Glossary of Lighting Terms

Lighting can be an immensely tricky business, so to help make things a little easier, we've put together a glossary of all the lighting terms you might encounter while browsing our site. If you're looking for a particular definition that we've missed or if you have any additional questions at all, please feel free to contact us with your query.

Accent Lighting

Additional lighting - often from directional light sources - used to highlight particular areas or objects. Commonly used in retail displays and galleries.

Aluminium Reflector

Many spot and flood lamps are manufactured using a thin film of reflective aluminium deposited on the inside of the glass. Unlike dichroic reflectors, this reflects both light and heat forwards out of the lamp.

Ambient Temperature

The air temperature in the vicinity of a light fitting. Extreme temperatures can affect the performance of a light bulb, so many have heat sinks built into them - particularly LED bulbs - to help dissipate heat.


Also known as 'amps.' Refers to the amount of electric current flowing through a circuit, and is used to calculate the wattage of a lamp.


A type of light bulb cap, also known as a small bayonet cap (abbreviated to SBC). It is a 'push-and twist' type of cap commonly used on candle bulbs and others that require a smaller fitting. The number 15 refers to the diameter of the cap in mm.


A type of light bulb cap, also known as a bayonet cap (abbreviated to BC). It is a 'push-and twist' type of cap and is the most common type of fitting in the UK. The number 22 refers to the diameter of the cap in mm.


Part of a fluorescent lamp fitting which regulates the current flow through a fluorescent lamp.


See cap.

Bayonet Cap

The most common type of light bulb cap used in the UK. Also known as a B22d or BC cap. It is a 'push-and-twist' type of cap, with two short horizontal spikes protruding from its base.


See bayonet cap.

Beam Angle

Describes the spread of a directional light source, in degrees. A light with a small beam angle (e.g. 30°) would have a very narrow, focused beam of light, while a light with a larger beam angle (e.g. 120°) would cover a much wider area.

Black Light Blue

Specialist fluorescent lamp designed to emit invisible ultraviolet (UV) light.


A popular term for the electric lamp, derived from the bulb-shaped glass envelope in which lamps are often enclosed.

Burning Position

The burning position defines the position in which lamps should be installed and operated and are sometimes defined by code letters as follows:-

  • h = vertical (base upwards)
  • s = vertical (base downwards)
  • p = horizontal


The candela (cd) is a standard measure of luminous intensity to allow direct comparison of the 'brightness' of different light sources. For any physicists who may be reading this...

“The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 Hertz and that has a radiant intensity in that direction of 1/683 Watt per steradian.”


A common light bulb shape, often used in smaller lamps and decorative fittings. So named because the bulb resembles the shape of a candle flame.


The electrical connection and mechanical fixing for a lamp is often referred to either as a cap or base. Lamps use a wide variety of fittings, partly to meet the appropriate electrical and safety requirements and partly to ensure that luminaires can only accept the appropriate lamp (to prevent using low voltage lamps in mains fittings, for example). Common caps for standard incandescent lamps include the ubiquitous 22mm 'Bayonet' fitting (otherwise known as 'B22d' or 'BC') and the Edison Screw (or 'ES') fitting, named after the early pioneer in electric lamp development, Thomas Edison. To ensure compatibility between different manufacturers’ products, international standards for lamp bases have been agreed by the IEC under the IEC 60061 standard. For further information see our Caps and Bases page.

Ceiling Light

Any light fitting designed to be hung directly from the ceiling.


See Compact Fluorescent Lamp

Chrome Cap

Also known as 'crown silvered' lamps, these are incandescent lamps with a highly reflective coating on the front of the glass. This reflects much of the light back towards the lamp fitting.

Originally designed for use in spotlight fittings with parabolic reflectors, particularly for shop window displays, these are also used in many contemporary designer light fittings to reduce glare and create spectacular effects where the lamp itself is visible.

Since the reflective coating also reflects heat back in to the luminaire, these lamps can cause overheating and should only be used with luminaires that have been designed to accept them. 

Colour Rendering Index

Abbreviated to CRI, the colour rendering index is a scale that measures how accurately a light source reproduces the colours of the environment around it. It is measured up to 100, where 100 is perfect reproduction. Many halogen light bulbs have a naturally high CRI, often of 100, whereas the best LED light bulbs have a CRI of <97. The CRI of standard LEDs is around <80. CRI is an important characteristic for many types of commercial lighting, such as in retail displays, galleries, and for photography studios.

