The optical property of the substrate governs the reproduction characteristics and quality of printed paper. Paper is a very complex substrate that is designed for different application areas. In the simplest case it is a one-layer structure of fibers. In more complex structures the substrate may be build from several layers with different properties. In both situations it is vital to be able to control the manufacturing in order to achieve the desired product. This paper deals with simulation of differently composed structures that is difficult to accomplish in complex cases that goes beyond the Kubelka-Munk original theory. The paper describes qualitative results concerning the reproduction due to inhomogeneities in the layers themselves and the layer structures. More specifically we will study two simple structures, a one-layer substrate with inclusions ( for example recycled paper) and a two-layer substrate in which the coating layer thickness varies. In the second example the optical properties for the coating layer are supposed to be different from the base. The qualitative results show the degradations that can be expected due to inhomogeneities. The simulations will also allow for computation of the tone value increase in the simulated instances.