The optical properties of inhomogeneous materials can be described by so-called effective dielectric functions if the wavelength of the probing radiation is much larger than the typical sizes of the inhomogeneities of the system [Theiss 1994A, Theiss 1995B]. In this case retardation effects can be neglected and the response of the mixed material to an incoming electromagnetic wave can be calculated in a quasi-static approximation, i.e. one has to answer the question which volume averaged polarization will exist in the sample in response to an applied static electric field.

Macroscopically the inhomogeneities cannot be seen (there is no light scattering in the long wavelength limit) and the system can be treated then as being quasi-homogeneous. An effective dielectric function εeff can be introduced which is an non-trivial average of the dielectric functions of the individual components.

For the case of a two-phase composite the situation is shown below:

Obviously the microgeometry plays an important role for the effective dielectric function: if, for example, the embedded particles are metallic and the host material is an insulator the effective medium can show metallic or insulating behaviour, depending on wether there is a percolating network of the embedded particles or not. But also other properties of the topology may influence the effective dielectric function significantly.

Several very simple 'mixing formulas' have been established, the most prominent being the ones of Maxwell Garnett [Maxwell Garnett 1903A] and Bruggeman [Bruggeman 1935A], both being implemented in SCOUT. The model due to Looyenga [Looyenga] has been added also since it leads for some typical microstructures to good results. All these simple effective medium concepts use just one parameter to characterize the microgeometry, namely the volume fraction f of the embedded particles. Due to this simplication in many cases quite wrong results are the consequence. The most general concept due to Bergman [Bergman 1978A][Theiss 1994A] holds in any case (as long as the electrostatic approximation is valid) and is therefore included in SCOUT with best recommendations from the author.

All effective dielectric function objects need to point to two dielectric functions, namely those of the host material and the embedded particles. The assignments are usually done using drag&drop operations like in the assignment of dielectric functions to layers in layer stacks. To do so, you have to open both the dielectric function list and the parameter window of the effective dielectric function. In case of a 'Bruggeman object' this looks like the following:

Start the drag operation by pointing on the wanted material in the dielectric function list, press the left mouse button and keep it down while you move the mouse cursor to the effective medium parameter window. If the material is going to be the matrix material place the cursor in the white rectange below the text 'Matrix material' and release the mouse button. In the case shown above vacuum was selected as matrix material. In an analogous way, drag and drop the particle material to the white area under the text 'Particle material'.