Colour Temperature

The colour temperature of a lamp is a measure of the 'warmth' or 'coldness' of the light that it produces. Lamps that produce a 'warm' or 'yellowish' light (for example the sodium lamps used for street lighting which appear orange) have a lower colour temperature. Lamps producing a pure white or bluish tinged light have a higher colour temperature. For a full explanation of colour temperature, see our Colour Temperature guide.

Colour temperature is measured in degrees Kelvin (K) and is defined as “the temperature of a black body having the same colour appearance as the type of lamp being considered”.

  • Lamps with a colour temperature of less than 3500K are usually described as having a 'warm' appearance.
  • Lamps with a colour temperature between 3500 and 5000K are considered to have a 'cool' appearance.
  • Lamps having a colour temperature over 5000K are said to have a 'cold' or 'daylight' appearance.

Typical colour temperatures are:-

  • High pressure sodium lamps: 2000K to 2200K
  • Tungsten filament light bulb: 2700K
  • Tungsten halogen lamps: 3000K
  • Fluorescent lamps: 2700K to 6500K
  • Metal halide lamps: 3000K - 5600K
  • Daylight: 5500 - 6500K

Cool White

A term used to describe a particular colour temperature range. Cool white lighting has a colour temperature of between 3500K-5000K, and emits a bluer, more intense light suited for kitchens, bathrooms and work areas.

Compact Fluorescent Lamp

Often also known as 'low energy' bulbs, compact fluorescent lamps use the same technology as fluorescent tubes but with the tubes folded into a much more compact design. The main advantages of compact fluorescents are:-

  • Lower power consumption – typically 80% less than equivalent incandescent lamps.
  • Longer operating life - typically 3 to 15 times longer than conventional incandescent lamps.
  • Lower heat output than incandescent lamps, making them ideal for use in enclosed fittings or close to delicate lampshades.

There are two distinct categories of compact fluorescents:-

  • Integrated: The lamp includes both the fluorescent tube and the necessary control gear in one package. These are ideal for use in direct replacement for conventional incandescent lamps and are often produced with standard BC or ES fittings.
  • Non-integrated: The control gear is part of the fitting or luminaire and the lamp is just the folded tube. These used to be fitted mostly in commercial and industrial fittings but are more frequently now being used in homes as well as in exterior lighting.

CFLs are still widely available, although they have been superseded by LED lighting from both a performance and energy efficiency perspective.

Control Gear for Fluorescent Lamps

Fluorescent lamps require special circuits to start the lamp when power is applied (the starter) and then to limit the electrical current once it is running (the ballast). This is often referred to as the control gear. In conventional fluorescent fittings, the ballast is integrated into the light fitting and the starter is generally a small user-replaceable plug-in tubular component.

In 'high frequency' or 'electronic' fluorescent fittings, the control gear uses an electronic circuit to perform these functions. This makes the lamp faster to start, more efficient and virtually flicker free. In some compact fluorescent fittings, this circuitry is fully integrated into the lamp itself.

Certain types of electronic control gear also allow fluorescent tubes to be dimmed.


See Colour Rendering Index

Crown Silvered

See Chrome Cap.

Delivered Light

The amount of light delivered to a surface by a light source. Measured in lux.


Sometimes known as 'cool beam' lamps, dichroic lamps have a special multi-layer coating on the reflector of the lamp.

This means that visible light is reflected forwards whilst the heat is transmitted backwards through the rear of the lamp. The word dichroic literally means 'two coloured'.


A cover or case over a light source designed to soften (or diffuse) the light it emits.

Dimmer Switch

A switch that allows you to adjust the brightness of a light bulb or fitting. There are two types available: leading edge and trailing edge dimmers. LED light bulbs must be used with compatible dimmer switches.


A type of light bulb cap, also known as a small Edison screw (SES). This type of cap simply screws into the fitting, and is most commonly used on light bulbs designed for small and decorative lamps. The number 14 refers to the diameter of the cap, in mm.


A type of light bulb cap, also known as an Edison screw (ES). This type of cap simply screws into the fitting, and is one of the most commonly used caps available today. The number 27 refers to the diameter of the cap, in mm.


A type of light bulb cap, also known as a giant Edison screw (GES). This type of cap simply screws into the fitting, and is most commonly used for specialist commercial and industrial purposes. The number 40 refers to the diameter of the cap, in mm.

Edison Screw

One of the most common cap types used today. As its name suggests, this type of cap simply screws into its fitting. Also known as an ES or E27 cap.

