LED Lighting Jargon Buster

Don’t know your ‘GU10s’ from your ‘PARs’? Get the answer to these and more in our handy guide to LED lighting jargon buster.


  • 2D lights are a style of light which is flat and wide. 2D lights are commonly fluorescent and are bent into a shape similar to a pretzel.
  • LED replacements consist of several LEDs on a circular board. LED replacements use less energy and last longer than fluorescent counterparts.
  • A 2D light is a common feature of a bulkhead, though not all bulkheads are 2Ds.

Accent Lighting

  • A lighting system used to highlight an object.
  • Eg. Coloured lights to highlight a company sign.

Ambient (General) Lighting

  • A lighting system which provides the main light source for a room.
  • This is the most common use of lighting.
  • Eg. A bulb suspended from the ceiling.


  • AR111 is a particular type of lamp, which can be thought of as a large spotlight. AR111s are used to focus light on a larger area than conventional spotlights.
  • They are frequently used for accent lighting, as they light a medium sized area and can deal with prolonged periods use.
  • The AR in AR111 stands for Aluminzed Reflector, and like a PAR lamp, this refers to the reflective surface surrounding the light source. 111 stands for 111mm, this is the diameter of the lamp.
  • See our Bulb Finder for more information.


  • A ballast is an electrical component which provides a short surge of voltage, and maintains a constant current, in order to run fluorescent tubes.
  • Switching to LED tubes very often requires bypassing a ballast, as the short surge in voltage can burn out LED tubes. See ‘Do I need to bypass a ballast to fit my LED tube?’ in our LED FAQs.

Bayonet Cap (BC, SBC)

  • Bayonet cap bulbs are placed in the fitting and then twisted into the fitting to secure it.
  • Bayonet caps come in two sizes, B22 and B15, sometimes referred to as BC (Bayonet Cap) and SBC (Small Bayonet Cap) respectively. The B in B22 and B15 stands for bayonet, while the 22 and 15 stand for 22mm and 15mm respectively. These measurements refer to the diameter of the bayonet fitting.
  • See our Bulb Finder for more information.

Beam Angle

  • The measure of how far light spreads over a given area from a lamp, it is measured in degrees.
  • A large number covers a large space. A small number will create a narrow spot of light.
  • A light’s beam angle is officially measured as where the intensity drops to half of the intensity in the centre.


  • Stands for: Building Research Establishment’s Energy Assessment Method
  • This is the industry leading environmental rating system that sets the standard for best practice in assessing how sustainable a building is.
  • Not just specific to lighting, the system gives an overall rating for a building marked on how environmentally friendly it is.
  • For more information visit the BREEAM website.


  • Used interchangeably with lamp, this is a general term used to describe a small to medium light source and its housing.


  • Bulkheads are lights designed for long life and high intensity.
  • They commonly have a high IP rating and durable designs, making them suitable for outdoor and tough working environments.

Candela (Luminous Intensity)

  • The recognised measurement for light intensity (luminous intensity), ie. the amount of light delivered to an area which the human eye easily sees.
  • This is in contrast to Lux, as lux accounts for light humans struggle to see as well.
  • Because candelas are limited to the light humans easily see, candelas are the most accurate measurement for how intense a light appears to us.
  • It is loosely based on the number of candles it would take to produce the intensity. A common candle measures at 1 candela.

Colour Rendering Index (CRI)

  • A scale varying from 0 to 100 which describes how accurately colours appear under a light. The higher the number the better.
  • For example, under a low CRI – below 60 – green objects can appear brown. A high CRI lamp will display the object as green.
  • The scale is not an ideal index but a decent indicator.
  • High quality LED lights above 95 CRI are well suited to environments where rendering colours accurately is important.

Colour Temperature

  • Measured in Kelvin (K), this describes how the light looks, often phrased as ‘warm’, ‘white’, and ‘cool’.
  • Measurements vary from 1900K for very warm to 5000K+ for very cool lights.
  • Warm lighting will appear golden and yellow in colour, similar to a burning candle.
  • White lighting appears white, and cool lighting appears almost blue.

Compact Fluorescent Lighting (CFL)

  • CFL lights, marketed as ‘energy saving’, emit light by the excitement of a gas in a tube.
  • Although they are more efficient than incandescent lighting, CFLs are less efficient than LED and last only 8,000 hours compared with more than 30,000 hours for a typical LED source.
  • CFL sources are slow to produce their maximum light output and often fade toward the end of their life.
  • CFL sources also contain mercury and their disposal is more complex.

Cool White

  • Cool white describes a colour temperature of 4000K+ which appears slightly blue.


  • Stands for: Digital Addressable Lighting Interface.
  • This is a protocol for lighting controls and dimming set up by manufacturers who sanction the use of the DALI trademark.
  • It works as a reassurance of quality on lighting controls.


  • This means the light can be used in a fitting connected to a dimming switch.
  • This allows the user to change the intensity of the light according to different occasions.
  • However, LEDs may not be compatible with some dimming units. This is because the lower wattage of LEDs may not be recognised as a sufficient load for the dimming unit.


