Topic of your interest

Illustration of two gold effects

Communication of characteristic values describing the metallic appearance of prints

Short name: Characterization of metallic prints

Fogra no. 34.021
Project leader: J. Gemeinhardt
Funding: BMWK (IGF) via AiF

Timescale: 01.11.2018 - 31.10.2020

Research Report

Objective and relevance

Metallic effects are often used in packaging. Communication of these effects is problematic in the workflow from product planning to the actual printing process. So far, there are no production-oriented parameters that allow a clear description of the visual appearance. In addition, both the colour measuring devices commonly used in the printing industry and the viewing booths are unsuitable for characterizing metallic print samples.

Therefore, the research project aims to establish objective parameters that correlate with the visual appearance of printed metal surfaces. Furthermore, a viewing technique is to be developed that allows a clear assessment of the metallic effect.

Jürgen Gemeinhardt

Head of Printing Techniques

+49 89 431 82 - 256

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Solution steps

Colour measurements with different measuring devices and geometries are to be carried out on an extensive selection of metallised print samples. For this purpose, multi-angle colour measuring devices, a goniospectrophotometer and a spherical measuring device are to be used. Additionally gloss measurements are carried out.

Furthermore, a special viewing device with adjustable illumination and viewing angle is to be constructed. The visual evaluations of the metallic samples achieved with this device are to be compared to the metrological results.

Sampling metallic effect

Achieved results

It turned out that the angle-dependent measured value curves of the tristimulus value Y on the metallic samples can be adapted very well by superimposing two exponentiated cosine functions. One of the two summands describes a narrow peak with high reflection and a maximum at the specular angle, while the other describes a comparatively broad peak with comparatively low reflection and a maximum at positive effect angles. They can be understood as the specular and diffuse portion of the reflected light.

For the silver samples, the maximum brightness measured with the goniospectrophotometer shows a good correlation with the perceived metallic effect. This increases with increasing brightness. In the case of gold, the angle-dependent colour shift and the half-value width of the brightness must also be included. The former can be described with the help of the course in the u'v' chromaticity diagram. In this case, large colour changes lead to a devaluation of the metallic effect. High half-widths, on the other hand, cause an enhancement.