Someone suggested that The Light Review should have an explainer for all those head-scratching technical terms that the industry is awash with. I agreed – and then I thought about it. Then I spoke to my friends at the Society of Lighting and Lighting . . .
. . . because you must have heard that there’s a new SLL Lighting Handbook, published at the end of last year. And it has an extensive Glossary – and its reproduced here!
Thanks to all concerned.
from The SLL Lighting Handbook: Appendix 4: Glossary of terms
A4.1 Introduction
The following definitions for lighting terms are given to assist the reader’s understanding of the terms used throughout this Handbook. For the precise mathematical definitions of some of the terms, refer to the glossary in the SLL Code for Lighting (SLL, 2004) or in BS EN 12665: 2018:International Electrotechnical Vocabulary (IEC, online).
The list includes definitions of some terms that are not used within the Handbook but are here for information.
A4.2 Glossary
absence detection
The verification of continued presence in a space after the luminaires have been manually switched on inorder to maintain the ‘on’ state of the luminaires.
adaptation
Adaptation is the ability of the human eye to adjust to various levels of light and is a process which takes place as the visual system adjusts to the luminance and colour of the visual field or the final state of this process.
In a light-adapted eye the cones in the retina are fully active and detail can be seen easily. The rod pigments are bleached out from having been in a bright area. Moving from a well-lit area to a dark area means that initially little can be seen as the rods are initially not functional. Once in the dark, the sensitivity of the retina increases over time (this can take many minutes). At the start of the adaptation process the pupil dilates as a reflexive change.
Conversely, when going from a dark area into a bright area. The bright light momentarily dazzles us because the sensitivity of the receptors is set to dim light. Rods and cones are both stimulated and large amounts of the photopigment are broken down instantaneously, producing a flood of signals resulting in the glare.
Within about one minute the cones are sufficiently excited by the bright light to take over. Visual accuracy and colour vision continue to improve over the next ten minutes.
annual operating time (to)
Number of hours per annum for which the lamps are operating (unit: h).
astronomical time clock
A timing device or software function designed to switch lighting on at dusk and off at dawn in relation to the day of the year at a given geographical location.
average illuminance (–E)
Illuminance averaged over the specified surface area (unit: lx).
Note: in practice this can be derived either from the total luminous flux falling on the surface divided by the total area of the surface or, alternatively, from an average of the illuminances at a representative number of points on the surface.
average luminance (–L)
Luminance averaged over the specified surface (unit: cd·m–2)
Note: in practice, this may be approximated by an average of the luminances at a representative number of point on the surface.
automatic test system (ATS)
System consisting of various parts (such as timers, current detectors, changeover switches) that can automatically carry out the routine testing requirements of emergency lighting luminaires and indicate the test results.
Note: the system may be manually initiated.
background area
Area in the workplace adjacent to the immediate surrounding area.
ballast
Device connected between the supply and one or more discharge lamps that serves mainly to limit the current of the lamp(s) to the required value.
Note: a ballast may also include means for transforming the supply voltage, correcting the power factor and, either alone or in combination with a starting device, provide the necessary conditions for starting the lamp(s). A ballast may also contain an analogue or digital input to control the functionality of the luminaire.
ballast lumen factor (Fballast)
Ratio of the luminous flux emitted by a reference lamp when operated with a particular production ballast to the luminous flux emitted by the same lamp when operated with its reference ballast.
Note: ballast lumen factor is sometimes signified by the abbreviation BLF.
brightness
Attribute of a visual perception according to which an area appears to emit (or reflect) more or less light.
brightness contrast
Subjective assessment of the difference in brightness between two or more surfaces seen simultaneously or successively.
built-in luminaire
Fixed luminaire installed into structure or equipment to provide illumination.
carriageway
Part of the road normally used by vehicular traffic.
centrally supplied emergency luminaire
Luminaire that is energised from a central emergency power system that is not contained within the luminaire.
changeover operation
Automatic connection of the lamp to the emergency lighting supply when failure of the normal lighting supply occurs, and connecting automatically back to the normal lighting supply when it is restored.
chromaticity
Property of a colour stimulus defined by its chromaticity coordinates, or by its dominant or complementary wavelength and purity taken together.
Note: see also CIE 15: 2004 (CIE, 2004).
colorimeter
Instrument for measuring colorimetric quantities, such as the tristimulus values of a colour stimulus.
colour contrast
Subjective assessment of the difference in colour between two or more surfaces seen simultaneously or successively.
colour rendering
Effect of an illuminant on the colour appearance of objects by conscious or subconscious comparison with their colour appearance under a reference illuminant.
