If sheets are pulled freshly printed from the delivery and measured, the ink is still wet and has a shiny surface. While drying, the ink penetrates the paper (absorption) and loses its gloss. This not only changes the color tone, but also the density. It is only possible to a limited extent for the press operator to use densitometry to compare wet sheets with the reference values, which also refer to dry ink.

Why Polarizing filters?

To get round this problem, two linear polarizing filters at right angles to one another are placed in the path of the densitometer. Polarizing filters only permit light waves oscillating in a certain direction to pass. Part of the resultant aligned beam of light is reflected by the surface of the ink, but its direction of oscillation remains unchanged. The second polarizing filter is rotated 90° in relation to the first, which means that these reflected light waves are blocked.

Fig1. Polarizing filter

However, if the light is only reflected after it penetrates the film of ink, either by the ink or the paper, it loses its uniform direction of oscillation (polarization). Consequently, part of it passes through the second polarizing filter and can be measured.

Filtering out the light reflected by the glossy surface of the wet ink thus has the effect of making the densitometric measurement values for wet and dry ink roughly equivalent.

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