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Lightbulb sockets, lamp sockets or lampholders provide electrical connections to the lamps and support it in the lighting fixture. The use of sockets allows lamps to be safely and conveniently replaced (re-lamping). There are many different standards for these lampholders, created by de facto and by various standards bodies. A general coding system is a letter or abbreviation followed by a number. Some miniature lamps have wire leads suitable for direct connection to wires; some reflector lamps have screw terminals for wire connections.
The most common type of sockets for mains electricity are Edison screws, used in continental Europe and North America, while bayonet mounts dominate in the Commonwealth countries and in the automotive industry. Fluorescent lamps require a different, typically four-pin design. A broad variety of other socket designs exists, applied for specialized lamp constructions.
Early experimental incandescent lamps had their wire leads connected to screw terminals, but this was inconvenient for commercial use. The Edison organization used simple wooden receptacles with copper strips inside them, when putting lamps on the commercial steamship Columbia (owned and operated by the Oregon Railroad and Navigation Company). These sockets included key switches, but were only useful for upright installations of the bulb. The Edison organization developed a screw-base in 1880, initially also of wood but later made with plaster of Paris.  Many competitive designs of lamp sockets flourished in the early era of incandescent lighting, using mutually incompatible electrical contact and mechanical fixing arrangements.
Construction and materials
The construction of a lampholder socket defines and limits its intended use. Ceramic insulation can withstand considerably higher operating temperatures than bakelite or other plastics. The electrical components and wires must be designed to carry the intended current plus a safety factor.
The contact surface area, thickness and conductivity of the metal, connection methods and maximum operating temperature must all be considered in the design of a new socket. In addition, mechanical factors such as shape of the socket, fixture mounting and attachment, lamp support, ease of re-lamping and total cost of manufacture must be considered. Sockets designed for ordinary household and industrial use have much more design leeway than those used in precision applications.
The lampholder must be located far enough from the filament that the metals with the lowest melting point will remain solid. Historically this metal was a tin/lead solder whose melting point might be as low as 180 °C (360 °F). Due to the thermal changes from ambient temperature to full operating temperature, the design of a socket must allow for a considerable amount of expansion and contraction. Spring elements are required to accommodate these dimensional changes. However - the temperature at which a metal loses its spring is far below the melting point. This is why some older sockets that no longer work can be restored by prying up the base spring slightly.
Lampholder failures are usually caused by mechanical abuse or by overheating. A socket with a built-in switch is far more likely to fail in normal use as the switch parts wear out. Insulation failures are usually caused by impacts or by difficulty inserting or removing a lamp. Sockets used outdoors or in damp areas often suffer from corrosion which can cause the lamp to "stick" in the socket and attempts to change a lamp can result in breakage of either the lamp or the lampholder. The corrosion is not only environmentally produced but may be a result of the current flowing through the parts when there is appreciable resistance between the parts. Fixtures in such environments may require gaskets or other waterproofing methods to prevent buildup of moisture in the socket area.
Edison screw bases
- E10 Miniature (Flashlight lamp)
- E11 Miniature
- E12 Candelabra
- E14 European
- E17 Intermediate
- E26 Medium
- E27 Medium
- E40 Mogul
- 3-Lite (modified medium or mogul socket with additional ring contact for 3-way lamps)
- Skirted (PAR-38)
The light bulb commonly used since the early 20th century for general-purpose lighting applications, with a pear-like shape and an Edison screw base, is referred to as an "A-series light bulb."
- Miniature bayonet
- Bayonet candelabra
- Bayonet Candelabra with prefocusing collar
- Medium prefocus
- Mogul prefocus
With bi-post bases, lamp orientation is fixed so filament will always be in the focal plane. Filament configurations such as the C13D (coiled, zig-zagged) emit far more light perpendicular to the zig-zag than parallel to it.
- Mogul bi-post (G38) can handle up to 100 Amps and is used with searchlights, film & stage lighting fixtures 1000 watts or larger. Incandescent, halogen and HMI light sources all use this design.
