Named after its lens, the Fresnel (fra-'nel) [the "s" is not pronounced] is probably the second most popular type of theatrical spotlight. Augustin J. Fresnel created the fresnel lens in the 1820s for use in lighthouses. (See this link.) There were problems with the purity of glass in the 1800's and a large solid lens placed in a lighthouse would break because of the heat of the light source. Since lenses work by refraction, it's the surfaces of a lens which do the work, the glass in the middle doesn't really matter. Fresnel took a convex lens and cut a series of concentric grooves into it, essentially removing the glass from the middle. His new lens would still approximate the focus of the original lens but was easier to fabricate and had less light loss through the glass. A side effect of Fresnel's lens is that some light hits the vertical portions of those grooves and bounces off at random stray angles. Around 1930, the Fresnel lens was placed into a spotlight for use in the motion picture industry. While Kliegl Brothers claim to have invented it in 1929, Mole-Richardson won an Academy Award (Oscar) for the use of the Fresnel for motion pictures in 1935.
The modern Fresnel has changed very little in the last seventy years. The fixture consists of a simple metal housing, a small spherical reflector and lamp socket that both slide toward or away from a lens. The back of the lens is "Pebbled" meaning it has a series of small bumps which diffuse the light before it goes into the lens. The reflector and lamp slide forward and backward which narrows or widens the beam.
Fresnels are classified by the diameter of their lens. Fresnels have been made in sizes from 3 inches to 36 inches, using incandescent lamps from 100W to 10,000W. Today most theatrical Fresnels are either 6 inch or 8 inch, and 500W-2000W.
Fresnels are used for a soft artistic feeling light. They have a defined hotspot in the center, but the edges blend and fade with no discernible outer edge. They are a short-throw instrument with a maximum useful range of about 25 feet for an 8-inch Fresnel. Due to the way the light spreads without ever focusing like in an ERS, one can not put a gobo or shutter into a Fresnel. In order to control the stray light, Fresnels use Barn Doors or Top Hats.
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