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Glossary of Lighting Terms

There is a lot of jargon and many, many acronyms to decipher when it comes to shopping for light bulbs. We want to do everything we can to make it easier to find the right bulb, so we have collated a glossary of all the lighting terms and acronyms you might come across while browsing our site.

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A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Directional lighting used to highlight particular areas or objects. Used in retail displays and galleries.

Describes the temperature of the air around a light fitting. Ambient temperature can affect the performance of LED lighting more than does other forms of light bulb. LED bulbs now often have heat sinks built into them to help dissipate the heat that they generate.

A measurement of the amount of electric current flowing through a circuit. It is often shortened to ‘amp’. Amps are used, along with volts, to calculate watts. Watts represent how much power a light source uses.

A device used to manage the power to fluorescent and high intensity discharge lamps. It controls the current and provides the power necessary for these light sources to operate. A ballast is an alternative to a starter switch.

The bayonet cap (BC) is the most commonly used fitting in homes and workplaces. It is a 'push-and-twist' type of cap, with two small horizontal spikes protruding from the base of the bulb. The versatility of the fitting means that there is a wide range of light bulbs available with this cap type. Also known as a B22d cap, it has a variant called the small bayonet cap.

Measures the spread of a light source. A standard GU10 bulb has a beam angle of around 38 degrees. Lighting with a wider beam angle is more suitable for general purpose. Narrower beam angles are good for highlighting specific objects or areas in a room.

A bi-coloured LED contains two separate colours of LED. This is most commonly red and green. These two LEDs can be lit separately or combined to create over 256 different shades of colour.

The process of sorting LEDs. They can be divided using many different criteria, such as colour temperature and brightness. Binning ensures the quality and consistency of LED lights. Exactly how LEDs are sorted will depend on their eventual application.

Used to describe the visual intensity of a display screen. Brightness is measured in Nits. A Nit is the same as one candela per square metre. Brightness is often confused with luminous flux, which measures the intensity of a light source.

The base of a bulb secures it in place and connects it to a power source. There are many types and sizes. Different types of light bulbs are often identified by their bases. Common types include bayonet cap (BC) and Edison screw (ES). See here for more information about bulb cap types and bases.

This describes the amount of light emitted in a particular direction. A candle, for example, emits one candela of light in all directions.

A standard light bulb shape, so named because the bulb resembles the shape of a candle flame. These are mainly used in decorative lamps, or in light fixtures where the bulb itself is on display. Candle light bulbs are available in a range of bayonet cap and Edison screw caps.


The cap is the part of the light bulb that connects it to its fitting. There are many cap types available, but the most common are B22d bayonet caps (BC) and E27 Edison Screw caps (ES).

Any luminaire that is designed to be fitted to a ceiling. The most common type of light fitting, found in most rooms in domestic and commercial environments.

A type of bulb, also known as CFL, that is used as an energy efficient alternative to incandescent lighting. CFLs are fluorescent tubes turned into more compact shapes (hence their name). However, LED lights have a longer lifespan and use less electricity, so are quickly becoming more popular than CFLs and incandescent bulbs.

This indicates how clearly a light bulb replicates the colour of objects it is illuminating. It works on a scale of 1-100, and the higher the rating, the more faithfully that colours are reproduced. High CRI bulbs are mainly used for photography and exhibition purposes, where it is important that colours are accurately reproduced.

A measure of how warm or cold a white light source appears. Warm light sources offer a softer, yellower light, while cool light bulbs emit a bluer, more intense light. Colour temperature is measured in Kelvins (K), and the lower the number, the warmer the light. A typical warm white light bulb for the home will range between 2700K-3200K, while cool white bulbs can be anything above 4000K. See here for more information about colour temperature.

A term used to describe the colour temperature of a particular type of bulb. Cool white light bulbs emit a bluer, more intense light suited for bathrooms, kitchens, and workspaces. See here for more information about colour temperature.

Describes the amount of light delivered to a surface by a luminaire. It is measured in lux, which measures the brightness of light over an area.

A kind of glass used as a reflector in some LED lights. It acts as a filter, letting visible light pass forward, and reflecting infrared light backwards. This means less heat is directed towards the subject of the lamp.

A diffuser is applied to a light source to spread light more evenly. It is made of translucent glass or plastic, and gives a much softer light. This is useful for general lighting, where the light needs to be less directed.

Switches that allow you to adjust the brightness of a bulb or a group of bulbs. The two common types are leading edge and trailing edge dimmers, although dimmable LED light bulbs must be used with LED-specific dimmer switches.