For design purposes, colour rendering requirements shall be specified using the general colour rendering index and shall take one of the following values of Ra: 20, 40, 60, 80, 90.
colour rendering index (Ra)
Value intended to specify the degree to which objects illuminated by a light source have an expected colour relative to their colour under a reference light source.
Note: Ra is derived from the colour rendering indices for a specified set of eight test colour samples. Ra has a maximum of 100, which generally occurs when the spectral distributions of the light source and the reference light source are substantially identical.
colour stimulus
Visible radiation entering the eye and producing a sensation of colour, either chromatic or achromatic.
colour temperature (Tc)
Temperature of a Planckian radiator whose radiation has the same chromaticity as that of a given stimulus (unit: K).
Note: the reciprocal colour temperature is also used, unit: K–1.
competent person
Person with the relevant current training and experience, and with access to the requisite tools, equipment and information, and capable of carrying out a defined task.
combined emergency luminaire
Luminaire containing two or more lamps at least one of which is operating from the emergency lighting supply and the other(s) from the normal lighting supply.
Note: a combined emergency luminaire is either maintained or non-maintained.
commissioning
The verification against design intent, setting to work and relevant operator training in respect of a lighting installation.
constant illumination
The measurement and regulation of excess illumination to the required level. Constant illumination is useful for saving energy during the early part of a lamp’s operational life, before lamp depreciation
becomes apparent.
contrast
(1) In the perceptual sense: assessment of the difference in appearance of two or more parts of a field seen simultaneously or successively (hence: brightness contrast, lightness contrast, colour contrast, simultaneous contrast, successive contrast, etc).
(2) In the physical sense: quantity intended to correlate with the perceived brightness contrast, usually defined by one of a number of formulae which involve the luminances of the stimuli considered, for example: ΔL/L near the luminance threshold, or L1/L2 for much higher luminances.
control gear
Components required to control the electrical operation of the lamp(s).
Note: control gear may also include means for transforming the supply voltage, correcting the power factor and, either alone or in combination with a starting device, provide the necessary conditions for starting the lamp(s).
correlated colour temperature (CCT) (Tcp)
The temperature of the Planckian radiator whose perceived colour most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions (unit: K).
cosine correction
Correction of a detector for the influence of the incident direction of the light.
Note: for the ideal detector, the measured illuminance is proportional to the cosine of the angle of incidence of the light. The angle of incidence is the angle between the direction of the light and the normal to the surface of the detector.
critical flicker frequency
See fusion frequency.
curfew
Time period during which stricter requirements (for the control of obtrusive light) will apply.
Note: it is often a condition of use of lighting applied by a government controlling authority, usually the local government.
cut-off
Technique used for concealing lamps and surfaces of high luminance from direct view in order to reduce glare.
Note: this term has been replaced by luminous intensity classes.
cut-off angle (of a luminaire)
Angle, measured up from nadir, between the vertical axis and the first line of sight at which the lamps and the surfaces of high luminance are not visible (unit: degree).
cylindrical illuminance (at a point, for a direction) (Ez)
Total luminous flux falling on the curved surface of a very small cylinder located at the specified point divided by the curved surface area of the cylinder (unit: lx).
daylight
Visible part of global solar radiation capable of causing a visual sensation.
daylight dependency factor (FD)
Level of efficiency that a control system or control strategy exploits the saving potential of daylight in a space.
daylight factor (D or DF)
Ratio of the illuminance at a point on a given plane due to the light received directly and indirectly from a sky of assumed or known luminance distribution, to the illuminance on a horizontal plane due to an unobstructed hemisphere of this sky, where the contribution of direct sunlight to both illuminances is excluded.
Note 1: glazing transmission losses, dirt effects, etc. are included.
Note 2: when calculating the lighting of interiors using this method, the contribution of direct sunlight needs to be considered separately.
daylight linking
Measurement of available natural light in real time at the working plane with automated control to electric lighting in order to ‘blend’ both sources to achieve the required illumination level.
daylight time usage (tD)
Annual operating hours during the daylight time, measured in hours (unit: h).
design speed
Speed adopted for a particular stated purpose in designing a road (unit: km·h–1).
diffuse sky radiation
That part of solar radiation which reaches the earth as a result of being scattered by the air molecules, aerosol particles, cloud particles or other particles.
diffused lighting
Lighting in which the light on the working plane or on an object is not incident predominantly from a particular direction.
direct lighting
Lighting by means of luminaires having a distribution of luminous intensity such that the fraction of the emitted luminous flux directly reaching the working plane, assumed to be of infinite extent, is 90% to 100%.
direct solar radiation
That part of the extraterrestrial solar radiation which as a collimated beam reaches the earth’s surface after selective attenuation by the atmosphere.
directional lighting
Lighting in which the light on the working plane or on an object is incident predominantly from particular direction.
disability glare
Glare that impairs the vision of objects without necessarily causing discomfort. Disability glare can be produced directly or by reflection.
discomfort glare
Glare that causes discomfort without necessarily impairing the vision of objects. Discomfort glare can be produced directly or by reflection.
display screen equipment
Alphanumeric or graphic display screen, regardless of the display process employed
Note: display screen equipment is sometimes signified by the abbreviation DSE.