- Medium bi-post (G22) is used with film and stage lighting fixtures between 250 and 1000 watts (the development of the T14 base up to horizon design for ellipsoidal spotlights was one of the most important innovations of the mid 20th century).
- Mini bi-post (G4-G6)
- Medium bi-pin is used on each end of a T12 fluorescent lamp
- mini bi-pin is used with MR16 halogen lamps
The 2 pin socket is an update of the bi-post design with smaller pins designed to reduce the cost of manufacture. The 1000 watt FEL medium 2 pin base halogen lamp allows designers to insert the lamp into the end of the ellipsoidal reflector through a smaller hole than previously possible with conventional incandescent lamps. This improves efficiency compared to the older side inserted lamp or a double ended lamp which requires two holes. One variation is the Polarized 2 pin socket - used primarily in projectors, which defines the exact positioning of the filament on one side. This improves the "point source" characteristic necessary for building complex optical systems.
Another facet of the two pin design is that many new designs of lamps use baseless glass envelopes. The wire leads are thickened and crimped in the glass envelope of the lamp base. The MR16 is an example of this design. You may argue that the reflector itself is the base, but the industry has decided that the reflector is an assembly where the actual lamp is inserted into the reflector with the leads sticking out and a ceramic paste used to glue it in.
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Miniature lamps may have a wedge base made of glass or plastic.
Fluorescent tubular lamp standards
- T-5 mini
- T-8 medium
- T-12 large
Lamp base styles
|DC Bay||Double contact bayonet candelabra|
|DC Pf||Double contact prefocus candelabra|
|EMEP||Extended mogul end prong ferrule contact|
|Med Bp||Medium bipost|
|Med Pf||Medium Prefocus|
|Med Skt||Medium Skirted|
|Med 2P||Medium two pin|
|MEP||Mogul End Prong|
|Mog Bp||Mogul bipost|
|Mog Pf||Mogul prefocus|
|MS||Miniature screw (with reference shoulder)|
|MSP||Medium side prong|
|G38 Bp||G38 mogul bipost|
|R7S||mainly used at linear halogen lamps measuring 118mm or 78mm. Also known as a double ended halogen lamp. It may be replaced with identical size LED lamps.|
|Rect RSC||rectangular recessed single contact|
|RM2P||Rim mount two pin|
|RSC||Recessed single contact|
|SC Bay||Single contact bayonet|
|SC Pf||Single contact prefocus|
|SFc 10-4||Sleeve with threaded pin|
|SFc 15, 5-6||Sleeve with threaded pin|
|TB2P||TruBeam two pin|
|Tf||Trufocus (also four pin)|
|TLMS||Tru-Loc miniature screw|
|2PAG||Two pin all glass|
|2PAGC||Two pin all glass with ceramic cover|
|2PM||Two pin miniature|
|2PP||Two pin prefocus|
Some of the above base styles are now obsolete. The trend in recent years[when?] has been to design newer bases to reduce waste of raw materials and simplify the replacement process.
The United States standard for lamp sockets is published by ANSI and developed by NEMA. It is standard number C81.64, titled Guidelines and General Information for Electric Lamp Bases, Lampholders, and Gauges and outlines the dimensions and tolerances of standardized lamp sockets. Lamp bases are standardized in C81.61 Standard for Electrical Lamp Bases - Specifications for Bases (Caps) for Electric Lamps.  Both standards are harmonized with IEC standard 600061 and include dimensional data sheets as printed in the IEC standard.
- "The Advantages of Using LED Verlichting". The Difference of Connections. LED Spots & LED Inbouwspots. Retrieved 16 August 2013.
- Robert Friedel, Paul Israel, Edison's Electric Light: Biography of an Invention, Rutgers University Press, 1986, ISBN 0-8135-1118-6, pp. 169-171
- ANSI Std. C81.61-2009
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