A necessary condition for quality control, as well as for the correct calculation of the specifications of gas-filled materials (including polymeric foams), is a rigorous quantitative evaluation of the apparent density distribution over the bulk. In the case of classical foams, a consideration of the density distribution in a plane, i.e., a two-dimensional distribution of the ϱ-values, would be quite sufficient. Evidently, with the integral foam structures (see Fig. 1.1-1.3), one has to consider a volume density distribution, i.e., a three-dimensional variation of the ϱ-values. Since a graphic representation of the behavior of any IF property in three-dimensional space is not always clearly comprehended, a two-dimensional “property vs. density” plot for a particular cross section, most often one normal to the surface skin, is used in practice.