  • Downlights are enclosed units which combine a fixture-housing and a light source all in one.
  • They are commonly fitted directly in the ceiling plasterboard.
  • Downlights typically have very long lives as the whole fixture must be replaced when it fails rather than just the light source.

Driver Rating

  • This outlines the particular specifications of a driver.


  • This is an extra device needed to run certain types of LEDs. These include MR16s, Panels, and Downlights.
  • They change the voltage from the mains to a lower voltage suitable for the particular light.

Edison Screw (ES, SES)

  • Edison Screw is a type of fitting common on light bulbs which screws into a fitting.
  • Edison Screw bulbs come in two sizes, E27 and E14, sometimes referred to as ES (Edison Screw) and SES (Small Edison Screw) respectively. The E in E27 and E14 stands for Edison, while the 27 and 14 stand for 27mm and 14mm respectively. These measurements refer to the diameter of the screw connection on the bulb.
  • See our Bulb Finder for more information.

Efficacy (Luminous Efficacy)

  • Efficacy can be roughly described as how energy efficient a light source is. It describes how much light you get out from the power you put in.
  • It is a ratio of lumens (total light emitted) to power. Eg. 90lm/W (90 lumens per watt).
  • The higher the number of lumens compared to the power, the greater the efficacy.

Equivalent Wattage

  • LED lighting requires much less power than conventional incandescent lamps.
  • Equivalent wattage is the power an incandescent lamp would need to run at to be as bright as the comparable LED lamp.
  • For example, the equivalent wattage of a 6W LED bulb is commonly a 50W incandescent bulb.


  • This is when a light regularly and quickly changes intensity.


  • GU10 is a common spotlight fitting comprising of two metal prongs that twist into a fixture. The 10 in GU10 refers to the distance in mm between the two prongs.
  • GU10s run on mains voltage and do not require a driver.
  • See our Bulb Finder for more information.

Halogen Lighting

  • Halogen lighting is a variation of incandescent lighting.
  • They are used because a halogen gas, such as iodine, is enclosed in the lamp in small quantities to allow the internal filament to burn hotter and for longer.
  • This makes halogen lamps have a longer operating life but they are still extremely inefficient compared with LED.

Heat Sink

  • This is a piece of conductive material which transfers heat away from electrical equipment and into the surrounding air.
  • It prevents appliances from overheating.

High bay/Low bay

  • High and low bay lighting refers to the height of the fixture in an industry environment.
  • High bay lighting is for high ceilings (8m or more), and is manufactured particularly to enable shelving as well as floor-space to be illuminated.
  • Low bay lighting provides a similar function, but is designed for lower ceiling mounting heights.

Incandescent Lighting

  • Light sources with an internal filament which is heated to produce light.
  • Extremely inefficient: they turn only 10% into light.


  • How strong (bright) the light output from a source appears.
  • Measured in Candelas

IP Rating

  • Stands for: Ingress Protection marking.
  • The IP rating indicates how resistant a fitting is to irritants. Areas often wet or dusty need higher IP rated products.
  • IP ratings consist of two numbers, eg. 65. The first number (from 0 to 6) refers to how well the fitting is resistant to solid particles, eg. dust.
  • The second number (from 0 to 9) refers to how well the fitting is resistant to liquids.
  • The higher the number the more resistant the fitting is, with IP65 ratings suitable for outdoor uses.
  • For more information, Wikipedia has an extensive page.

Kilowatt Hour (kWh)

  • A kilowatt is 1000 watts. Kilowatt hour is a unit of energy for watts used over time. One kilowatt hour is 1 kilowatt used for 1 hour.
  • For example, a 1000 watt heater, used for two hours would use 2 kWh in energy.
  • Utility companies commonly measure domestic usage in kWhs, and charge on average £0.12 per kWh.
  • A 60W incandescent light used for four hours a day would use 0.24kWhs. Whereas a 9W LED light would only use 0.036kWhs in four hours.


  • Used interchangeably with bulb, this is a general term used to describe a small to medium light source and its housing.

LED Lighting

  • Stands for: Light Emitting Diode
  • LEDs are electrical components which convert electricity into light with very little waste energy. LED lights combine several of these components to make up a light source suitable for general use.
  • LEDs have a lifespan of more than 30,000 hours and use little energy, meaning they pay for themselves with savings on electricity bills.
  • Compared to Incandescent Lighting, LEDs can save up to 90% on energy consumption.

Life (Rated Life / Life Span)

  • This is an indication of the lifespan of the bulb, it is given in hours.
  • It gives the amount of time the bulb will run before it drops below a particular percentage of its output – this is typically 70% (L70). This means the bulb can be expected to work for much longer than its ‘life’, but not as brightly as when new.
  • It differs from a warranty as rated life is not a contractual assurance, just an indication from scientific testing of the device.


  • Load is the total number of appliances which draw power from a circuit.
  • For example, a light will contribute to the load of a circuit when turned on.
  • As LEDs have a smaller load, some dimming devices will not work with LEDs as the dimming device has a minimum load higher than the load of the lights.

Lumen (Luminous Flux)

  • The recognised measurement of total light emitted from a source (luminous flux), ie. the total amount of light emitted.
  • This is in contrast to Lux, as lux measures the light directed to a particular area, whereas lumens can be dispersed in all directions.


  • The term used to describe a whole lighting fixture including the light source, the socket, and any housing around the light source.

Lux (Luminous Emittance)

  • Lux is a measurement of light delivered to an area (luminous emittance), ie. the total amount of light delivered to an area.
  • This is in contrast to Lumens, as lumens are a measure of light emitted which can be dispersed in all directions.
  • It also contrasts with Candelas, as lux measures all light delivered to an area, not just that which the human eye easily sees.

MR11 (GU4)

  • MR11 is a type of spotlight fitting comprising of two thin metal prongs which are pushed into a fixture. Uncommonly referred to as GU4s, the 4 in GU4 refers to the distance in mm between the two prongs.
  • MR11s, similar to MR16s, require an additional driver to power them, this sits between the lamp and the mains voltage.
  • See our Bulb Finder for more information.

MR16 (GU5.3)

  • MR16 is a common spotlight fitting comprising of two thin metal prongs which are pushed into a fixture. Uncommonly referred to as GU5.3s, the 5.3 in GU5.3 refers to the distance in mm between the two prongs.
  • MR16s require an additional driver to power them, this sits between the lamp and the mains voltage.
  • See our Bulb Finder for more information.


  • Stands for: Parabolic Aluminized Reflector
  • This refers to a reflective housing around the light source which spreads the light.
  • Numbers following PAR (eg. PAR16) are the number of eighths of an inch the bulb is in diameter. Eg (PAR16 = 2 inches/50mm)

PIR Sensor

  • Stands for: Passive Infrared Sensor.
  • These sensors monitor movement in a room in order to automatically turn on lighting when people are present, and off when nobody is present. They are more commonly referred to as motion sensors.
  • They help reduce consumption by ensuring lights are turned off when they are not needed.

Power Factor

  • This is a ratio of power used by an electrical device, against the amount of power needed in a circuit to run that device.
  • This is relevant to LEDs because they reflect some power back to the grid and need more power sent to them than they actually use.
  • If you are buying for your house or a small building this is irrelevant, as you only pay the electricity company for what you use and not what is sent to the building.
  • Some large buildings such as hotels might need this information as their energy bills are calculated differently. This information is available direct from the manufacturer.


  • Stands for: Red Green Blue
  • This is a feature of commercial LED lights, which combines red, green, and blue lights so that the resulting light appears white and can be general purpose.
  • It sometimes refers to lamps which can switch between a red, a green, and a blue light emitted.

Rotatable End Caps

  • Traditional tube fittings slot in to fixtures at a set angle. Rotatable end caps allow the tube to be turned to better angle the light to where it is needed.


  • Barely noticeable, frequent small changes in light intensity.
    T4, T5, T8, T12
  • Lighting tubes come in fittings labelled T4, T5, T8 etc.
  • The number stands for eighth of an inch diameter, eg. T8 = one inch diameter. The T fitting code therefore describes the width of the tube.
  • Tubes vary in length and will be described separately as 2 ft, 4 ft, 6 ft, etc.

Task (Directional) Lighting

  • A lighting system used to light a specific area.
  • Eg. A spotlight focused on the centre of a table.

Thermal Management

  • The process of directing heat generated by a light source away from the fixture.
  • This is commonly achieved by a Heat Sink in LED lights.


  • This is the amount of leeway given either side of a measurement in a specification.
  • No product will exactly meet the specification quoted because of unavoidable irregularities in production. This is why manufacturers say their product is a certain specification, within a certain tolerance.
  • For example, the quoted colour temperature of a lamp is 2700K. The manufacturer of the lamp gives a tolerance of 200K, so this lamp could be anywhere between 2600K and 2800K. This is because the difference between 2600 and 2800 is 200.
  • This seems like quite a large tolerance, but in a string of lights you would not notice the variation. For special requirements some manufacturers will use a smaller tolerance, but this comes at a significant cost.

Warm White

  • Warm white describes a colour temperature of around 2700K which appears slightly yellow.


  • This is the amount of time the manufacturer guarantees the product will work when fitted properly.
  • It is given in years from purchase and is typically 3 years.
  • This is different from Life, which is given in hours, as the warranty is valid regardless of the total number of hours the lamp is on for.
  • If a product fails during its warranty, the manufacturer is obliged to replace it.

Watt (W)

  • The recognised measurement of power. In lighting, watts measure the amount of energy drawn by the light source.
  • Incandescent Lights commonly use the number of watts as a rough measure of intensity.
  • The relationship between watts and intensity is less accurate in LED Lighting.


  • White describes a colour temperature of around 3000K which appears white.
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