An Etymological Dictionary of Astronomy and Astrophysics

A method used to test the → image quality of → mirrors and → lenses. The test is performed by moving a knife edge laterally into the → image of a small → point source. The → eye, or a → camera, is placed
immediately behind the knife edge, and the → exit pupil of the system is observed.

See also: Named after the French physicist Léon Foucault (1819-1868),
who invented the method; → knife; → edge; → test.