Fresnel lantern

A Fresnel lantern (pronounced frəˈnɛl or fruh-nel) is a common lantern 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 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.

These lenses are named after their inventor, Augustin-Jean Fresnel, who developed the lens initially for use in lighthouses.

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.

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Wikipedia