Indicatrix
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. In all other cases, in which the ellipsoid has three distinct axes, the material is called biaxial.
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