Emergency Lighting

Lighting designed to function in the event of a power outage. Many commercial LED light fittings feature integrated, battery-operated emergency drivers, so the light uses mains power during normal use, but automatically reverts to the emergency driver when mains power is cut.

Equivalent Wattage

After the development of energy-saving light bulbs such as LED and CFLs, wattage alone is not an adequate indicator of a bulb's brightness. Equivalent wattages are used to compare like-for-like bulbs. For example, a 5W LED light bulb is roughly equivalent to a 60W incandescent bulb, and will be around the same brightness.


See Edison Screw.

Festoon Lighting

Light strings commonly used for decorative and celebratory purposes. Festoon lighting consists of a cable to which light bulbs are fixed at regular intervals.


The filament is the part of an incandescent lamp that glows as an electrical current is passed through it. In very early lamps the filament was made of carbon. Most incandescent lamps now use tungsten filaments.

Fluorescent Lamps

Fluorescent lamps are a type of gas discharge lamp which generate invisible ultra-violet radiation and then use a phosphor coating on the inside of the lamp glass to convert this into visible light using a process known as 'fluorescence'. By using different coating materials, fluorescent lamps can be made to generate light in a range of different colour temperatures. Typically these include Warm White, White, Cool White and Daylight versions.

The tube diameter is often expressed in eighths of an inch (e.g. T5 = 5/8” = 16 mm).

Advantages of fluorescent tubes include:-

  • High energy efficiency.
  • Long operating life (typically over 10,000 hrs).
  • Even light distribution.

Some fluorescent lamps are manufactured without the fluorescent coating and use a bluish black glass in order to deliberately emit only light in the ultra-violet (UV) spectrum. These are used for specialist applications such as banknote forgery detectors and nightclub special effects lighting.

Fluorescent lamps are generally designed to work directly from the mains (110 or 240V AC power) but require a starter and ballast or electronic gear to operate. Electronic control gear (ECG) converts mains power to a very much higher frequency and gives several advantages over conventional ballasts and starters including:-

  • Greater efficiency and hence lower power consumption.
  • Reduced flickering
  • Faster starting

Usually manufactured in a tube shape, fluorescent lamps can also be found in circular, “U”-shaped and a variety of other specialist shapes.


See Giant Edison Screw.

Giant Edison Screw

A larger variant of the standard Edison screw cap. Uncommon on household bulbs but often used for industrial lighting purposes on lamps over 500 watts. Also known as a GES or E40 cap.


GLS stands for General Lighting Service. These are the standard, pear-shaped light bulbs that are present in almost every home. Also known as 'A-shape' or 'A60' light bulbs.


See Tungsten Halogen

Heat Sink

An electrical component integrated into many LED light bulbs and fittings. Heat sinks are designed to draw heat away from the delicate componentry of a bulb, helping to preserve it and extending its lifespan.

High Frequency Fluorescents


See High Intensity Discharge.

High Intensty Discharge

High Intensity Discharge lamps are used to illuminate larger spaces like car parks and warehouses. More energy efficient than incandescent bulbs and much brighter.

International Electrotechnical Commission (IEC)

The International Electrotechnical Commission ( is the authoritative worldwide body responsible for developing consensus global standards in the electrotechnical field. IEC is dedicated to the harmonization and voluntary adoption of these standards, supporting the transfer of electrotechnology, assisting certification and promoting international trade.


Most “standard” light bulbs are incandescent lamps. These use a filament (usually tungsten wire) inside an enclosed glass bulb that glows white hot as an electrical current is passed through it. The filament is prevented from burning by either creating a vacuum inside the bulb or filling it with inert gas.

Incandescent lamps are not very efficient since most of the radiation is in the infra-red spectrum (ie heat rather than visible light). The light produced is quite “warm” with a colour temperature of around 2700°K.

Ingress Protection

Ingress Protection ratings denote how protected a bulb or fitting is from water and foreign objects. IP ratings consist of two numbers: the first specifying how protected an object is from the ingress of solid objects, while the second specifies how protected it is from water ingress. See our guide to IP ratings and bathroom zones for more information.

IP rating

See Ingress Protection.


See Kelvin.


Units used to measure the a light's colour temperature. Abbreviated to K.


Lamp is the generic term for a light source. Light bulbs, tubes, capsules and spots are all types of lamp.

Leading Edge Dimmer

A common type of dimmer switch, not compatible with most LED light bulbs. See out dimming guide for more details.


A light-emitting diode (LED) is a semiconductor light source. LED Light Bulbs offer long life and high energy efficiency, but initial costs are higher than those of compact fluorescent and incandescent lamps. Life cycle of LED lamps is multiple compared to incandescent lamps, however, degradation of LED chips reduces luminous flux over life cycle as with conventional lamps.

