This is necessary because applications like interferometric measurements are sensitive to variations in the sixth digit of the index of refraction. Cold processing steps like sawing, cutting, grinding, lapping, and polishing convert the fine annealed raw glass into the required optical components to enable the optical functionalities as described above. This chapter provides a brief overview about the variety of optical glasses and their production procedure including melting, coarse, and fine annealing. Further, the author introduces the definition of the Abbe number. This value describes the dispersion and defines together with the refractive index the optical position.
The Abbe diagram depicts the optical position of various optical glasses. This diagram explains the naming conventions of optical glasses. Finally, the measurement techniques in an industrial environment for the main properties of optical glasses are explained. Glass has many unique properties. For each application mentioned in this book, the supplying industry breeds special glass types with adapted features to serve the requirements of their application.
The most obvious feature of optical glass for a human eye is the high transparency as indicated in Figure 3. Compared to window glass, the optical path in optical glass is more than 30 times longer for achieving the same transmission, which is a huge difference. In order to achieve such high grades of transmission, the requirements on the purity of the ingredients, the haze level, the number of bubbles, and the inclusions are significantly stronger than for window glass.
Figure 4 shows the internal transmittance over the visible spectrum and the near-infrared regime. Spectral development of the internal transmittance and refractive index of an optical glass e. Data taken from . In contrast to the transmission, the optical position of an optical glass is not obvious for a human eye. Hereby, the refractive index at a specific wavelength and the Abbe number describing the dispersion define the optical position. The graphic in Figure 4 shows the decrease of the refractive index starting from the ultraviolet over the visible spectrum to the near-infrared regime.
Dispersion is the name of this spectral refractive index development. The value n d at the d-line This dispersion is one of the main reasons why we need optical glasses and high sophisticated lens systems for the photonic products at all. If a single lens focuses a blue light ray e. Therefore, the focus position of both colors differs. If an optical designer combines a flint and a crown glass lens in a proper way, the designer achieves that the focus of the blue and the red ray overlaps.
This doublet is an achromatic system.
Unfortunately, the focus position of other colors still varies. Therefore, a further chromatic correction and other aberrations require a complex multi-lens design as depicted in Figures 5 and 2 right [ 8 ]. Such lens system design relies on a broad portfolio of optical glass that spread widely in their optical position. Exploded view on a lens system consisting of 10 different lenses made of different optical glass types. This depicts the complexity of such setups .
This diagram maps the different optical glass types by using the refractive index and Abbe number as coordinates. Besides the rough differentiation between crowns and flints, the map shows further areas of similar chemical composition, e. According to their position in the diagram, the glasses get their labels, e.
The number at the very end of the glass-type label is without any further information and counts of the developments in the relevant area seldom followed by a letter indicating a new version.
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All explanations are valid for the optical glass manufacturer SCHOTT as a reference but are in general transferable to other manufactures as well. In earlier days, optical glass manufacturer filled pots with the ingredients of the optical glass composition, melted the raw material, reduced the bubble content by refining processes, mixed the liquid composition, casted the glass, and filled up the pot again [ 12 ]. The state-of-the-art method is to melt glass in a continuous process in a tank production. Figure 7 shows a sketch of such a tank.
Compared to other glass industries mentioned in this book, the optical glass production is rather tiny. Seldom, the overall volume inside such tank exceeds more than 5 tons. Sketch of a melting tank for optical glasses including the spatial temperature profile.
The overall time consumption from the raw material melting to the casting takes several hours with courtesy of SCHOTT. A glass manufacturer feeds continuously the ingredients into the melting chamber. Gas burners and electrodes heat up and finally melt the ingredients. The picture in Figure 8 shows some still solid raw material on the liquid surface in the melting chamber. View inside the melting chamber with some still solid raw material on the surface of the melted material with courtesy of SCHOTT.
The melted material contains some bubbles due to residual air inside the raw material and due to chemical reactions between the ingredients. Just driven by convection, the liquefied material flows into the neighboring refining chamber. The increased temperature in the refining chamber leads to growing gas bubbles and so to a larger buoyancy. Additionally, the higher temperature, the reduced viscosity of the melt supports this upthrust, and the gas bubbles vanish.
Afterward, the melted material flows into the mixing chamber. A mechanical stirrer homogenizes the melt by rotational motion. The decreased temperature in the mixing chamber and feeder increases the viscosity of the melt in order to enable a proper hot forming during the casting. Figure 9 shows the hot forming process of a strip production. The glass is not yet frozen and still glowing red due to black body radiation.
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The still liquid glass is glowing due to the black body radiation . The outer part is already frozen, but the inner part of a strip is still liquid. So, the volume change during the freezing of the inner part cannot be compensated by the already solid outer part. If this stress exceeds a certain threshold, some cracks or breakage occurs. With increasing thickness of a strip, this risk of damage rises. Therefore, a controlled cooling process is necessary for optical glass. An annealing lehr of several meter lengths after the casting minimizes the risk of damage.
At the hot end of the annealing lehr has a similar temperature as the feeder and at the other end a few hundred degrees. Figure 10 shows a view through such an annealing lehr with an endless strip of optical glass inside. After the coarse annealing in the lehr, the glass manufacturer breaks or saws the glass strip into manageable length depending on the final application. Actually, the annealing rate has a significant impact on the optical position of the glass.
Controlling the chemical composition tightly is mandatory to hit the target values of the refractive index and the Abbe number. The annealing velocity influences the internal glass structure and so the optical features. The fine adjustment of the refractive index takes place in the so-called fine annealing. Therefore, ovens heat up each piece of glass again. At a target temperature around the glass-type specific transformation temperature, the stress inside the glass relaxes.
Figure 11 shows the influence of the annealing rate on the optical position. The red cross in the center of the diagram corresponds to the target value that is mentioned, e. The figure also contains the preferred tolerance steps for refractive index and Abbe number from ISO that specifies raw optical glass bluish boxes [ 14 ]. The green square depicts a piece of glass, which was annealed with a cooling rate of 0.
Obviously, the optical position is not within the accepted maximum tolerance range dark blue box. This piece of glass could be annealed again reversible process with a higher annealing rate along the annealing line inside the tolerance range [ 12 ]. So, the refractive index and Abbe number are fine-tuned by the annealing process. The printed annealing line is constant for a specific glass type but differs significantly from one glass type to another. Unfortunately, there are boundaries to the annealing rate. Below a glass-type specific annealing rate, the piece of glass tends to crystallize, which would lead to significant haze.
A thickness of meters? Not cm, but meters? How do you make high-purity glass, then? Glass is made of silica sand, quartzite, and other stones or sands from mine. Those stones or sands already look like glasses when mined. Glass is made from a mixture of sands and other materials. Glass is made of sands! Sounds interesting.
Each sand has a difference in the proportion of impurities. The higher the purity of the sand, the higher the purity of the glass. Sand that contains less impurities are expensive. Opticical glass is made of different quality of sand from other glass.
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In addition to that, as you mentioned, neither bubbles nor flaws are allowed in optical glass.
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