The knowledge of the index of refraction of optical materials is essential when dealing with optical instruments and problems. Different interpolation-formulas are used, the best known is the Laurent-formula as used in the Schott-catalogue. Unfortunately, the range of this formula is limited to 365 nm up to 1014 nm. But often problems must be solved, where a spectral region wider than the above one is used. In our company we introduced an interpolation which covers a wide range; not only usual glasses are well interpolated, but even special materials such as ZnS are described correctly. Some of the common formulas are compared, regarding the usable spectral range and the smoothness of the interpolation. For all Schott-glasses it is possible to interpolate at minimum a region of 365 nm up to 2335 nm, for some other materials (Zns, ZnSe, Saphir, Silicon, ...) much larger regions are usable.