The Edison screw (ES) cap is one of the most common types in use today. It is named after the inventor Thomas Edison, and like its name suggests, it is the type of cap that simply screws into its fitting. Many ceiling lights, floor lamps and wall light use this type of fitting, so there are a wide variety of bulbs available with this cap. Also known as an E27 cap, variants include the small Edison screw (SES) and giant Edison screw (GES).

Lighting that is designed to work when mains electricity fails, usually powered by a rechargeable battery. Emergency lighting is useful when normal power may be cut off, and it is a health and safety requirement for areas around fire exits.

The 'equivalent wattage' makes it easier for consumers to understand the brightness of an LED light bulb by comparing its wattage to an incandescent bulb. For many years, people used a bulb's wattage to determine its brightness. A 100W bulb is brighter than a 40W bulb, for example. However, with the development of low-power, energy-saving LED light bulbs, wattage alone can't be used as a measure of a bulb's brightness. For example, a 5W LED light bulb is just as bright as a 40W incandescent.

A strong of lights, commonly used for decorative outdoor lighting or seasonal light displays. Festoon lighting is simply made up of a cable on which are fixed light bulb fittings at various points.

The thin wire enclosed in incandescent bulbs that generates light (and heat) when a current is passed through it. The filament is a delicate component and is usually the main reason an incandescent light bulb fails. LED bulbs do not use filaments, although there are some available that are designed to mimic the look of traditional incandescent bulbs.

A kind of tube shaped light, often used for kitchen lighting and in offices. Fluorescent lights contain mercury gases, which are released if the lamp is broken. Mercury gas can be dangerous, so LEDs provide a safer and more environmentally friendly option.

The kind of lighting used to illuminate rooms and open spaces. GLS and candle light bulbs are the most common types.

A faint glow from a light source after it has been turned off. It is caused by power remaining in the circuit for a short period of time.

The giant Edison screw (GES) is a larger cap type mainly used in commercial lamps over 500 watts. Like the regular Edison screw cap, it simply screws into the fitting. It is difficult to mistake a giant Edison screw fitting, as it is noticeably larger than the standard Edison screw, and it is used on comparatively few bulbs. Also known as an E40 cap.


The ‘classic’ light bulb shape, a pear-shaped bulb, tapering to a narrow base. Stands for General Lighting Service. They are also sometimes known as 'A-shape' or 'A60' light bulbs.

Halogens are a group of five related elements on the periodic table: Fluorine, Chlorine, Bromine, Iodine, and Astatine. Halogen gases - particularly iodine and bromine - are used in a particular type of incandescent light bulb, known as halogen bulbs. An EU directive has banned the manufacture of halogen light bulbs in Europe; the last stage of the ban affects all non-directional bulbs (e.g. GLS, candle) and will take effect in September 2018.

A heat sink is used to draw heat away from certain parts of an electronic circuit. Because LEDs are sensitive to heat, a heat sink is often an important part of an LED luminaire. Heat sinks help to preserve the lifespan of LED lights.


HID stands for High Intensity Discharge. It describes a kind of light source used in warehouses, football stadiums, and large public areas. They are typically very bright and are also energy efficient. They are often used as car headlights.

A very bright LED to create the latest lamps. These lamps provide a light source that is closer to traditional lighting such as CFL (Compact Fluorescent Lamp) and incandescent lights.

The oldest kind of electrical light bulb. Incandescents are very inefficient, as 90% of the energy they produce is lost to heat. Because they get so hot, incandescent lights can be considered a fire hazard. Incandescent bulbs also have a short life span.

Ingress Protection (IP) ratings describe the level of protection that a bulb or electrical fitting has. It is a two-digit number, the first of which refers to how protected it is against the ingress of solid objects, and the second of which describes its protection from water. Any light bulbs used in wet areas, such as bathrooms or gardens, must have a good IP rating. See here for more information about IP ratings.

The unit of measurement for a light’s colour temperature (see above).

A standard unit for measuring electrical usage. Equal to the power consumption of 1000 watts for one hour. Energy suppliers use kilowatt-hours (kWh) as the basis for energy tariffs.

Leading edge dimmers are the most common type of dimmer switch and are used to vary the amount of power to a light source. This is useful to change the intensity of the light produced. Leading edge dimmers are not compatible with LED light bulbs.

An LED driver is a device which controls the amount of power to an LED bulb or fitting, protecting it from sudden increases in voltage. Drivers are required to use low voltage LED bulbs and they can improve the performance and lifespan of LED light bulbs.


