Glass is such a fundamental material that most of us take it for granted. Have a look around and you may notice that your computer display screen is made of glass, the windows in your office are made of glass, and perhaps you are using eyeglasses, just to name a few examples. These are items you can see on a daily basis, but spend most of the time looking through them in search of something else. What you cannot see as clearly are the small lenses in your DVD players, or the optics in your new cell phones turning you into an instant photographer at will, or the optical glass fiber your email message is sent through. Looking deeper you will realize that glass has had a significant impact on mankind. In 2009 we celebrated the 400th anniversary of the telescope, invented by Galileo Galilei, who started a scientific revolution that changed the way we view the world and the universe. Later developments of glass lenses made microscopes available to us with superior quality revolutionizing medicine and providing us with, e.g. methods for developing vaccines. Besides the Nobel Prize in Physics being awarded to Charles K. Kao for his work on optical glass fibers, 2009 was also the 125-year anniversary celebrating the time when Otto Schott joined forces with Ernst Abbe and Carl Zeiss, forming the company known today as SCHOTT. They set out to systematically develop glass and the glass manufacturing process and introduced a number of new glasses essential for scientific development.
In this review on optical glass and glass ceramics formed by melting, Hartmann, Jedamzik, Reichel, and Schreder take you on a fascinating journey, starting with a review of the main milestones in glass history followed by important developments as well as the current challenges in glass science and technology performed at SCHOTT. Different events in history, trends and driving forces in the glass industry are discussed, including the quest for lead- and arsenic-free glass, optics for microlithography, and glasses suitable for precise press-forming that maintain cost efficiency in production. The main challenge in glass fabrication is the ability to reproducibly produce glass with suitable refractive index, dispersion, and transmission properties, while guaranteeing high homogeneity, low bubble content, minimum striae, and reduced stress-induced birefringence. The availability of glass with a wide variety of refractive index and dispersion values is of crucial importance for optical lens system designers. From this point of view the authors discuss the Abbe diagram and the approach, solutions, and technological difficulties of making glasses in the extremes of this diagram. With improved material purification and processing techniques, modern glass has extended the compositions and range available in the Abbe diagram from the dominating silicate glasses to include most oxides, but also halides, calcogenides, and nitrogen-containing glass. The review also discusses the challenges in producing and characterizing extra large optical glass items, requiring the simultaneous process development in attainable size as well as the means to characterize these large samples. The authors describe state-of-the-art systems for characterizing very large glass items regarding homogeneity and stress birefringence as well as a method for real physical quantification of striae, in contrast to traditional shadowgraphs. Because of size limitations of refracting optics, modern extremely large telescopes (ELT) use reflective optics based on ultra-low thermal expansion ZERODUR glass ceramic, with a typical size of the mirror blanks in mass-production on the order of 2 m. Here special techniques have been developed to characterize and model the mirror blanks regarding thermal expansion and homogeneity in order to ensure high-quality reproducibility in production over decades. The review also describes the progress and applications of light weight ZERODUR blanks for space applications.
I highly recommend this review as it gives a very comprehensive historical as well as recent view of optical glass on a level that would interest both experts of diverse fields of science as well as a novice. Glass has shaped our lives, and I cannot image what I would be doing today if it had not existed.
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