Optical glass: spatial refractive index homogeneity from small to large parts - an overview

The refractive index, Abbe number and transmittance are the most important properties of optical glass. Nevertheless, in many applications the spatial refractive index variation - called homogeneity – is of highest priority. The index homogeneity of optical glass is a key property in industrial metrology devices and professional movie cameras for example. These applications often use small to medium sized lenses produced from cold cuts or hot pressings. On the other side of the range, scientific instruments of large astronomical telescopes require optical glasses with excellent index homogeneities on large apertures of several 100 mm up to even meter sized lenses e.g. for atmospheric dispersion correction. The challenge of enabling highest refractive index homogeneities in small and large dimensions requires tight control of all production steps from melting to hot forming and fine annealing. This paper gives an insight overview on the process of generating high quality glass part from small glass pressings to meter sized glass blanks. It gives suitable help for the interpretation of refractive index homogeneity of optical glass in relation to their dimensions. Latest results added to this overview reflect the current state of this topic at the optical glass manufacturer SCHOTT.