How exactly can printing ink be measured?

For some years now, the possibilities of colorimetric measurement of printing inks have become simpler and cheaper. And so it is often believed that measuring printing inks is simple, inexpensive and, above all, highly accurate. And this also across a wide variety of brands and generations of measuring devices. Is that true?

If you look at a few studies, that does not necessarily seem to be the case. IFRA, for example, requires that when measuring BCRA ceramic tiles the colour differences between different measuring instruments should be below Delta-E 0.3. In reality, however, things looked different. In a Nussbaum study, 8 out of 9 measurements were for a Delta-E greater than 2.0; in a Wyble & Rich study, the deviations were between Delta-E 0.76 and 1.68. But why are the deviations so large?

On the one hand, the measuring instruments differ in the way they illuminate the surfaces to be measured. This is important in two respects: On the one hand, measurements can vary greatly depending on the material, for example, because light is emitted and measured from only one light source onto the measuring surface. If a measuring instrument has only one lamp, which, for example, radiates at an angle of 45 degrees onto the measuring surface and whose reflection is measured, then the measurement can deviate by up to Delta-E 3.0 if you only rotate the measuring instrument about its own axis. If a left-handed person and a right-handed person measure the same tiles with the same measuring device, then just by holding the measuring device differently and by the different lighting angles of the tiles a measurement can be completely different.

The solution for this: In a measuring device, several light sources are distributed or, in the best case, the illumination is emitted directly circular at an angle of 45 degrees in order to minimize such effects.

The illuminants

But also the way of lighting plays a role. In the past, tungsten lamps were mainly used for lighting, which emitted a good spectrum but whose spectrum changed considerably over the life of the lamp, especially in the UV range, and could therefore lead to incorrect measurements. And there is also the problem that a fresh print from the press with inks still wet has a significantly higher gloss and reflection factor, i.e. the same print when it is delivered to the customer a week later. Up to now, these differences in gloss have been eliminated as far as possible via polarizing filters.

Since the new D50 lighting standard ISO 13655:2009, lighting with D50 neon tubes would be ideal, but they do not really fit into handy measuring devices. Therefore, different lighting conditions were defined: M0, M1, M2 and M3.

M0 is the traditional tungsten illumination as in the I1 Pro or DTP 41 or DTP 70 without UV filter and without pole filter.

M2 describes the same but with UV-Cut filter as used in the I1 Pro UV-Cut, for example. Advantages of the UV filter: harmonious results even with papers with optical brightener. Disadvantage: Typical production papers today contain many optical brighteners that make paper appear whiter by absorbing UV light components and reflecting them as blue light. Papers can therefore look completely identical under UV cut, but they can look extremely different in daylight or under D50. And in practice, printed products are almost never viewed under UV-cut conditions.

M3 meters have both a UV filter and a pole filter.

The M1 mode is achieved by using a D50 light source according to the specifications of ISO 3664:2009. However, since the LEDs used in measuring devices are not yet capable of reproducing the full spectrum of D50 and neon tubes are simply too large, some measuring device manufacturers make do with one trick: On the one hand, they emit D50 light without UV components in order to achieve the most harmonious measurement possible. On the other hand, they only emit UV light to measure the effect of the optical brighteners. Since currently no LED is able to emit clean UV light, but the paper is still illuminated by secondary light and the result would be falsified, some work with a trick: They measure at high speed quasi flickering permanent UV cut and the bad UV variant, and calculate from it a result that would correspond to an illumination with D50 light.

Conclusion: Measurements with various measuring instruments are currently still very unreliable and do not meet the required low tolerances as gloss, UV components and spectral deficiencies and degeneration of the light sources pose difficulties. However, there are promising approaches to at least compensate for these difficulties in future generations of measuring instruments.

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