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A Fresnel lens is a glass or plastic lens originally invented to improve lighthouse capabilities. Made up of concentric angled rings of material, a Fresnel lens uses these rings to focus light onto the central part of the lens. Although Fresnel lenses are not common in lighthouses today, there have many other applications.
Augustin-Jean Fresnel is attributed the invention of the Fresnel lens. A French inventor fascinated with the properties of light, Fresnel worked extensively on scientific theories of light acting as a wave. Experts believe it was his research into the properties of light that gave him the idea for the lens that would bear his name; the first of his lenses was installed around 1823 at the lighthouse of Cordouan in France.
Until the development of the Fresnel, lighthouse range was a serious problem for ships at sea. The Fresnel lens greatly extended the capability of lighthouses, creating beams visible for more than 20 miles (32 km). Unlike traditional plano-convex lenses previously popular in lighthouses, the concentric circles of a Fresnel reduced the required thickness of a lens and was often considerably lighter than the burdensome, single-piece plano-convex models.
Today, Fresnels are made from glass or plastic and have found many modern uses. Thanks to mechanical glass-milling processes, the lens angles and concentric design can be etched into a single piece of material, rather than individually carved and assembled. One of the most common uses of the Fresnel is in traffic lights, where a strong but not particularly high-quality beam of light is required to alert traffic.
Fresnels are popular in theater and film productions for several purposes. Lenses developed for movies and theater are extremely flexible in design, allowing lighting technicians to easily adjust the light from a clear focused beam to a flooded position that allows a wash of light. The lights are also highly prized for their ability to create hard-edged, clean shadows on stage or in the frame. These shadows allow the light to be better manipulated and controlled, giving lighting technicians the ability to create specific nuances of light.
One common mistake when speaking of a Fresnel lens is mispronunciation. It is the mark of the beginner to pronounce the word as it is written, with a vocalized "s." The word is almost constantly pronounced as "Fre-nel," with the "s" remaining silent.