Optical Glasses

The most common types of glasses used in optics are crown glasses and flint glasses, designations based on their dispersions. Flint glasses contain lead. These designations are further subdivided by composition and have letter designations and number designations called "glass numbers".

Example data for glasses: Glass Glass Number Density gm/cm3 Borosilicate BK7 517642 2.51 Crown K5 522595 2.59 Dense barium crown SK4 618551 3.57 Dense flint SF6 805254 5.18

Common crown glasses have indices of refraction around 1.5-1.6, while extra dense flint glass may have an index as high as 1.75 . Lenses of crown and flint glasses are often used in multi-component lenses because of their complementary properties. For example, a strong positive crown lens with its low dispersion may be used in a doublet with a weaker negative lens of flint glass (high dispersion) to correct for chromatic aberration.

The design of multi-component lenses requires very exacting specifications for the glasses used. Professional optics books have detailed tables of glasses with their glass numbers, densities, softening temperatures, etc. For example, Table 11.6a in Waynant & Ediger.

Composition of Glasses

Glasses can be considered to be solutions, rather than chemical compounds. According to Meyer-Arendt, about 95% of all glasses are of the "soda-lime" type, containing silicon dioxide (silica), Na2O (soda), and CaO (lime). Crown glass is a soda-lime-silica composite. Flint glasses contain 45-65% lead oxide - they are high-density, high-dispersion, high-refractive-index glasses. There are glasses which have barium oxide rather than lead oxide; they are called barium glasses. Barium glasses have refractive indices comparable to the flints, but have lower dispersions. Other heavy elements are used to make flint glasses, such as lanthanum and the rare earths.

As alternatives to silica glasses, other "glass formers" such as boron oxide (B2O5), phosphorus pentoxide (P2O5) and germanium oxide (GeO2) can be used.