23 September 2015 Optical lead flint glasses: key material in optics since centuries and in future
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Abstract
About 350 years ago a new kind of glass types was invented for decorative purposes such as drinking glasses, bowls and vases. It needed more than 70 years until the capability of these lead flint glasses was discovered to improve the performance of optical systems markedly. Color correction enabled images with resolution more than ten times better than earlier systems opening the view of researchers for new fields in the micro and macro world. Within the next 150 years the progress in optical glass production concentrated on improving quality especially homogeneity, characterization of its properties and achieving larger lenses. The introduction of glass types with considerably different compositions in the 1880s led to complementation of the glass program but not to a replacement of the lead flint glasses. Their outstanding optical properties together with their favorable melting behavior kept them being workhorses in optical systems design. One of the outstanding properties of lead flint glasses is their capability of being cast in large volumes. The size development reached a summit by the end of the 19th century with the lenses of the largest refracting telescopes. Their use as radiation shielding glasses since the second half of the 20th century led to even bigger castings of up to two tons of weight. In the 1990s the other outstanding property made lead flint glass types playing an important role in microlithography. Transmissive optics working with the mercury i-line needs crown and flint glass for dispersion correction of the comparatively broad i-line. The flint glasses had to have utmost transmission in the near UV to reduce thermal lensing as far as possible. This combination of requirements on dispersion and transmission could be fulfilled only by using lead flint glasses. It remains valid in fluorescence microscopy. Here the trend goes to an ever broader spectral range extending from the IR into the UV allowing diffraction limited resolution for many fluorescence light bands simultaneously. These outstanding properties of the lead flint glass types caused SCHOTT to keep them in the glass program and not to replace them completely as other glass companies have done. The improvements of the last two decades with respect to homogeneity and transmittance underline their suitability for future extreme quality optics with applications in medical and general research and in astronomy for large beam shaping and atmospheric dispersion correction.
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Peter Hartmann, "Optical lead flint glasses: key material in optics since centuries and in future", Proc. SPIE 9626, Optical Systems Design 2015: Optical Design and Engineering VI, 96260R (23 September 2015); doi: 10.1117/12.2190326; http://dx.doi.org/10.1117/12.2190326
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KEYWORDS
Glasses

Lead

Flint glass

Lenses

Refractive index

Optical lithography

Annealing

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