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Lamp/Globe

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

  1. Grog12

    Grog12 CBMod CB Mods Premium Member

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    =Lamp Parts=
    What is commonly refered to as a light bulb in hardware stores.
    Lamps are comprised of three parts.
    ==1. The bulb or envelope==
    The glass envelope that encloses the inert gas or vacuum. In household bulbs is usually made of regular glass.

    We use Tungsten-Halogen (T/H) lamps that have a Quartz envelope and have halogen gas (usually bromine) inside of them.

    ********DO NOT TOUCH THE GLASS********

    The oil from your hand has the potential to heat up and boil off, which weakens the envelope. This can cause the lamp to decompress and fill with oxygen which will cause the filament to burn up and the lamp to fail. Sometimes the envelope can be weakened to the point where the lamp explodes.

    Envelopes come in many shapes and sizes, with each type being designated by a letter or letters and a number which designates the lamp diameter in eighths of an inch. Typical household light bulbs come in either A (arbitrary) PS (pear-shape with straight sides) or G (globe-shaped). In the theatre we use primarily T (tubular) shaped and PAR lamps. You may come across lamps like the PAR 64, the 64 denotes that the lamp is 64/8 of an inch in diameter or 8". Many cyc lights take a T3 lamp, or a tubular lamp that is 3/8" in diameter. The most popular household light bulb is the A19, measuring 2 3/8" across its largest diameter.

    ==2. The base==
    The base holds the lamp in proper position and provides electrical contact.
    Bases have three important functions:
    • It precisely holds the lamp in a predetermined position, critical to the proper operation of a reflector
    • It conducts electrical current from the socket to the filament
    • It allows for quick and easy lamp replacement.
    Normally the size of a base varies with the wattage of a lamp. Large bases are called “mogul,” middle-sized are called “medium,” and small bases are called “miniature” or “candelabra.” Common base styles are bipin. bipost, prefocus, bayonet, end prong, and screw-base. The same bases are often refered to other ways. The bipin base of the FEL-lamp family, fits in a TP22 or TP220 or G9.5 socket. A mogul bipost socket is a G38.

    ==3. The filament==
    The Filament is a piece of thin wire, often coiled, that when electrical current is passed through it heats up to incandescence, meaning that it glows. It is typically made out of Tungsten alloy. When the tungsten heats up it is thrown off the filament, but it reacts with the halogen gases within the envelope and is redeposited on the filament. This prolongs lamp, but the [[Tungsten Cycle]] may only work when the lamp is operation is at near 100% output.

    There are many forms of filaments. Theatrical fixtures primarily use a “coiled coil” which is longer and narrower than other filaments. The ideal filament would be a theoretical point source, for most efficient use of the reflector.

    The filament becomes supple when it heats up, as much as 800°F. In this state any excessive jarring can cause the filament to break.

    ===Light-Center Length===
    L.C.L. refers to the distance from the center of the filament to a predetermined place in the base which varies from base to base. It’s important to be aware of it when working with a reflector or lens so you can align it with the focal point of the fixtures reflector.

    ==The ANSI Code==
    The American National Standards Institute has established a system for identifying lamps using a three-letter code. If one lamp differs in ANY way it is assigned a separate ANSI code. The codes are non-descriptive by themselves. But they do specify a certain lamp and make it easier to order. Prior to the three-letter codes a lamp was often designated by its wattage, shape, envelope size, and a modifier, such as "750T12/9"--the most common incandescent lamp for Radial ERSs, or "500T20/8"--for 6" Fresnels. The modern T/H replacements are EGG and BTL, respectively.

    ===Specifying/Purchasing Lamps===
    Almost always, a compromise must be made between lamp life and output. Since theatre lamps are rarely run at 100% for their entire life, one expects lamps to last longer than their published life. Most buy the brightest lamp for a given wattage. The exception would be fixtures that are difficult to access for relamping, or in institutions where budget is a critical factor. Long-life lamps often are not significantly more expensive than regular ones, but do suffer a loss of output, and often a lower color temperature as well.

    Other factors:
    Color Temperature, Nominal Operating Voltage, MOL (Maximum Overall Length), domestic vs. imported.
     
    Last edited by a moderator: Feb 2, 2009

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