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In glass lenses a distinction is made between mineral glass (Abbe number > 55) and flint glass (Abbe number < 50).
The term "crown glass" (B 270) was first introduced to describe the appearance of the blown, circular glass plates with crown-shaped attachments once made in England. The name flint glass comes from an earlier production technique in which pure, brightly coloured flint stone (quartz) was used.
Before 1886 the standard crown and flint glasses were the only types known. The development of new materials started after this date. Research into ways of producing new types of glass, and types with a higher refractive index in particular, is continuing to this very day. The goal is to achieve as low a dispersion as possible by the addition of suitable substances, even for materials displaying a high refractive index. At the same time, a high level of hardness and chemical resistance must also be guaranteed for the finished spectacle lenses.
The term "glass" is used to denote all materials whose chemical structure is similar to that of a liquid, but whose viscosity at a normal temperature is so high that they can be described as solids.
Glass has an amorphous (non-crystalline) structure, i. e. the configuration of the atoms or molecules does not follow any principle of periodic arrangement. A large number of different types of glass exist, from white to tinted, and from clear to opaque.
Mineral glass is obtained by overcooling a melt and is hence also often described as an overcooled liquid. In actual fact, glass is not a solid but even in its solid state displays a certain viscosity which is not noticeable in everyday use.
Mineral glass is the product of a melting process. The composition of the glass melt is as follows:
70% glass former (quartz)
20% fluxing material (potash and soda)
10% glass hardener (oxides)
By the addition of different metal oxides and fluorides (1%), the optical properties and colour of the glass can be deliberately changed. The addition of lead, titanium and lanthanum oxide increases, for example, the refractive index, while barium oxide and fluoride reduce dispersion. The glass melt can also be dyed for tinted sunglass lenses by the use of iron, cobalt, vanadium and manganese. To obtain photochromic properties, metal compounds are added with fluorine, chlorine and bromine (halides) to the melt.
All of the substances required to produce the required glass are melted in a furnace at temperatures of between 1400 and 1500 °C. The gas bubbles contained in the viscous melt can be removed by the addition of so-called fining agents. Stirring for several hours after the fining procedure prevents streaks, inclusions and colour casts. After the melting process, the glass melt is directed – at a slightly lower temperature – through a dosage unit to an automatic press where the pressings are then produced. These pressings are cooled in a special oven known as a lehr before being processed into finished spectacle lenses.
|Glass or mineral lenses
|Very high refractive indices allow the production of thin lenses, even for high prescriptions||Large range of refractive indices from n = 1.5 to n = 1.9
|Resistant to scratches, hence greater durability and longer lens life||Good surface hardness
|Fewer colour fringes than plastic lenses with same refractive index||Low dispersion, even with high refractive index
|No palpable edges in bifocal and trifocal lenses||Good fusability of different materials|
|Unproblematic disposal of by-products resulting from manufacturing process||Good environmental compatibility of manufacturing process|
|No deformation and therefore no impairment of optical properties at high temperatures||High thermal resistance
|Equitint lenses and cemented segments possible, e. g. with different prismatic powers in near and distance portions||Good cementing properties of the materials
|Plastic or organic lenses|
|High refractive indices allow the production of thin lenses, even for higher prescriptions||Range of refractive indices from n = 1.5 to n = 1.74
|Lightweight spectacles which are comfortable to wear||Low density
|Very suitable for sports and children’s spectacles||High resistance to breakage
|Tinting using dipping process, irrespective of prescription, in whatever colour the wearer requires||Extensive tinting possibilities
|Uniform darkening of plastic photochromic lenses, irrespective of power||Incorporation of photochromic substances in lens surface
|No damage to the lens in welding or grinding work||Very resistant to sparks|
|A hard coating is necessary to achieve a similar hardness to that of glass lenses||Low surface hardness
|Materials for glass lenses
||Examples of lenses in which used
||Mean refractive index nd
||Abbe number νe
||ZEISS Single Vision Sph Mineral 1.5
||First glass material used for spectacle lenses
||ZEISS Single Vision Sph Mineral 1.6||1.604||43.8||Differs optically from crown glass: greater refraction with low colour despersion|
||ZEISS Single Vision
- Sph Mineral 1.7
- Sph Mineral 1.8
- Sph Mineral 1.9
|High-index material for high prescriptions. Additives include titanium and lanthanum. In 1973 Schott received an award for being the first company to develope these lenses|
||ZEISS Single Vision
- Sph Mineral 1.5 Umbramatic brown
- Sph Mineral 1.6 Umbramatic brown
|Addition of silver chloride and silver bromide produces the photochromic properties. A further additive is boric acid|
|Barium flint||ZEISS Bifocal Classic CT25 Mineral 1.6||1.684
|Segment materials for bifocal and trifocal|