Light Emitting Diode. A highly efficient form of lighting, which lasts 3 times longer than a CFL (Compact Fluorescent Lamp), and 30 times longer than incandescent lights. LED lights come in many different forms and are suitable for any application.

Units of luminous flux, a measurement of the visible intensity of a source of light. A bulb’s lumens (lm) rating is the best way to determine its brightness. A bulb for general household use might have a lumens rating between 300-800lm, while high-powered floodlights could emit in excess of 20,000lm.

A term used to describe the intensity of light given off from a point on a surface, in a particular direction. It is used to measure how reflective a surface is, and is measured in either foot lamberts (imperial units) or candelas per square meter (metric units).

A common term for a light fitting.

Used to describe the efficiency of a light source. It compares light output (measured in lumens) to the amount of power used (measured in watts). This gives you a lumens per watt rating (lm/w). LED lights have a very high lm/w rating.

A term used to describe a small, halogen lamp with a reflector that creates a narrow beam of light. Halogen multifaceted reflector lamps are more energy efficient than incandescent bulbs, but far less efficient than CFLs (Compact Fluorescent Lamps) or LED lighting.

A common name for a light that hangs from the ceiling on a length of cable, usually with a lamp shade attached.

A method used to generate white light with LEDs. Blue or near ultra-violet LEDs are covered in phosphor, which glows white in reaction to the light produced by the LED. It is the most efficient method of producing white light from an LED.

A device used to control the intensity of a luminaire. It detects light, and adjusts the amount of power to the light source accordingly. This is useful as it gives a more constant level of lighting.


Stands for Passive Infrared detection device. In lighting, it is connected to a luminaire. A PIR detection device turns the light on or off when there is a large change in infrared activity, usually when someone enters or exits a room. This helps to save energy. PIRs are frequently used in security lighting.

A bulb's lifespan is measured in hours. Typically, LED light bulbs last 10-20 times longer than their incandescent equivalents, and they never burn out (although they can malfunction if faulty). LED bulbs get gradually dimmer over time (too slowly for us to be aware of), and are considered to be at the end of their life when they reach a certain percentage of their original brightness, usually 70%. So although LED bulbs may continue to operate for many years after, they should be replaced at this point

A bulb which has been silvered on its inside layer. It produces a soft-edged beam and is generally smaller in size than other lamps. Reflector lamps are available in a variety of colours and bases, so are suitable for many applications.

A term used to describe new equipment that has been designed to be compatible with existing components. LED light bulbs are retrofitted to be compatible with existing light fittings.


Stands for red, green, and blue, and is often used when the colour of a bulb can be changed. These three colours can combine in different ways to create various colour options, including white light.

The small bayonet cap (SBC) is a smaller cap type mainly used in table lamps and wall lights, or any other fixture where a smaller light bulb is needed. Like the regular bayonet cap, it is a 'push-and-twist' type of cap, with two small horizontal spikes protruding from the base of the bulb. Small bayonet caps are noticeably narrower than the standard bayonet cap. Also known as a B15d cap.

The small Edison screw is a smaller cap type mainly used in table lamps and wall lights, or any other fixture where a smaller light bulb is needed. Like the regular Edison screw cap, it simply screws into the fitting. Small Edison screws are noticeably narrower than the standard Edison screw. Also known as an E14 cap.

Refers to lights that do not have any moving parts, or any parts that can break. Solid State Lighting is more efficient and safer than other kinds of lighting, such as fluorescent and incandescent. LEDs are a type of Solid State Lighting.

Required to start fluorescent lamps. Unlike other forms of lighting, fluorescent lights need a starter switch to produce light and control their power. Electronic ballasts are often used as alternatives.

Used to change the voltage of an electrical supply. This is important, as different bulbs require different voltage levels to function properly. Transformers are used to lower voltage to 12v or 24v lights.

These lamps work in the same way as incandescent lighting and are similar in design. Tungsten halogen lamps are not as energy efficient as either CFL (Compact Fluorescent Lamp) or LED lighting.

A term used to describe the colour temperature of a particular type of bulb. Warm light sources offer a softer, yellower light suited to relaxed spaces like living rooms and bedrooms. Warm white bulbs typically have a colour temperature between 2700K-3200K. See here for more information about colour temperatures.

Units that measure the rate at which power is generated or consumed. As a general rule, the higher the wattage, the brighter the light bulb, although this is not an exact way of measuring a bulb's brightness (for that, see 'lumens).'