crown glass

English

So called from a crown-like shape given to it in the process of blowing.

crown glass (usually uncountable, plural crown glasses)

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.
(optics) 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.

type of window glass

variety of optical glass