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In the graphics industry it often happens that a transparent layer is applied over a print. There are various reasons for this, e.g. to improve the abrasion resistance or to increase “optical appearance”. The thickness of the transparent layers ranges from a few micrometres to several tenths of a millimetre. The colour appearance is changed by the gloss and lateral light propagation. The desired colour is still achieved today by an iterative process.
To avoid this cost-intensive process, an algorithmic method will be developed. The thickness of the transparent layer, the spectral reflection and absorption behaviour of the involved transparent and opaque surfaces, the lateral light propagation and the refractive index of the transparent layer will be used as measurement parameters for the modelling. These data are to be provided to industry as material-specific characteristic values for colour profiling.
Solution steps
First, components of market-typical samples (coated or foil-laminated print samples, ID cards) are identified and characterised. The following determinations are made for this purpose:
Refractive indices of the various transparent liquid and solid materials
Colour measurements on coated and uncoated samples
Angle dependent gloss
Diffuse reflection and transmission spectra with calculation of the dispersive reflection and absorption coefficients
Lateral light propagation (by using a specially designed measuring device and correspondingly developed measuring method)
Based on the data and knowledge gained, a colour prognosis model is developed based on the 4-channel theory. The colour changes to be expected with transparent coatings and different spot sizes are simulated on the basis of colour measurements of uncoated samples and verified on the real samples.
Achieved results
The project aims to develop a model that allows the prognosis of colour changes through matt and gloss coatings of known thickness. As central measurement variables for the modelling, besides the thickness of the transparent layer, the spectral reflection and absorption behaviour of the involved transparent and opaque surfaces and the refractive index of the transparent layer are used.
This allows reliable colour location forecasts to be made for transparent coated surfaces and other measuring aperture sizes after printing. This means that tedious customer coordination about the colour of the finished product could be a thing of the past.