Fresnel lantern Totally Explained

Everything about Fresnel Lantern totally explained

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 apposed to being less intense around the edges as in an ERS. The stepped lens design causes less heat build up than a plano-convex lens of the same angle.

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's 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 can't move independently, and remain a fixed unit inside the housing. It is this unit that's 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', for example with the bulb standing upwards. Burning these lamps upside down will shorten lamp life significantly.
   Fresnels are not very efficient. The reflector can't be larger than the lens aperture, and thus all the radiated light that's neither redirected forward by the spherical reflector behind the bulb or 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.

Use

In the theater/dance world, Fresnels are most often used for top or back lighting, at medium throw distances. In small venues, they're 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 can't use internal shutters, such as those found in an ellipsoidal spotlight, they're 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.

Further Information

Get more info on 'Fresnel Lantern'.

External Link Exchanges

Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:

<a href="http://fresnel_lantern.totallyexplained.com">Fresnel lantern Totally Explained</a>

Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned.