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Lighting Glossary



A

AC (Alternating Current)   
The flow of electricity (electric current) in a circuit that alternates direction every second with a standard current frequency of 50 Hz (Hertz or cycles/second).

Accent Lighting   
Lighting that is used to accent a particular object or feature and for it to be effective, accent lighting should be approximately four or five times the level of ambient light within the space.

Ambient Lighting   
Uniform background lighting of a space without providing for any localised directional lighting requirements.

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B

Beam Angle   
The angle between the opposed points either side of the beam where the luminous intensity reaches a value of 50% that of the peak beam intensity.

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C

Candela (cd)   
The measure of luminous intensity of a light source in a given direction.

Chromaticity   
Is an objective specification of the quality of colour regardless of its luminance.

Colour Rendering   
The ability of a light source to render colours accurately and is measured by the Colour Rendering Index in “Ra” on a scale of 0-­100. The higher the number the more accurately the light source can render colour.

Colour Temperature   
The measure of the colour appearance of a light source which describes the apparent warmth or coolness of that light source. Generally, light sources are described as either “warm”, “intermediate”, “cool” or “cold”. The letter “K” refers to Kelvin. The term can also be referred to as Correlated Colour Temperature (CCT). Learn more in our Light Lab

Constant Current   
A circuit in which the current remains constant but the voltage may vary.

Constant Voltage   
A circuit in which the voltage remains constant but the current may vary.

Cylindrical Illuminance   
The amount of vertical illuminance that falls on a cylindrical surface positioned within a space.

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D

DALI (Digital Addressable Lighting Interface)   
DALI is a two way communication system protocol for lighting that allows addressed ballasts to receive and send signals in order to provide a flexible controlled lighting solution.

DC (Direct Current)    
An electric current that moves in one direction without changing or cycling and is usually supplied from a battery, DC Transformer or Photovoltaic (PV) Cells.

Dichroic Reflector   
A reflector or filter that transmits certain wavelengths but reflects other wavelengths. In lighting fixtures, dichroic reflectors transmit infra­red light backward through the rear of the lamp whilst reflecting visible light forward, resulting in cooler visible light source and can be considered as "Cool Beam".

Dimmer   
A device in an electrical circuit used for varying the brightness of a light source within a lighting installation.

Direct Lighting   
Lighting provided from a light source without reflection that distributes at least 90% of luminous flux in a downward direction.

Downlight   
A ceiling based light fitting either, recessed or surface mounted which concentrates the light in a downward direction.

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E

Emergency Lighting   
Lighting used in the event of power failure for whatever reason. Luminaire types are usually either stand alone Non­maintained operation Emergency use only) or mains converted Maintained operation (Mains + Emergency use).

Extra Low Voltage   
IEC defined where the electrical potential does not exceed 25 Volts RMS for AC or 60Volts DC.

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F

Fire Rated   
A recessed fitting that maintains the integrity of a fire rated ceiling in compliance of the Building Regulations – Part B on Fire Safety.

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G

Glare   
Glare is an interference with visual perception with the result of excessive contrast of luminance within the field of view which can result in anything from a mild discomfort through to impairment of vision. There are two forms of glare – Discomfort Glare & Disability Glare.

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H

High Frequency (HF) Control Gear   
Electronic control equipment that operates fluorescent lamps at a high frequency of 30­60kHz compared to a mains supply 50Hz. Increasing the frequency provides instant starting, removes stroboscopic effect (flickering) and provides for a more energy efficient circuit with increased quality of illumination. Can also be used in conjunction with some HID lamps.

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I

Illuminance   
Also referred to as “Luminous Flux Density” and is the total amount of visible light illuminating a point on a surface (either physical or an imaginary plane) from all angles. The standard unit of measurement is “Lux” which equates to “lumens per square metre”. Typical Values – Full Moonlight=1Lux / Street Lighting =10Lux / Workplace Lighting =100­1,000Lux / Office Lighting =300­500Lux / Operating Theatre table = 10,000­100,000Lux, / Full Sunshine = 100,000Lux.

Indirect Lighting   
Lighting provided by reflection, usually from wall or ceiling surfaces. Luminaires are usually either wall mounted or ceiling suspended distributing light upward which is reflected by the ceiling and walls to provide ambient illumination.

Ingress Protection (IP)   
Ratings, developed by the European Committee for Electro Technical Standardisation, are used to specify the environmental protection an enclosure provides to the electrical equipment inside it. An IP Rating normally has two numbers associated with it: (1) protection from solid objects or materials (like dust) and (2) protection from liquids (like water).

Intumescent material   
Material used within Fire Rated Downlights and gaskets, it expands to form a fire rated barrier once exposed to fire.

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K

Kelvin   
The standard unit for the measurement of Colour Temperature. The unit “Kelvin” is the basis upon which all temperature measurement is undertaken starting at “absolute zero”. The Kelvin chart mirrors that of degrees Celsius where zero Kelvin equates to ­273.15°Celsius.

