Where relevant, the definition of each term is followed by the definition of its unit.
QUANTITIES AND UNITS OF LIGHT
The light emitted by source, or received by a surface (expressed in lumens). The quantity is
derived from radiant flux (power) by evaluations the radiation in accordance with the spectral
sensitivity of the “standard” eye.
S1 unit of luminous flux: used in describing the quantity of light emitted by a source or
received by a surface (a 100W GLS lamp, for example, emits about 1200 m.). One lumen is
the luminous flux emitted within unit solid angle (one steradian) by a point source having a
uniform luminous intensity of one candela.
(intensity symbol: I)
The quantity which described the power of a source or illuminated surface to emit light in a
given direction. It is the luminous flux emitted in a very narrow cone containing the given
direction divided by the solid angle of the cone: the result is expressed in candelas.
SI unit of luminous intensity, 1 candela = 1 lumen per steradian
The process of lighting an object.
(unit: lux) (lumen per sq m.)
The luminous flux density at a surface, i.e. the luminous flux incident per unit area. (The
quantity was formerly known as the “illumination value” or “illumination level”)
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Standard maintained illuminance
The service illuminance recommended for standard conditions it is subject to modification for
special circumstances (indicated in the Flow Chart text book page 150 or CIBSE Code for
SI unit of illuminance, equal to one lumen per square metre.
Lumen per square foot
(Unit: lm/sq ft)
A non-metric unit of illuminance, equal to 10.764 lux (This has been superseded in the UK
by the metric unit lux. Previously called the foot-candle.
(units: candela per square metre. cd/m2)
A term which expressed the intensity of the light emitted in a given direction by unit area of a
luminous or reflecting surface. It is the luminous flux emitted in the given direction from a
surface element, divided by the product of the projected area of that element perpendicular to
the prescribed direction and the solid angle containing the direction. Thus defined,
luminance is expressed in lumens per square metre per steradian which, by definition, is the
equivalent of candelas per square metre.
Candela per square metre
(symbol: cd/m2 )
Si unit of luminance in lumens per square metre per steradian. Unit luminance in this system
is that of a uniform plane diffuser emitting lumens per square metre.
The process which takes place as the eye adjusts to the brightness or the colour of the visual
field. The term is also used, usually qualified, to denote the final state of the process. For
example, ‘dark adaptation` denotes the state of the eye when it has become adapted to very
A term that is used subjectively and objectively. Subjectively, it describes the difference in
appearance of two parts of a visual field seen simultaneously or successively. The difference
may by one of brightness or colour or both. Objectively, the term expresses the luminance
difference\ ratio numerically by such relations as:
contrast = (Lt - Lb ) divided by Lb .
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The discomfort or impairment of vision experienced when parts of the visual field are
excessively bright in relation to the general surroundings.
Glare which impairs the ability to see detail without necessarily causing visual discomfort.
Glare which causes visual discomfort without necessarily impairing the ability to see detail.
Correlated colour temperature (CCT)
The temperature of a full radiator which emits radiation having a chromaticity nearest to that
of the light source being considered, e.g. the colour of a full radiator at 3400K is the nearest
match to that of a White fluorescent lamp.
A general expression for the colour appearance of objects when illuminated by light from a
given source compared consciously or unconsciously, with the appearance under light from
some reference source. Good ‘colour rendering` implies similarity of appearance to that
under an acceptable light source, such as daylight. The colour rendering properties of a lamp
relates to this effect under specified conditions.
Of a light source. Objectively the chromaticity of a truly white surface, illuminated by the
source. Subjectively, the hue of a white surface illuminated by the source; the degree of
warmth associated with the source colour. (Lamps of low correlated colour appearance, and
lamps of high correlated colour temperature as having a cool colour appearance.)
Colour rendering index
A measure of the degree to which the measured colours of surfaced illuminated by a given
light source conform to those of the same surfaces under a reference illuminant.
(unit: lumen per watt, lm/W)
The ratio of the luminous flux emitted by a lamp to the power consumed by it
The luminous flux from a lamp after 100 hours of operation.
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OPTICAL PROPERTIES OF MATERIALS
Specular reflection (regular reflection)
Reflection without diffusion in accordance with the laws of optical reflection, as in a mirror
Reflection in which the reflected light is diffuses and there is no significant specular
reflection, as from a matt paint.
Partly specular and partly diffused reflection, as from a smooth non-gloss paint.
The ratio of the flux reflected from a surface to the flux incident on it. Except for matt
surfaced, the reflectance depends on how the surface is illuminated and especially on the
directions of the incident light and its spectral distribution. The value is always less than
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TYPES OF LIGHTING
Luminaires from which light is emitted only within relatively small angles to the downward
Luminaires which direct most of the light upwards onto the ceiling cavity in order to
illuminate the working plane by reflection.
Lighting designed to illuminate the whole of an area without provision for special local
Lighting designed to illuminate an interior and at the same time to provide higher illuminance
over a particular part or parts of the area.
