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.
Read more about this topic: Fresnel Lantern
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