Glass

Materials used for Glass Lenses

Glass
Glass spectacle lenses from ZEISS with different refractive indices from 1.5 to 1.9 (top) and Umbramatic spectacle lenses (bottom)

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.

What is Glass?

Optical glass
Optical glass (Photo courtesy of Schott Glas, Mainz)

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.

What is mineral glass?

Mineral glass

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 invalid tinted sunglass lenses by the use of iron, cobalt, vanadium and manganese. To obtain invalid 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 during the melting process in the melting tank  (Photo courtesy of Schott Glas, Mainz)
Glass during the melting process in the melting tank (Photo courtesy of Schott Glas, Mainz)

Glass or plastic?

Tips and advice

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

Examples of lens types

Materials for glass lenses Examples of lenses in which used Mean refractive index nd Abbe number νe  

Crown glass

ZEISS Single Vision Sph Mineral 1.5

1.525

58.3

First glass material used for spectacle lenses

Barium crown 

ZEISS Single Vision Sph Mineral 1.6

1.604

43.8

Differs optically from crown glass: greater refraction with low colour despersion

Heavy flint

ZEISS SingleVision-

Sph Mineral 1.7-

Sph Mineral 1.8-

Sph Mineral 1.9

 

1.706

1.800

1.893

 

39.3

35.4

30.4

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

Borosilicate

ZEISS Single Vision- Sph Mineral 1.5 Umbramatic brown- Sph Mineral 1.6 Umbramatic brown

1.525

1.604

56.5

42.8

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

1.755

44.2

38.1

Segment materials for bifocal and trifocal