Fresnel lantern

Fresnel lantern


plano-convex lens of equivalent power

A Fresnel lantern (or merely Fresnel) is a spotlight used in theatre, which employs a Fresnel lens to wash light over an area of the stage. The lens produces a wider, soft-edged beam of light, which is commonly used for back light and top light.

The distinctive lens, named after Augustin-Jean Fresnel, has a 'stepped' appearance instead of the 'full' or 'smooth' appearance of other lenses. This allows the lens to have a much greater curvature than would otherwise be practical. The lens focuses the light by tilting each ring of glass slightly more towards the center as the distance is increased from the center of the lens. If the glass were completely flat, this would cause a corresponding pattern of circles of light, so Fresnel lenses are usually stippled on the flat side. This pattern of small bumps helps to break up the light passing through the lens and gives Fresnels their characteristic soft beam. This means that the intensity of the light is consistent across the spread of the beam of light, as opposed to being less intense around the edges as in an ERS. The stepped lens design causes less heat buildup than a plano-convex lens of the same angle. [cite web
last =Brain
first =Marshall
authorlink =
coauthors = and staff
title =How does a Fresnel Lens work?
work =
publisher =HowStuffWorks
date =
url =http://science.howstuffworks.com/question244.htm
format =
doi =
accessdate =2006-12-12
]

Construction

Theatrical Fresnels are typically made in 8, 6 or 3 inch varieties, referring to the diameter of the lens, with lamps ranging in power from 150 W (typically with a 3-inch fresnel) to 2000 W (with an 8-inch fresnel). The 3-inch variety is referred to as an inkie. Fresnel lenses can operate close to the light source and are very cheap to produce, so the lanterns tend to be small and cheap.

In film lighting, a much greater range of lens and lamp sizes are produced and used. For commonly available fixtures, lenses range in size from 2 to 24 inches, and lamp power range between 200 W and 20,000 W.

Fresnels use a spherical reflector, with the filament of the lamp at the focus point of the reflector. The reflector effectively doubles the light output of the fixture, as all light that is emitted backwards into the reflector is reflected back through the filament of the lamp and out the front. As with most lighting fixtures, the lamp and reflector cannot move independently, and remain a fixed unit inside the housing. It is this unit that is moved back and forth inside the lamp to focus the light. This is done by a slider on the bottom of the lantern, or by a worm track with a crank in the back of the unit. The lamps are almost always mounted 'base down', i.e. with the bulb standing upwards. Burning these lamps upside down will shorten lamp life significantly.

Fresnels are not very efficient. The reflector cannot be larger than the lens aperture, and thus all the radiated light that is neither redirected forward by the spherical reflector behind the bulb nor emitted directly through the lens is absorbed by the casing as waste heat.

The degree to which the lamp may be focused is limited by the length of the housing. To reduce the width of the beam, the lamp and reflector are moved further back from the lens (spot focus). However, the farther back in the housing the lamp is placed, the more light is wasted in the housing. [cite book
last =Box
first =Harry C
authorlink =
coauthors =
title =Set Lighting Technician's Handbook: Film Lighting Equipment, Practice, and Electrical Distribution
publisher =Focal Press
date =1993
location =Boston
pages =
url =
doi =
id =ISBN 0-240-80161-X
]

Use

In the theater and dance world, Fresnels are most often used for top or back lighting, at medium throw distances. In small venues, they are occasionally used for frontlight, although the relative lack of control when compared to an ellipsoidal is a major disadvantage. The distinctive soft beam of a fresnel can make it very useful as a special in certain situations. While the focus can alter the size of the beam, the distinctive scatter of light that the Fresnel lens emits also requires a way of controlling its shape. Since Fresnels cannot use internal shutters, such as those found in an ellipsoidal spotlight, they are often fitted with distinctive barn doors to control the spill and shape the beam of light. These are large metal flaps that may be mounted just beyond the color slot at the front of the lantern.

Electronic Theatre Controls introduced its Source Four PARNel as an alternative to Fresnel lanterns. The PARNel uses an enhanced aluminum reflector and has a wider focus range than standard fresnel lanterns.Fact|date=September 2008

References


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • Fresnel (disambiguation) — Fresnel can refer to physicist Augustin Jean Fresnel, or to the following topics associated with him:*Fresnel equations, describing light reflection and refraction *Huygens Fresnel principle, a description of wave propagation *Fresnel diffraction …   Wikipedia

  • Fresnel lens — A Fresnel lens (pronounced [ frɛz.nəl] or [freɪ nɛl] ) is a type of lens invented by French physicist Augustin Jean Fresnel. Originally developed for lighthouses, the design enables the construction of lenses of large aperture and short focal… …   Wikipedia

  • Fresnel equations — The Fresnel equations, deduced by Augustin Jean Fresnel (pronEng|freɪˈnɛl), describe the behaviour of light when moving between media of differing refractive indices. The reflection of light that the equations predict is known as Fresnel… …   Wikipedia

  • Fresnel lamp — Fres nel lamp , Fres nel lan tern Fres nel lan tern[From Fresnel the inventor, a French physicist.] A lantern having a lamp surrounded by a hollow cylindrical Fresnel lens. [1913 Webster] …   The Collaborative International Dictionary of English

  • Fresnel lens — Optics. a large lens with a surface of concentric grooves of prismatic profile, used in automobile headlights, searchlights, spotlights, etc. [1840 50; see FRESNEL] * * * Series of concentric rings, each consisting of a thin part of a simple lens …   Universalium

  • Augustin-Jean Fresnel — Infobox Scientist name = Augustin Jean Fresnel box width = image width =150px caption = Augustin Jean Fresnel birth date = Birth date|1788|5|10 birth place = Broglie (Eure) death date = death date and age|1827|7|14|1788|5|10 death place =… …   Wikipedia

  • Bird-cage lantern — A bird cage lantern was the style of lantern common to American lighthouses in the early years of the nineteenth century. The lanterns received their name because of their appearance; they are shaped like wire bird cages. When Fresnel lenses were …   Wikipedia

  • Lens lantern — A lens lantern is a small, self contained lamp structure which may sometimes be used to serve as a lighthouse. Unlike a regular Fresnel lens, the lantern requires no housing to protect it from the weather; its glass sides would refract and… …   Wikipedia

  • Stage lighting instrument — Stage lighting instruments are used in stage lighting to illuminate theatrical productions, rock concerts and other performances taking place in live performance venues. They are also used to light television studios and sound stages.Terminology… …   Wikipedia

  • Minot's Ledge Light — Minots Ledge Light Minots Light in a storm Location Offshore Cohasset, Massachusetts Year first constructed 1850 (first tower) Year first lit 1860 (current tower) Automated 1947 Deactivated 1851 1860 Foundation …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”