| Materials for glass lenses | Mean refractive index nd | Abbe number νe | |
| Crown glass | 1.525 | 58.3 | First glass material used for spectacle lenses |
| Barium crown | 1.604 | 43.8 | Differs optically from crown glass: greater refraction with low color dispersion |
| Heavy flint | 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 develop these lenses |
| Borosilicate | 1.5251.604 | 56.542.8 | Addition of silver chloride and silver bromide produces the photochromic properties. A further additive is boric acid |
| Barium flint | 1.684 1.755 |
44.2 38.1 |
Segment materials for bifocal and trifocal |
- Abbe number is the constringency of transparency
Glass is a term to describe material that whose chemical structure is similar to liquid but can be described as a solid. Glass has a viscosity that is high at room temperature which allows it to become a solid. Glass is a non-crystalline or amorphous structure. The atoms arrangement does not allow any type of periodic arrangement. A number of different types of glass exist due to different elemental combinations. Hardness and chemical resistance will vary due to the chemical composition.
Glass can have different metal oxides and fluorides added to change its optical properties and color. The addition of titanium, and lead can increase the refractive power. Barium oxide and floured can reduce dispersion. Glass must be melted to add dye for tinted sunglasses. Elements like iron and cobalt are uses to make glasses darker.
Furnaces, ovens and other heating apparatuses used to melt glass must maintain temperatures between 1400- 1500 C°. Gas bubbles contained in the glass melt can be removed by refining agents. This gives glass it smooth clear appearance. Stirring glass allows dyes to penetrate the glass melt and color it. Once melted, the temperature of the glass is gently lowered to allow formation of lenses in molds. The molds then compress the glass into shape and tempering can occur.