Uniaxial birefringence is classified as positive when the extraordinary index of refraction is greater than the ordinary index . Negative birefringence means that is less than zero.Brad Amos. Birefringence for facetors I: what is birefringence? First published in StoneChat, the Journal of the UK Facet Cutter's Guild. January–March. edition 2005. In other words, the polarization of the fast (or slow) wave is perpendicular to the optic axis when the birefringence of the crystal is positive (or negative, respectively). In the case of biaxial crystals, all three of the principal axes have different refractive indices, so this designation does not apply. But for any defined ray direction one can just as well designate the fast and slow ray polarizations.
