Sheets of material are often used in various industries and in a variety of ways. These materials can include paper, plastic, and other materials manufactured or processed in webs or sheets. As a particular example, long sheets of paper or other single layer or multi-layer products can be manufactured and collected in reels.
The “Z-structure” or cross-sectional structure of paper and other sheet materials is often a determining factor in numerous quality properties for the materials. For example, the distribution of voids between fibers in paper products typically affects bulk and opacity, and the distribution of fillers in paper products typically affects printing quality. Only fillers near the surface typically affect surface smoothness and ink permeability, and asymmetric filler distribution can cause the color of the surfaces of a paper sheet to differ.
Papermakers are often interested in the distribution of fillers because there are several process adjustments available to influence it. As a result, accurate measurements of the Z-structure of a paper sheet could lead to the identification of filler distribution problems and allow timely adjustments to the paper-making process. Another issue may arise for intrinsically multi-layer sheets of material, such as those formed by splicing together multiple formed sheets or by coating a formed sheet with a polymer. In these cases, it is often desirable to know the thicknesses of individual layers or differences between exterior layers and interior layers.