ R(460nm, ..., 730nm) ≥ 0.79

Please refer to manufacturer specifications of available whitebackings to see whether they conform to the tolerances of the revised ISO 13655:2017. 

Additionally, proof papers which fulfill the above mentioned requirements can also be used as a whitebacking. Please refer to our proof paper database to find out which substrates are suitable and how many layers of the respective substrate are required for this purpose. (May 2018: Due to the recent discussion in the Digital Printing Working Group – see below – the database for proofing papers currently bases on the old tolerances according to ISO 13655:2009.) 

Do you still use self backing [sb]? More information on that topic.

Comparison of old and new tolerances (ISO 13655)

We would like to share some evaluations with you regarding the compliance of different backing materials according the new tolerances of ISO 13655:2017. Please find the XLS file here. (Update 15.05.2018: Following the discussion in the DPWG mailing list, the measurements with the i1pro2 were repeated and the evaluation was revised accordingly.)

A brief video showing what we did is here:



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