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Special-effect varnishes – looks can be deceiving


Special-effect varnishes, which are based on interference pigments and exhibit different colours from different perspectives, are growing more and more popular. So far, however, they have been a problem for metrology as both their colour and their saturation and brightness change according to the perspective and the light conditions. To solve this problem, a new robot-based measuring set-up has been established for the characterisation of their visual appearance: With two additional camera systems, it allows an improved metrological assessment of colours, varnishes and coatings.

Special-effect varnishing “Metallic Fantasy Emerald Green”: According to the perspective and the light conditions, the colour impression changes. (Photo: Höpe/PTB)

For industry, it is difficult and time-consuming to guarantee a constant colour quality for special-effect varnishes within the production chain – from the pigment to the varnish and then to the finished product. It is nearly impossible to reproduce existing special-effect varnishes whose exact composition is unknown.

As numerous wavelengths have to be determined for an angular resolved absolute reflection measurement with a defined angle of incidence and of reflection, such measurements have so far taken several hours or even days. A line scan camera at the new robot-based measuring set-up “ARGon3” (3D Appearance Robot-based Gonioreflectometer) of PTB now reduces the measuring time to just a few minutes, as it can detect a whole spectrum at the same time. It is supplemented by a luminance camera that improves the previous spatial resolution from 20 mm to about 30 µm and thus provides clearly more detailed colour information from a test sample.

The colour coordinates determined here ensure that industrially used colour-measuring instruments are able to provide more precise and comparable data. Thus, the quality control of colours can be improved – both for conventional and for special-effect varnishes.


Andreas Höpe
Department 4.5 Optical Technologies
Phone: +49 (0) 531 592-4520
E-Mail: andreas.hoepe(at)ptb.de