Long Life

Many different types of lamp may be manufactured in “long life” versions. The design often means using better quality components, different construction and manufacturing techniques as well as more stringent quality control. Long life is a relative term and depends very much on the technology of the lamp as shown in the table below...

Technology 'Standard Life' 'Long Life'
Incandescent 1000 to 2000 hours 3000 to 3500 hours
Tungsten Halogen 1500 to 2000 hours 5000 to 6000 hours
Fluorescent Tube 8000 to 10000 hours       15000 to 20000 hours
Compact Fluorescent           3000 hours 12000 to 15000 hours


Low Energy

See compact fluorescent.


Click Here for a full explanation of what a lumen is and why they are becoming increasingly more important with the emergence of LED bulbs.


Luminaire is the general term for the fitting into which a lamp is installed. This will always include a lamp holder with the electrical connection to the lamp and may also include shades, reflectors or diffusers along with decorative, protective and/or safety features.

Luminous efficacy

Described the efficiency of a light source. Compares light output (lumens) with power input (watts) to determine a lumens/watt ratio (lm/w).

Metal Halide Lamps

Metal halide lamps are a type of discharge lamp that creates an electrical arc inside a high-pressure gas capsule containing various rare earth compounds. Usually used in retail, showroom and factory applications, they can create a very crisp white light with colour temperatures ranging from 3000 to 10,000°K.

Multifaceted Reflector

Halogen lamps that contain a small reflector to more efficiently control light distribution. These types of bulb often have a narrow beam angle, providing a bright, focused source of light.

Passive Infrared Detection

A type of motion-detection commonly used on outdoor floodlights and commercial light fittings. PIR lights activate when the sensor detects a significant change in surrounding infrared, such as when a person walks past.


A type of luminaire attached to a cable, most commonly hung from the ceiling.

Phosphor Conversion

A process used to generate white LED light. Blue LEDs are coated in phosphor to cause a reaction that produces white light. It is one of the most efficient methods for producing white LED light.


An electronic component that helps to regulate the light output from a particular source. It does this by detecting light levels around the lamp and adjusting power input accordingly.


See Passive Infrared Detection

Rated Life

A measurement that denotes how long a light bulb or fitting will last before it no longer operates. For incandescent bulbs, this usually means when the filament will burn out, but as this doesn't happen with LED lights, their lifespan is calculated differently.

Although it's too slow to notice, LEDs gradually get dimmer over time, and they are deemed to be at the end of their life when they drop to a certain percentage of their initial brightness. This is most commonly 70% brightness (known as an L70 measurement), but this can vary. The bulb will continue to operate past this point, but should be replaced as soon as possible.


See Multifaceted Reflector.


Used to describe new light bulbs and fittings that are compatible with older light fixtures. The majority of LED light bulbs are retrofit, making it easy to replace older incandescent bulbs.


Red, Green, and Blue. Used to describe coloured or colour-changing lighting. These three colours can combine to create any other colour on the spectrum.


See Small Edison Screw.


See Control Gear for fluorescent lamps.

Tungsten Halogen

Tungsten halogen lamps are similar to incandescent lamps but instead of just being filled with inert gas they also have small quantities of halogens (bromine, chlorine and iodine) or their compounds added to the filler gas and use quartz instead of glass for the lamp capsule. Benefits of tungsten halogen lamps include:-

  • A slightly higher colour temperature than ordinary tungsten incandescent lamps, giving a bright, white light, ideal for shop display lighting.
  • Physically much smaller than equivalent tungsten incandescent lamps.
  • Longer rated life – typically 2000 to 5000 hours or more. glossary-of-lighting-terms One disadvantage of tungsten halogen lamps is that the quartz envelope is much more sensitive to contamination than ordinary glass bulbs. The quartz capsule must not be touched with bare hands since contamination can lead to overheating and severely reduced life. Some designs get around this by encapsulating the lamp inside a glass envelope.


Named after James Watt, the inventor of the steam engine, the Watt (W) is a standard international unit of measurement for power. In the case of electric lamps, the 'wattage' of the lamp defines the amount of electrical power that it consumes and also is related to the brightness of the lamp. However since different types of lamp are more or less efficient in converting electrical power into light, the Watt is not in itself a measure of brightness. For example, a 20W compact fluorescent lamp is approximately as bright as a 100W incandescent lamp yet it consumes only one fifth of the power.