For technical reasons the maximum ink film thickness in offset printing is about 3.5 μm. For coated paper and process colours according to DIN 16 539 the correct colour locations should be achieved with ink film thicknesses between 0.7 and 1.1 μm. 

If unsuitable lithography’s, inappropriate printing stock or unsuitable printing ink are used, however, it may happen that the standardized points at the corners of the CIE chromaticity diagram are not reached. The range of reproducible colors also decreases if the saturation is insufficient. In the illustration the white area shows how the range of colors narrows with the under inking of each of the three process colors. 

In terms of physics the influence of the ink film thickness on the optical appearance can be explained as follows. 

Printing inks do not cover the paper; they are, rather, transparent. The light penetrates the ink. In passing through the ink it encounters pigments which absorb to a greater or lesser extent certain wavelengths.  The higher the pigment concentration and the thickness of the ink film, the more pigments are hit by the incident light and, consequently, the more of it is absorbed. 

Finally, the light rays reach the (white) surface of the printing stock and are reflected. On its way back the light has to pass through the ink film again and only then can it reach the observer’s eye. 

A thick printing ink film absorbs more light and reflects less than a thin one; the observer therefore perceives a darker, more saturated, color hue. The portion of light reaching the eye thus serves as a basis for the assessment of each color.

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