Lighting designed to illuminate a particular small area which usually does not extend far
outside the visual task (e.g. a desk light)
Lighting in which the greater part of the flux from the luminaires reaches a surface (usually
the working place) directly, i.e. without reflection from surrounding surfaces. Luminaires
with a flux fraction ratio less than 0.1 are usually regarded as direct.
Lighting in which the greater part of the flux reached a surface (usually the working plane)
only after reflection at other surfaces and particularly at the roof or ceiling. Luminaires with
a flux fraction ratio greater then 10 are usually regarded as indirect.
Lighting designed to illuminate a task or surface predominantly from some preferred
Lighting in which the flux comes from many direction, none of which predominates.
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The ratio of the illuminance at a point on a given plane within an interior due to the light
received directly and indirectly from a day of assumed or known luminance distribution, to
that on a horizontal plane due to an unobstructed hemisphere of this sky. Direct sunlight is
excluded from both values of illuminance.
Average daylight factor
The ratio of the average illuminance on all the interior room surfaces to that on a horizontal
place due to an unobstructed hemisphere of sky.
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EMERGENCY LIGHTING DEFINITIONS
The luminous flux density at a surface, i.e. the luminous flux incidence per unit area. The
unit of illuminance is lux.
The illuminance on a surface that is due to light flux directly from the luminaires of the
installation. Excludes light from all surfaces.
An apparatus which distributes, filters and transforms the lighting given by a lamp or lamps
and includes all the items necessary for fixing and protecting these lamps and for connecting
them to the supply circuit. Note that internally illuminated signs are a special type of
Maintained emergency luminaire
A luminaire containing one or more lamps all of which operate from normal supply or from
the emergency supply at all material times.
Self-contained emergency luminaire or single point luminaire
A luminaire or sign providing maintained or non-maintained emergency lighting in which all
the elements such as the battery, the lamp and the control unit are contained within the
housing or within one metre of the housing.
Non- maintained emergency luminaire
A luminaire containing one or more lamps, which operate from the emergency supply only
upon failure of the normal mains supply.
Slave or centrally supplied luminaire
An emergency luminaire without its own batteries to work with a central battery system.
All permanently installed artificial lighting operating from the normal electrical supply, that
in the absence of adequate daylight, is intended for use during the whole time that the
premises are occupied.
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That part of emergency lighting provided to enable normal activities to continue in the event
of failure of the normal mains supply.
Emergency escape lighting
That part of emergency lighting provided to enable safe exit of failure of the normal supply.
Escape route lighting
That part of emergency lighting provided to enable safe exit for building occupants by
providing appropriate visual conditions and direction finding on escape routes and in special
areas/locations, and to ensure that fire fighting and safety equipment can be readily located
Open area (or anti-panic area) lighting
That part of emergency lighting provided to reduce the likelihood of panic and to enable safe
movement of occupants towards escape routes by providing appropriate visual conditions and
High risk task area lighting
That part of emergency lighting provided to ensure the safety of people involved in a
potentially dangerous process or situation and to enable proper shut down procedures to be
carried out for the safety of other occupants of the premises.
The vertical distance between the luminaire and the working plane. Note that the floor is
taken to be the working plane for emergency lighting.
The relationship between the height, length and width of a room used for illuminance
Controls the operation of a fluorescent lamp from a specified AC or DC source (typically
between 12 and 240 volts). It can also include elements for starting the lamp, for power
factor correction or radio frequency interference suppression.
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Ballast lumen factor (blf)
The ratio of the light output of the lamp when the ballast under test is operated at its design
voltage, compared with the light output of the same lamp operated with the appropriate
reference ballast supplied at its rated voltage and frequency.
Secondary cells providing the source of power during mains failure.
Battery sealed (recombination)
A battery that is totally sealed, or constructed so that no provision is made for replacement of
Battery unsealed (vented)
A battery that requires replacement of electrolyte at regular periods.
The discharge capability of a battery, being a product of average current and time, expressed
as ampere hours over a stated duration. Note that shorter total discharge period gives rise to a
smaller available capacity.
The manufacturers declared duration, specifying the time for which the emergency lighting
will provide the rated lumen output after mains failure. This may be for any reasonable
period but is normally one or three hours.
The maximum load that may be connected to the system and will be supplied for the rated
The time necessary for the batteries to regain sufficient capacity to achieve their rated
Central battery system
A system in which the batteries for a number of luminaires are housed in one location are
housed in one location, usually for all the emergency luminaires in one lighting sub-circuit,
sometimes for all emergency luminaires in a complete building.
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Combined emergency luminaire (sustained)
Contains two or more lamps at least one of which is energised from the emergency supply
and the remainder from the normal supply. The lamp energised from the emergency supply
in a combined emergency luminaire is either maintained or non-maintained.
The voltage declared by the manufacturer to which all the ballast characteristics are related.
A way out which is intended to be used any time that he premises are occupied.
The terminal point of an escape route, beyond which persons are no longer in danger from
fire or any other hazard requiring evacuation of the building.
Shows the luminaires can be mounted on combustible surfaces. It does not show that the
luminaire is fire retardant.
Fire retardant housing 850oc test
All emergency luminaires on escape routes must pass this test as specified in EN 60 598-2-
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