Interfaces of type 'Specular and diffuse' are used to combine specular and diffuse (Lambertian) reflectance (and transmittance). The dialog has four buttons to open the corresponding windows:
For a specular reflectance of 0.3, specular transmittance of 0.2, diffuse reflectance of 0.2 and diffuse transmittance of 0.2 (i.e. the probability for absorption at the interface is 0.1) the interface behaves like this:
