There are also many other bayonet cap variants including the 3-pin BC B22d-3 sometimes used on Fireglow lamps but perhaps more commonly on High-pressure mercury lamps for industrial applications. The BY22d is used on some low-pressure sodium (SOX) lamps.
Edison Screw Cap
Named after the pioneering inventor Thomas Edison, the Edison Screw cap or “ES” lamp fitting is used worldwide in a vast range of applications.
The most popular ES or E27 fitting is 27mm diameter and is widely used in UK and Europe. This cap is the standard 27mm diameter screw cap for UK 240V light bulbs – not to be confused with E26, which is a 26mm size and is designed for the 120V US market.
Miniature halogen capsule light bulbs are generally used in integrated fittings and appliances, such as cooker hoods and kitchen cabinet lights.
Capsule light bulbs can be identified by their miniature size and they all have 2-pins extruding out of the base of the capsule. The model of the capsule is designated by the measurement in mm between these two pins.
The different distances between the pins prevent the wrong type of capsule light bulb being inserted into the wrong fitting – such as a 12V G4 capsule being inserted into a 240V G9 light fitting.
The G4 is specifically used for 12V capsule light bulbs. These capsules require a 12V transformer or driver to operate – this is usually built into the fitting. The capsule can be identified by measuring the distance between the centre points of the pins. The distance of a G4 measures 4mm.
The GY6.35 is specifically used for 12V/24V capsule light bulbs. These capsules require a 12V transformer or driver to operate – this is usually built into the fitting. These capsules are often used for task lighting. The capsule can be identified by measuring the distance between the centre points of the pins. The distance of a GY6.35 measures 6.35mm.
LED Capsule Bulbs are now also available as an energy-efficient alternative to halogen light bulbs. The fitments on LED capsules are exactly the same as those on halogen capsules, so if you’re replacing your current light bulbs with LED light bulbs and your fitting accepts one of the standard types listed here, you should be able to install them without an issue.
Linear Halogen light bulbs for floodlights and up-lighters have a 7mm R7s cap at each end of a long quartz linear light bulb. All linear halogen light bulbs have the same cap but come in either 78mm lengths or 117mm length light bulbs. They also come in a variety of wattages, so it is important that a replacement linear halogen is like-for-like (same wattage, same length).
Featuring two pins that twist and lock the spotlight into the fitting, the GU10 is the most popular spotlights used in the UK. This spotlight is mains operated 240V and was initially developed as a halogen spotlight but is nor widely available in LED. Various wattages, colours and beam angles can now be purchased to provide lighting in many different applications.
This base of this lamp is technically named GU5.3, however, the whole spotlight is more commonly referred to as an MR16. Featuring two round pins that push into the fitting, this is the 2nd most popular type of spotlight. The MR16 spotlight operates at 12V so requires a 12V transformer or driver to run- these drivers are sometimes built into the fitting but can also be purchased separately.
The GZ10 spotlight looks very similar to the GU10 lamp, but there are two major differences. The GZ10 spotlight makes use of a dichroic reflector to direct all of the heat generated by the spotlight backwards, creating a cool beam spotlight. The GZ10 spotlight, therefore, features a square corner base (where the GU10 has a bevelled base). This stops the GZ10 being used in a light fitting designed for a GU10 where this backwards reflected heat could damage the fitting.
Note: although a GZ10 cannot be used in a GU10 fitting, a GU10 can be used in either a GU10 or GZ10 fitting.
Fluorescent Tubes and LED Tubes
Fluorescent and LED tubes have a two-pin fitting at both ends of the tube.
The most common size of fluorescent or LED tube is the T8 which uses a G13 two-pin cap on both ends of the tube. The distance between the pins is 13mm and when inserted into the fitting, the tube twists to lock into place.
Also found on T10 (tube diameter: 10/8 inch) and the larger T12 (tube diameter: 12/8 inch).