While the static electrical conductivity is vanishingly small in insulators (such as diamond or porcelain), the optical conductivity always remains finite in some frequency intervals (above the optical gap in the case of insulators).
In the simplest cases, this property can be considered as a complex valued scalar function of frequency only.
This formulation applies in the limit of long wavelengths, coarse grained structure, and cubic material symmetry.
The optical conductivity is most often measured in optical frequency ranges via the reflectivity of polished samples under normal incidence (in combination with a Kramers–Kronig analysis) or using variable incidence angles.
[5] For samples that can be prepared in thin slices, higher precision is obtainable using optical transmission experiments.
To fully determine the electronic properties of the material of interest, such measurements are combined with other techniques that work in different frequency ranges, e.g., in the static limit or at microwave frequencies.