Bulb Fitting (caps)

Index Technology Colour Hazards
Fittings Identification Levels Timeline

The cap connects the light bulb to the light socket and there are not as many different types as you might think. Sir Joseph Swan used a bayonet fitting for his first light bulbs and Thomas Edison used a screw connection. These developed into the two most common bulb connections today:

Name Bayonet   Edison Screw
Type designator BC   ES
Size designator B22   E27


These caps are used on all four main technologies:

  • Incandescent
  • Fluorescent (integrated forms only)
  • High Intensity Discharge
  • Light Emitting Diodes

They were developed for incandescent technologies which do not need a control system or any other treatment of the incoming current. Compact fluorescent technology, where a balast system is required, must incorperate a balast in the bulb, if they want to use these caps, but this makes the bulbs more expensive and compromises its performance. The solution was to develop a new type of cap only for these compact fluorescent lamps (CFL). They are generically called PL or plug in - light bulbs. We will look at these in a bit more detail later.


Traditional Variations

These two caps were quickly standardised as the electric lighting market developed but the one size did not fit every application. The two types were both scaled into the following variations:

Name Bayonet Small Bayonet   Giant Edison Screw Edison Screw Small Edison Screw
Type designation BC SBC   GES ES SES
Size designation B22 B15   ES40 ES27 ES14



Less common but still found are the incandescent strip lights:

Name Architectural   Picture
Type designation S14   S15


Halogen Fittings

Halogen light fixtures are broken into 2 categories, bulbs and capsules:

Base GU10   GU5.3   GU4   -   -   -
Capsule MR16   MR16   MR11   G9   GY6.35   G4
Voltage 240V   12V   12V   240V   varies   12V


You can find out more about the different features of all the bulbs in the section by looking in our identification guide.


Compact Fluorescent fittings

Integrated compact fluorescent lamps are designed to replace the existing incandescent bulbs so they use the bayonet and screw fittings above but non-integrated compact fluorescent lamps need a balast in the fitting. These non integrated CFLs come in a wide variety of shapes and sizes from the familiar looped tubes and spirals to the long thin tubes. If there is a connection at both ends then it is a standard fluorescent tube but if there is a single connection then it is a CFL:

G23d G23d-2 GX23d GX23d-2 G24d-1 G24d-2 G24d-3 GX32d-1 GX32d-2 GX32d-3 G24q-1 G24q-2 G24q-3 GX24q-2 GX24q-3 GX24q-4 2G-11