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, s, 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, e.g. constructed by rotating an ellipse around either the minor or major axis. In this case, there is only one optical axis; the axis of rotation. In such a case, the material is uniaxial and has only one principal symmetry axis.