crown glass

noun2

1. •
   A variety of glass used in lenses and other optical components due to its low refractive index and dispersion. Originally crown glass was made from alkali-lime silicates with approximately 10% potassium oxide, but similar properties may be attained with other recipes.

   • The convex half of an achromatic lens is composed of crown glass.
2. •
   An early type of window glass made by blowing glass into a hollow globe (crown) and then flattening it on a punty, reheating and spinning out into a flat disk by centrifugal force.