(spatial) distribution of luminous intensity (of a source)
Display, by means of curves or tables, of the value of the luminous intensity of the source as a function of direction in space.
diversity (luminance, illuminance) (Ud)
Ratio of minimum illuminance (luminance) to maximum illuminance (luminance) on (of) a surface.
See also uniformity
downward light output ratio (DLOR) (of a luminaire: RDLO)
Ratio of the downward flux of the luminaire, measured under specified practical conditions with its own lamps and equipment, to the sum of the individual luminous fluxes of the same lamps when operated outside the luminaire with the same equipment, under specified conditions.
Note: the luminaire attitude should be declared so that appropriate corrections to the DLOR can be made if in application the installed attitude is different.
driver
Device connected between the supply and one or more LED lamps which serves mainly to limit the current and/or regulate the voltage to the lamp(s) to the required value.
Note: a driver may also include means for transforming the supply voltage and correcting the power factor. A driver may also contain an analogue or digital input to control the functionality of the luminaire.
Efficacy
See luminous efficacy of a source
emergency ballast lumen factor (EBLF)
Ratio of the emergency luminous flux of the lamp supplied by the emergency control gear to the luminous flux of the same lamp operated with the appropriate reference ballast at its rated voltage and frequency.
Note: the emergency ballast lumen factor is the minimum of the values measured at the appropriate time after failure of the normal supply and continuously to the end of the rated time duration.
emergency escape lighting
Part of emergency lighting that provides illumination for visibility for people leaving a location and
attempting to terminate a potentially dangerous process before doing so. See high risk task area lighting.
emergency exit
Way out that is intended to be used during an emergency.
emergency lamp flux
See practical emergency lamp flux.
emergency lane (hard shoulder)
Lane parallel to the traffic lane(s) provided for emergency and/or broken-down vehicles only.
emergency lighting
Lighting provided automatically for use when the supply to the normal lighting fails.
emergency lighting charge time (tem)
Operating hours during which the emergency lighting batteries are being charged (unit: h).
emergency lighting charging power (Pei)
Input power to the charging circuit of emergency luminaires when the lamps are not operating (unit: W).
emergency lighting, total installed charging power
See total installed charging power of the emergency lighting luminaires in the room or zone.
emergency luminaire rated luminous flux
Lumen output in emergency operation as claimed by the luminaire manufacturer, 60 seconds (0.5 s for high-risk task-area luminaires) after failure of the normal supply, and continuously maintained to the end of rated duration of operation.
emergency safety lighting
Part of emergency lighting that provides illumination for the safety of people staying in a premise when the supply to the normal lighting fails.
energy consumption used for illumination (WL,t)
Energy consumed in period t by the luminaires when the lamps are operating to fulfil the illumination function and purpose in the building (unit: kW·h).
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equivalent veiling luminance (for disability glare or veiling reflections) (Lve)
Luminance that, when added by superposition to the luminance of both the adapting background and the object, makes the luminance threshold or the luminance difference threshold the same under the two following conditions: (1) glare present, but no additional luminance; (2) additional luminance present, but no glare (unit: cd·m–2).
escape route
Route designated for escape in the event of an emergency.
escape route lighting
Part of emergency escape lighting provided to ensure that the means of escape can be effectively identified and safely used when the location is occupied.
externally illuminated safety sign
Safety sign that is directly illuminated by both general and emergency light sources, when it is required, positioned within 2 m horizontally from the sign.
flicker
Impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time.
flicker frequency
See fusion frequency.
floodlighting
Lighting of a scene or object, usually by projectors, in order to increase considerably its illuminance relative to its surroundings.
flux
See luminous flux, rated lamp luminous flux.
fusion frequency
Critical flicker frequency (for a given set of conditions). Frequency of alternation of stimuli above which flicker is not perceptible (unit: Hz).
general colour rendering index
See colour rendering index.
general lighting
Substantially uniform lighting of an area without provision for special local requirements.
glare
Condition of vision in which there is discomfort or a reduction in the ability to see details or objects, caused by an unsuitable distribution or range of luminance, or to extreme contrasts.
