A fresnel lens (pronounced fre-NELL) is a special type of lens with raised circular ridges on its outer surface. The illustration on the left shows a cross-section of a fresnel lens.
Fresnel lenses are much thinner than conventional lenses, so they are lighter and lose less light through absorption.
Lights with fresnel lenses are often referred to simply as fresnels.
The light from a fresnel can be made to spread out (flood) or concentrate into a tight beam (spot) by adjusting the distance between the lamp and the lens.
The fresnel lens was invented by Augustin-Jean Fresnel in 1821 for use in lighthouses. Modern uses include overhead projectors, projection televisions and solar energy systems.