Crown glass (optics)

A molten blob of glass was attached to a pole and spun rapidly, flattening it out into a large disk from which windows were cut.

The center, called the "crown" or "bullseye", was too thick for windows, but was often used to make lenses or deck prisms.

Borosilicates contain about 10% boric oxide, have good optical and mechanical characteristics, and are resistant to chemical and environmental damage.

In that application, it gives better image quality and a round exit pupil.

The dispersions of the glasses partially compensate for each other, producing reduced chromatic aberration compared to a singlet lens with the same focal length.