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Glossary of Lighting Terms
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.
The angle between the opposed points either side of the beam axis where the luminous
intensity reaches a value of 50% that of the peak beam intensity.
Candela (cd)
The measure of luminous intensity of a light source in a given direction.
Is an objective specification of the quality of colour regardless of its luminance.
Colour Rendering Index
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).
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.
DALI (Digital Addressable Lighting Interface)
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 infra-red 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”.
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.
A ceiling based light fitting either, recessed or surface mounted which concentrates the
light in a downward direction.
Emergency Lighting
Lighting used in the event of power failure for whatever reason. Luminaire
types are usually either stand alone Non-maintained operation (Emergency use only) or mains
converted Maintained operation (Mains + Emergency use).
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.
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.
High Frequency (HF)
Control Gear Electronic control equipment that operates fluorescent lamps
at a high frequency of 30-60kHz 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.
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 =100-1,000Lux
/ Office Lighting =300-500Lux / Operating Theatre table = 10,000-100,000Lux, / Full Sunshine =
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).
material Material used within Fire Rated Downlights and gaskets, it expands to form
a fire rated barrier once exposed to fire.
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.
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.
Extra Low Voltage
IEC defined where the electrical potential does not exceed 25 Volts RMS for
AC or 60 Volts DC.
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.
LM-79 - LM-80 LM-79
lighting measurement standard published by the IESNA in 2008 that provide
specific practices for LED testing performance. LM-80 designates uniform test methods for measuring
lumen maintenance for LEDs.
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 LM-80).
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
Solar Disk at Noon = 1.6 x 109 cd/m²
Average Clear Sky = 8,000cd/m²
Average Cloudy Day = 2,000cd/m²
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.
Is the international unit for illumination measured in lumens per square metre.
The MacAdam Ellipse
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, 2-
3 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
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.
The use of light to create uniform illumination for three-dimensional objects.
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.
Task Lighting
Lighting that is designed into an application to illuminate an area used for carrying
out a specific task.
A device used to raise (step up) or lower (step down) the electric voltage.
The Unified Glare Rating (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.
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.
Visual Performance
The ability to perceive detail and carry out visual tasks within an environment.
Unit of electrical force or pressure. Voltage in the UK, nominally 230 Volts, varies between 230
Volts-6% 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.
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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