The Scattering Medium node is used to create the look of subsurface scattering. This is the phenomena where light rays enter a surface, are scattered within the material of surface and then exit again. It is the key to creating the look of realistic human skin and other organic surfaces.
The subsurface scattering effect is determined by balancing the scattering value and the phase function value. Colors and Textures can be connected to these inputs if desired.
The Scattering parameter determines how quickly light is scattered as it moves through the surface. A high value means that light is scattered sooner as it enters the surface, while a low value means that light passes deeper into the surface before it is scattered. A value of 0 disables scattering entirely.
The Phase function controls the direction of the light as it is scattered in the surface. A value of zero results in light being scattered equally in all directions, positive values result in forward scattering - where photons continue in roughly the same direction they were going when they entered the surface. Negative values result in backwards scattering - where light moves through the surface in the direction roughly opposite to the angle at which the enter the surface.
The Scattering node also has an absorption setting. Creating a desired look is achieved by balancing Absorption, Scattering and Phase.
Mediums are affected by the scale of the scene and the objects that have the shader applied to them. Use the Scale slider in the Medium node to tune the effect. In some cases, you may not see much of the effect unless you lower the Scale setting.
The Vol Step Len parameter Is used with Volume Mediums only.
Figure 1: The parameters for the Scattering medium