Drop-ball test

Optical glossary/Making and checking a lens

An impact-resistance test: a lens must survive a steel ball dropped onto it from a set height.

In more depth

In the United States, eyeglass lenses must resist impact before they are sold. The standard test drops a steel ball onto the lens's front surface from a fixed height; to pass, the lens must not fracture.

It is a minimum, not a measure of how strong a lens is. Materials differ widely above that line, which is why tougher materials are chosen for drilled rimless mounts, children's glasses and safety work, where the demand goes well beyond the test.