and the reduced displacement as
then the index ellipsoid is defined by the equation
The semiaxes of this ellipsoid are dielectric constants of the crystal.
This ellipsoid can be used to determine the polarization of an incoming wave with wave vector by taking the intersection of the plane with the index ellipsoid. The axes of the resulting ellipse are the resulting polarization directions.
An important special case of the index ellipsoid occurs when the ellipsoid is an ellipsoid of revolution, i.e. constructed by rotating an ellipse around either the minor or major axis, when two axes are equal and a third is different. In this case, there is only one optical axis, the axis of rotation, and the material is said to be uniaxial. When all axes of the index ellipsoid are equal, the material is isotropic, commonly known as cubic. In all other cases, in which the ellipsoid has three distinct axes, the material is called biaxial.