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Fresnel - Description :
Fresnels are typically 8-, 6- or 4-inch, referring to the diameter of the lens. This light is named after Augustin-Jean Fresnel who invented the distinctive Fresnel lens which has a 'stepped' appearance instead of the 'full' or 'smooth' appearance of other lenses. The stepped nature of the lens causes 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 into the lens to smooth out its eventual pattern.
Fresnels use a spherical reflector, with the filament of the bulb at the focus. Due to this, the bulb and reflector cannot move independently of one another, and remain a fixed unit inside the housing. It is this unit that is moved back and forth inside the lamp to focus the fresnel. This is done by a slider on the bottom of the light, or by a worm track.
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 or emitted directly through the lens is absorbed by the casing as waste heat. Additionally, the degree to which the lamp may be focused is limited by the length of the housing. The tighter the focus ('spotted in' by moving the lamp and reflector back from the lens) the less light is able to escape. Thus fresnels are not good for tight focus on small areas. Fresnels also lack internal shutters, and must rely on barndoors, large metal flaps that may be mounted just beyond the color slot at the front of the light. Due to these restrictions, fresnels are most often used at middling distances for area lighting. Fresnel lamps are almost always 'base down': mounted with the bulb up. Burning these lamps upside down will shorten lamp life significantly. (In UK: Fresnels are not spotlights, they are Fresnels.)