The microgeometry of the paper surface in the watermark area and beyond was investigated using an optical non-contact contactless profilometer. This made it possible to obtain quantitative characteristics of the surface roughness of the studied paper samples, to measure the height and depth of relief, to obtain the distribution of projections by area, as well as to construct a 2-D and 3-D images of the surface, to observe the interference images of the paper surface in white and in monochromatic light. It has been found that the structural parameters of the surface of the protected paper are not the same for areas with and without watermarks. The results of the interference-based measurement make it possible to determine promptly the roughness indices that are subsequently used to determine the surface quality of the paper and to calculate the strength of the ink layer in areas of paper without watermark and with a negative watermark. The constructed 3-D profiles of the paper surface clearly demonstrate the size and number of randomly arranged fibers and filler particles in the paper pulp in the watermark area and beyond, on a given scan path.
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