An Etymological Dictionary of Astronomy and Astrophysics

An optical lens composed of a series of rings of glass so curved that they all have the same focus. It is flat on one side and ridged on the other making it possible that nearly every ray of light from the source be re-directed out on a horizontal path.
This design enables the construction of lenses of large aperture and short focal length avoiding thus the large weights and volumes of material which would be required in conventional lenses. The first Fresnel lens, designed for use in a lighthouse on the river Gironde,
was installed in France in 1823, and by the 1850s many examples were in use everywhere. Fresnel lenses are most often used in light gathering applications, such as condenser systems or emitter/detector setups. They can also be used as magnifiers and projection lenses.
Nowadays, Fresnel lenses made of optical plastics are widely used for various applications.

See also: → Fresnel diffraction; → lens.