Lightbulb sockets, light 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.


bulb fitting guide 3bulb fitting guide 2


Edison Screw Bases

  • E10 Miniature (Flashlight lamp)
  • E11 Mini-Candelabra
  • E12 Candelabra
  • E14 European
  • E17 Intermediate
  • E26 Medium
  • E27 Medium
  • E39 Mogul
  • E40 Mogul
  • 3-Way (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.”

Bayonet Styles


A bayonet mount

  • BA9s Miniature bayonet
  • BA15s Single Contact Bayonet
  • BA15d Double Contact Bayonet
  • Bay15d Indexed DC Bayonet
  • Bay22 Double Contact Bayonet
  • Bayonet Candelabra with prefocusing collar
  • P28s Medium prefocus
  • P40s Mogul prefocus



115-volt airway beacon light bulb with a Mogul bi-post base

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)

Common types:

  • G4 – 4mm pin spacing
    • GU4 & GZ4 – are same as G4 and only denote what lamp mount clip is needed to hold the actual light bulb in place
  • G5.3 – 5.3mm pin spacing
    • GU5.3, GX5.3, GY5.3, GZ5.3 – all are same as G5.3 and only denote what lamp mount clip is needed to hold the actual light bulb in place
  • G6.35 – 6.35mm spacing
    • GY6.35 & GZ6.35 – all are same as G6.35 and only denote what lamp mount clip is needed to hold the actual light bulb in place
  • G8 – 8m pin spacing
    • GU8 – same as G8 but denoted what lamp mount clip is needed to hold the actual light bulb in place
  • GY8.6 – 8.6mm pin spacing
  • G9 – 9mm pin spacing
  • G12 – 12mm pin spacing

Bi-Pin Connector

Main article: Bi-pin connector


Metal halide lamp with G8.5 base

  • Medium bi-pin is used on each end of a T12 fluorescent lamp
  • Mini bi-pin is used with MR16 halogen lamps

The two-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 two-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 two-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; the actual lamp is inserted into the reflector with the leads sticking out and a ceramic paste used to glue it in.

Wedge Base

Miniature lamps may have a wedge base made of glass or plastic. The base may be an extension of the glass envelope of the bulb, with the wire leads of the lamp folded up at the base. Some wedge bases are made of plastic and slipped over the wire leads. A wedge base holds the lamp by spring compression in the socket. The lamp is inserted and removed without twisting. Wedge base lamps are widely used in automotive applications, and many Christmas lights strings use plastic wedge-based bulbs.

Fluorescent Tubular Lamp Standards

Fluorescent Linear Tube Light bulbs are measured in 8th’s of inches. So a T12 fluorescent is 12 – 8th’s of an inch in diameter or 12/8 = 1.50″

  • T4 – 4/8 or 0.500″ in diamater
  • T5 – 5/8 or 0.625″ in diameter
  • T8 – 8/8 or 1.00″ in diameter
  • T10 – 10/8 or 1.25″ in diameter
  • T12 – 12/8 or 1.50″ in diameter




By | 2018-07-23T13:50:22+00:00 August 8th, 2017|Blog|0 Comments

About the Author:

Leave A Comment