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Reflector

Discussion in 'Wiki' started by Grog12, Jul 14, 2007.

  1. Grog12

    Grog12 CBMod CB Mods Premium Member

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    ==Overview==
    Reflectors are the part of lighting instruments used to bounce stray light from the lamp out of the instrument in a predictable manner.

    Reflectors are typically made of a lightweight spun metal. After fabrication, this metal shell is given a highly reflective and durable surface treatment referred to as Alzak processing. Modern instruments (Such as the Source Four, and Shakespeare) have a dichroic-coated glass reflector that allows ultraviolet and infrared rays to pass out the back, reflecting only visible light. This results in a literally cooler beam of light as close to 40% of the heat causing EM radiation is filtered out. Some fixtures like the Selecon Pacific use a [[cold mirror]] in addition to the dichroic reflector. The mirror also has a dichroic coating that removes most of the rest of the heat energy from the beam. The Pacific features a cool enough temperature at the gate to be able to use transparency film as a projection media.

    ==Reflector Types==
    There are three types of reflectors commonly used in the theatre.

    ===Spherical===
    Spherical reflectors are most commonly used in Fresnels and PC Spots (Plano-Convex). The Reflector is shaped like part of a sphere, though not quite a hemisphere. The center of the imaginary sphere is the focal point of the reflector. Light rays from a source at the focal point travel away from the reflector and at it. Almost all the light that travels toward the reflector is reflected back along the exact same path through the source and out of the instrument. This Reflector type sends nearly 50% more light through the front of the fixture than would pass that direction without it.

    ===Parabolic===
    Parabolic reflectors are used in PAR-, MR-, and R type lamps. Like a spherical reflector, a parabolic reflector has a single focal point at shich the light source is places. The properties of a parabolic reflector are that all the light from the source that hits the reflector is reflected in parallel rays. This is the same principle behind satellite and microwave dishes. The parabolic reflector can be used to produce very narrow concentrated beams as in an [[ACL]], or through the use of diffusing lenses like in PAR lamps can produce wider beams.

    ===Ellipsoidal===
    Ellipsoidal reflectors are used in ellipsoidal reflector spotlights (funny thing about that), or "lekos," and most moving lights. The reflector is shaped like approximately half of an ellipse. By mathematical definition an ellipse has two foci, and in an ERS unit the lamp is placed at the focus closest to the reflector surface. The gate of the instrument is just beyond the second focus of the ellipse where the rays of light just pass/fit through it and the lens tube can be adjusted so that light that is focused by the reflector hits the lenses in the optimal place to be further focused dependent on the throw distance. Placing shutters or a template at the gate of the instrument allows highly focused light to pass through them, and then the lenses are able to focus the pattern on a surface.

    ==Inefficiencies==
    Reflectors are most efficient if used with a point source of light. A point source is a mathematically theoretical idea in that a point has no volume. Since we currently do not have any lamps that are a true point sources reflectors cannot be 100% efficient. To cope with the larger light sources and to make up for efficiency loss many reflectors are faceted. They are designed around the lamps that are recommended for the fixture to be as efficient as possible.
     
    Last edited by a moderator: Jul 14, 2007

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