The optical rotation of chiral crystals can only be measured accurately along the optic axis, because in any other direction the circular birefringence is obscured by the much larger linear birefringence. The optic axis is defined as the direction perpendicular to the central circular section(s) of the ellipsoid called the indicatrix. The semi-axes of this ellipsoid are oriented along the principal axes of the dielectric tensor ε r . For low-polar media, the dielectric tensor is related to the electric dipole-electric dipole polarizability tensor α (all in a.u.) as