Kilowatt (kW)   
Measure of electrical power which equals 1000 Watts.

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L

Lamp    
An artificial light source used in conjunction with appropriate fittings.

LED (Light ­emitting diode)   
Small semiconductor device that emits light with the advantages of low operation costs, compact size and long life.

(LENI) Lighting Energy Numeric Indicator   
Defined by the European Standard for addressing the Energy performance in Buildings Directive – BS EN15193:2007 as the measure for the lighting energy requirement for the building per square metre over a fixed time period (usually annually) and calculated – LENI = kilowatts/ metre²/Year.

Light Output Ratio (LOR)   
The ratio of light emitted by a luminaire to the total light output of the lamps it contains.

Lighting Efficacy   
A term relating to the amount of luminous flux emitted by a light source multiplied by the Light Output Ratio of the luminaire against the total amount of energy consumed (light source and any associated control gear) to produce the light.

LM­79 ­ LM­80   
LM­79 lighting measurement standard published by the IESNA in 2008 that provide specific practices for LED testing performance. LM­80 designates uniform test methods for measuring lumen maintenance for LEDs.

Lumen    
The SI derived unit of luminous flux, a basic measurement for light and is the amount of light emitted by a light source in all directions.

Lumen Maintenance   
A measurement of how a lamp maintains its light output over time (see LM­80). Learn more in our Light Lab

Luminance   
The amount of visible light leaving a point on a surface in a given direction, which can be either a physical or an imaginary plane and is the measured brightness of that surface which can be due to reflection and/or transmission and/or emission. The unit of measurement is candelas/metre²

Solar Disk at Noon = 1.6 x 10 cd/m²
Average Clear Sky = 8,000cd/m²
Average Cloudy Day = 2,000cd/m²

Luminaire    
A fitting which emits light by use of connecting a lamp to the required power supply.

Luminous Flux   
The total light output emitted by a light source or received by a surface. Commonly known as lumens. Unit of measurement is lumens.

Lux   
Lux is the international unit for illumination measured in lumens per square metre.

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M

The MacAdam Ellipse   
It is a system of colour measurement and measures how much colour variation is possible before the human eye detects the colour change.

A series of ellipses can be drawn around a target colour and the closer the lamp is to the target, the lower the colour deviation that will be experienced from the light source.

Measured in SDCM (Standard Deviation of Colour Matching) with 1 step having no difference, 2­3 steps will have an unnoticeable difference and 7 step is considered as the market norm in line with “Energy Star” requirements.

Maintenance Factor   
The result of time dependent depreciation effects must be considered in the initial design. Factor calculated into the initial installation to take account of the fitting and environment depreciation over time.

Maintained illuminance   
Maintained illuminance is the value below which the average illuminance is not allowed to fall. When a lighting installation is planned, account needs to be taken of the fact that as time moves forward, the environment depreciates and luminaires become less efficient. As a result, illuminance decreases and to compensate the new installation needs to be designed to a higher value.

Maintained Illuminance = Maintenance Factor x Illuminance on installation.

Modelling   
The use of light to create uniform illumination for three­ dimensional objects.

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P

Power Factor   
Is the ratio of the real power to apparent power and represents how much real power electrical equipment utilises. It is a measure of how effectively electrical power is being used. It is always between 0.0 & 1.0 and commercial installations with a low Power Factor and on a kVA tariff will have a greater energy consumption than a similar installation with a high Power Factor.

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T

Task Lighting   
Task lighting that is designed into an application to illuminate an area used for carrying out a specific task.

Transformer   
A device used to raise (step up) or lower (step down) the electric voltage.

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U

Unified Glare Rating (UGR)   
The UGR was developed by the International Commission on Illumination CIE (Commission Internationale de l’Eclairage). It is a complicated calculation being undertaken for each observer within the space taking into account the dimensions of the space and the position and type of all luminaires. The lower the UGR value, the lower the glare. The maximum accepted value for commercial offices is a value of 19. Learn more in our Light Lab

Uniformity    
The ratio of the minimum illuminance achieved within a space to that of the average achieved illuminance measured across the whole of the same space.

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V

Visual Performance   
The ability to perceive detail and carry out visual tasks within an environment.

Volt   
Unit of electrical force or pressure. Voltage in the UK, nominally 230 Volts, varies between 230 Volts­6% and 230 Volts+10%.

Volt Drop   
Difference in voltage along a circuit length. Voltage drop becomes a problem with Low Voltage wiring and small cable, resulting in a reduction of intensity of the light source at the far end of the run.

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W

Watt   
The unit for measuring electrical power. It defines the rate of energy consumption by an electrical device when it is in operation. In single phase circuits, it is related to volts and amps by the formula: Volts x Amps x P.F. = Watts. (Note: For AC circuits, P.F. must be included.)

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