On the inhomogeneity of refractive index of CaF2 crystals for high performance optics

CaF 2 -crystals belong to the standard materials, used in optical precision instruments. As well as the traditional optics for microscopy new developments in the field of DUV-lithography (projecting lenses made out of CaF 2 and quartz) need a homogeneity of the refractive index in the order of 10 -6 , but over much larger volumes (diameter of the projecting lenses of lithiography>100 mm). Some interesting physical connections between the real structure of the crystals and the homogeneity of refractive index are discussed