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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".
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.
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