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Summary:
In the past it has been quite difficult to measure and control metallic inks on press. Since 2002, various research projects have been carried out to find a way to control these inks on press. The main challenge is that the 0/45o or 45/0o measurement geometry commonly used in press rooms, does not necessarily lend itself to measure the density or the color of metallic inks accurately. The metallic flakes inside a metallic ink have different orientations and reflect the light in different directions resulting in different colors and different amounts of light send back to the instrument. Incorporating polarization filters into densitometers helped to minimize the effect of different amounts of light being reflected back to the sensor inside a densitometer. A 0/45o spectrophotometer can not be used to measure the color of a metallic ink. Metallic inks are commonly used in the automotive industry, which uses either a multi-angle goniometer to measure and control metallic inks. Another measurement device that can be used for measuring the color of a metallic ink is a sphere-geometry based spectrophotometer, also known as an instrument with a d/8o measurement geometry. This device uses indirect lighting vs. direct lighting used in common press room color measurement tools. The indirect lighting and the ability to exclude the gloss of a sample from the color measurements allows one to measure and control metallic inks. The only problem is that there is probably no d/8o spectrophotometer in a press room.
In 2009, ISO 13655 was released which specified different measurement modes. These modes, labelled M0, M1, M2 and M3, define measurement conditions. ISO 13655:2009 - Graphic technology -- Spectral measurement and colorimetric computation for graphic arts images, now specifies four measurement modes - M0, M1, M2 and M3. The M0 is a legacy mode and covers previously built instruments (i.e. X-Rite 428, 530 etc). The M1 mode specifies the amount of UV light in a D50 light source. The M2 mode is D50 without and UV light below 400 nm. The M3 mode describes the use of two polarization filters. The light source is the same as in the M2 mode. One polarization filter is in front of the light source and the grating of the second polarization filter is orthogonal to the first filter and sits in front of the measurement sensor. The M3 mode was originally designed to measure the dry-back effect of inks and also to control special effect inks. The M3 minimizes surface reflections that could influence any color measurements.
The Pantone metallic book contains 301 metallic colour that use either P877 silver or P874 as the basic metallic colors. These metallic base colors are mixed with the Pantone basic colors to create the various metallic inks. In this study, 9 metallic inks with various amounts of metallic silver and metallic gold were printed first on the Prufbau printability tester and then on a 2-color offset press. Two M3-mode capable spectrodensitometers and one sphere-geometry based spectrophotometer were used to measure the printed colors. Both M3-mode instruments were capable of not only measuring the colors, but also giving quite similar L*a*b*-values and density values, outperforming the sphere-geometry based instrument.
Instruments capable of using the M3 measurement mode can be used to measure and control metallic inks on press.