Comment on “Refractive indices of biaxial crystals evaluated from the refractive indices ellipsoid equation”

Abstract In 2007 Yin, Zhang and Tian [1] [Yin et al., 2007] derived the expressions of the refractive indices of biaxial crystals evaluated from the refractive indices ellipsoid equation. In the past we have researched about the simultaneous measurement of birefringence and optical activity in different crystals [2] [Hernandez-Rodriguez et al., 2000], [3] [Hernandez-Rodriguez and Gomez-Garrido, 2000], [4] [Herreros-Cedres et al., 2003], [5] [Herreros-Cedres et al., 2005] and [6] [Herreros-Cedres et al., 2007], and recently, when we used their methods for the study of nonlinear crystals such as KTiOAsO 4 (KTA) and KTiOPO 4 (KTP), we found some errors in some expressions in their paper which were used by other authors [7] [Gao et al., 2003].