Spectral measurements as a basis for standardization of colour images in reproduction processes

Colorimetric adjustment of the printing process is based on spectral characteristics, which are performed by strictly defined measurement conditions. The value of spectral data measurements and calculation of colorimetric characteristics of imprints for the processes of calibration and profiling of multi-colour printing is clarified.

Paper types containing bleaches present many challenges for printing companies seeking to control colour reproduction in their workflows. Optical Brighteners absorb photons of invisible UV radiation and transform them into radiation of the visible zone of the spectrum. The influence of optical brighteners, which are part of some types of paper, on the results of measurements is studied. An analysis of the colorimetric features of the target values of the colour coordinates of the ISO 12647-2 standard is carried out. It is shown that the colorimetric adjustment of the printing process is based on spectral characteristics, which are performed strictly under specific measurement conditions.

Only the colour coordinates in the systems of the International Illumination Commission CIE accurately describe the colour of the samples. The CMYK value is just a combination of the percentages of colours that are applied to the paper. The same CMYK values will produce a different colour on different types of paper. The colour of inks on different papers and different triads is associated with an unambiguous description of the colour on the print in IСС profiles, which contain volumetric tables of correlations between the values of percentages of inks and colour in L*a*b* units. The influence of the measurement conditions on the colour of the inks on the imprint in CIE L*a*b* units is studied. The values of colour differences were calculated for a large number of samples, the values of the colour coordinates of which are obtained under the conditions of measurement with the M1 and M0 filters. It is shown that measurements give different results.

The effect of fluorescence enhancement by the M1 filter applies specifically to bleached papers. Measurements of paper, which does not glow at all in ultraviolet, behind filters M0, M1 and M2 are equal in spectrum and colour, but the higher the level of fluorescence in the paper, the more filter M1 enhances the blue shift.

Keywords: colour reproduction, colour coordinates, spectral measurements, target values, standard, optical brighteners, fluorescence, colorimetric calculations.

doi: 10.32403/0554-4866-2023-2-86-13-27