Using a Polarization Filter

It is generally agreed that polarization filters can give less difference in densitometer measurements between a wet and a dried printed sheet (mainly used on coated paper and other non absorbing materials).

When a wet film of ink is applied to paper, the surface of the ink is fairly smooth. Since the densitometer illuminates the ink surface vertically and views the reflected light at 45° (or vice versa), the density measured approaches the true diffuse density of the body of the ink. As the ink dries, the surface becomes rougher and, under normal conditions, the density is lowered by the increase in surface reflections. The polarization filters remove these surface reflections to a substantial degree. Only the part of the light which reaches through the color layer and which is diffusely reflected is measured, in other words the color content and not the gloss content. This produces the following advantages:

The polarization filters reduce the measurement differences between wet and dry samples (wet/dry problem) especially with coated papers. With uncoated papers, wet values should also be determined using polarization filters.

The ANSI Status T standard used in the Americas, does not mention the use of polarization filters and they are generally not used in this part of the world. When using Status E density (DIN), these filters are generally used. When comparing data it is critical that filter state and status are the same.