In 1884 Otto Schott, Ernst Abbe and Carl and Roderich Zeiss founded the SCHOTT & Associates Glass Technology laboratory in Jena, Germany. Otto Schott’s target was to develop new glasses on optical positions defined by Ernst Abbe and high reproducible quality that would tremendously improve the image quality of Carl Zeiss microscopes and optical instruments. Prior to 1880, optical components were made from simple crown and flint glasses. The crown glasses were soda-lime silicates with low refractive indices and moderate Abbe numbers. The flint glasses were lead silicates with relatively low Abbe Numbers. By 1884 already two dozen flint and crown glasses were available for optical system designs. Still today photonics industry relies heavily on optical glasses to realize tightest optical design requirements of today photonics applications. The difference between today and the past is that the designer can choose from a variety of more than 120 optical glasses with differences in optical position, transmittance and very specific dispersion requirements.
The refractive index is the most important property of optical glass. Therefore, refractive index measurement is a key characterization method also for process control. The requirements for refractive index control demand a fast and accurate measurement method for production control and a very high accurate measurement method for tightest requirements. For refractive index measurement two different measurement setups are common: the v-block refractometer for economic refractive index control up to the fifths digit and the spectral goniometer for high precision index measurement over a broad wavelength range from 185 nm to 2.325 μm up to the sixth digit. Providing Sellmeier dispersion equation data for individual measurements enables accurate interpolation of index data for arbitrary wavelengths for high precision applications. To cope with specific production requirements and the required high reliability of the measurements SCHOTT develops its own measurement setups. Both, the v-block system and the high precision spectral goniometer have been updated in recent time. To evaluate the accuracy SCHOTT participates in international round-robin tests. SCHOTT is also participating in the standardization of refractive index measurement methods for the optical industry. In this paper the current status of refractive index measurement of optical glasses at SCHOTT is presented.
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