Visual and Numerical Evaluation of Metallic Inks and How They Compare to Numerical Colour Differences


Year: 2018
Pages: 15


Previous projects in 2010, 2016 and 2017 looked into methods for measuring and controlling metallic inks that have been printed on press. These projects have established controlled ways to measure printed metallic inks. The two most recent studies showed clearly that the M3 measurement condition, as outlined in ISO 13655, can be used to measure and control the printing of metallic inks. In 2009 the ISO released the procedure ISO 13655 that regulates various measurement conditions for the print industry. The measurement conditions are labelled M0, M1, M2 and M3. The M0 condition encompasses all legacy models that do not conform to ISO 13655, M1 uses lighting with a defined amount of UV in it, to excite optical brighteners that are present in the paper, M2 uses the same light source as M1, but without the UV portion and M3 is for the measurement of special effect inks. The M3 measurement condition uses the same light source as M2, but there are 2 polarization filters with orthogonal grating in the machine. One is in front of the light source and the other in front of the measurement sensor. The 2017 study also showed that it also is of no concern if the inks are made for the offset or flexographic printing process. The difference between the metallic inks for these two printing processes is the size of the metallic flakes in the ink. Flexographic ink allows the of use larger metallic flakes than for offset inks. The larger flakes result in a more metallic looking print. These larger flakes have also larger reflecting surfaces than the metallic flakes used in offset inks. These larger metallic flakes do not influence the color measurement under the M3 measurement condition. This has been proven in the 2017 study.

The follow-up project will consist of two parts. In the first part the metallic inks used in previous studies will be printed on an offset press in two different ways. The first way is to print to the same measured printed ink density read from the Pantone(r) metallic book. The second way is to achieve visual similarity to the color swatch in the Pantone(r) book. This test will be repeated three times for repeatability purposes. This can also be used to verify the usability of the M3 measurement condition on press. The printed samples will be marked and presented to human test subjects for evaluation of visual acceptance to the Pantone(r) metallic book swatch of the test colors. The results from the visual evaluation test will be correlated with the color difference values the samples have to the Pantone(r) metallic book.

In the 2016 and 2017 study it was established that a spectrodensitometer that is capable of conforming to the M3 measurement condition will be used to evaluate the test samples for their color difference to the reference in the Pantone(r) metallic book. In the end, the color difference values will be linked to the observations by the test subject. The test subjects will use a Likert-type scale to rate the color difference between the reference and the printed samples. Correlations will be drawn between the perceived color difference and the measured color difference values.

The visual ranking of the prints matching the printed ink density in the Pantone Metallic book did not correspond well with change in printed ink density.

The visual ranking of the prints achieving pleasing visual color corresponded well with the samples evaluated by the test subjects.