The most common types of glasses used in optics are crown glasses and flint glasses, designations based on their dispersions .
Flint glasses contain lead.
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. 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. Barium glasses have refractive indices comparable to the flints, but have lower dispersions.