Pure materials can be transmissive : the light is not only reflected by the material but it can be transmitted and go through the material. In these situations, the surface of a geometry defines an interface between two materials : the internal material and the external material.

By default, the default external medium is air. Interface materials enable you to create interfaces where the external material of a geometry is not air but some other Fresnel material.

If the coating is defined with a stack of thin materials, the materials are given from the closest to the external material (top of the list) to the closest to the internal material (bottom of the list). Each layer is defined by a material and a thickness, and can optionally have a thickness map : a texture containing a multiplicative factor for the thickness.

If the interface material defined bellow were applied to a sphere, the sphere would be made of BK7, coated by 1e-05m of red_glass and 1e-05m of blue_glass, surrounded by Water.

Usually when we define a material, we consider that it fills the geometry that it is assigned to. Pure materials can also be defined as “Thin” : in that case they have a given thickness and they only represent a thin layer of material on the surface of the geometry.

Just as for Pure materials, you can define the roughness of the material using two textures. See the Pure materials section for more details.