See also disability glare and discomfort glare.
glare rating limit (RGL)
Maximum allowed value given by the CIE Glare Rating system.
global solar radiation
Combined direct solar radiation and diffuse sky radiation.
grid points for measurement and calculation
Arrangement of calculation and measurement points and their number in each dimension of the reference surface or plane.
hemispherical illuminance (at a point) (Ehs)
Total luminous flux falling on the curved surface of a very small hemisphere located at the specified point divided by the curved surface area of the hemisphere (unit: lx).
high risk task area lighting
Part of emergency escape lighting that provides illumination for visibility for people involved in a potentially dangerous process or situation and facilitates safe termination of activities
Note: in sports lighting it is referred to as ‘Safety lighting for participants’.
illuminance (at a point of a surface) (E)
Quotient of the luminous flux incident on an element of the surface containing the point, by the area of that element (unit: lx = lm·m–2).
Note: the orientation of the surface may be defined, e.g. horizontal, vertical, hence horizontal illuminance, vertical illuminance.
illuminance meter
Instrument for measuring illuminance.
immediate surrounding area
See surrounding area.
indirect lighting
Lighting by means of luminaires having a distribution of luminous intensity such that the fraction of the emitted luminous flux directly reaching the working plane, assumed to be of infinite extent, is 0 to 10%.
inhibit mode
State of a self-contained emergency luminaire that is inhibited from operating by a remote device while the normal supply is on and in case of a normal supply failure the luminaire does not changeover to emergency mode.
Ingress Protection (IP) ratings
Numerical index used to define levels of sealing effectiveness of electrical enclosures, including luminaires, against intrusion from foreign bodies (tools, dirt etc) and moisture.
initial average luminance (–L)
Average luminance of the specified surface when the installation is new (unit: cd·m–2).
initial illuminance (Eav or –E)
Average illuminance on the specified surface when the installation is new (unit: lx).
installed loading
Installed power of the lighting installation per unit area (for interior and exterior areas) or per unit length (for road lighting) (unit: W·m–2 for areas; kW·km–1 for road lighting).
integral lighting system (of a machine)
Lighting system consisting of lamp(s), luminaire(s) and associated mechanical and electrical control devices which forms a permanent part of the machine, designed to provide illumination in and/or at the machine.
intensity
See luminous intensity.
intensity distribution
See luminous intensity distribution.
internally illuminated safety sign
Safety sign that is illuminated, when it is required, by an internal source.
IK rating
Numerical index used to define the degrees of protection provided by electrical enclosures (including luminaires) against external mechanical impacts.
IP rating
Numerical index used to define levels of sealing effectiveness of electrical enclosures, including
luminaires, against intrusion from foreign bodies (tools, dirt etc.) and moisture.
lamp
Source made in order to produce an optical radiation, usually visible.
Note: this term is also sometimes used for certain types of luminaires.
lamp code
Any combination of letters and numbers by which the lamp type is identified.
lamp dimensions
All dimensions of the lamp that are relevant for the luminaire design.
lamp lumen maintenance factor (FLLM)
Ratio of the luminous flux of a lamp at a given time in its life to the initial luminous flux.
Note: lamp lumen maintenance factor is sometimes signified by the abbreviation LLMF.
lamp luminous flux
See rated luminous flux.
lamp survival factor (FLS)
Fraction of the total number of lamps that continue to operate at a given time under defined conditions and switching frequency.
Note: lamp survival factor is sometimes signified by the abbreviation LSF.
lamp wattage
Wattage used by a lamp in operation (unit: W).
LED (light emitting diode)
Solid state device embodying a p–n junction, emitting optical radiation when excited by an electric current.
life of lighting installation
Period after which the installation cannot be restored to satisfy the required performance because of nonrecoverable deteriorations.
light loss factor
See maintenance factor.
light output ratio (of a luminaire) (RLO)
Ratio of the total luminous flux of the luminaire, measured under specified practical conditions with its own lamps and equipment, to the sum of the individual luminous fluxes of the same lamps when operated outside the luminaire with the same equipment, under specified conditions.
Note 1: for luminaires using incandescent lamps only, the optical light output ratio and the light output ratio are the same in practice.
Note 2: light output ratio is sometimes signified by the abbreviation LOR.
See also downward light output ratio and upward light output ratio.
light source
See source.
light source colour
The colour of a light source can be expressed by its correlated colour temperature.
Lighting energy numeric indicator (LENI)
Numeric indicator of the total annual lighting energy required in the building (unit: kW·h·m–2·year–1).
Note: the LENI can be used to make direct comparisons of the lighting energy used in buildings that have similar functions but are of different sizes and configurations.
loading
See installed loading.
local lighting
Lighting for a specific visual task, additional to and controlled separately from the general lighting.
localised lighting
Lighting designed to illuminate an area with a higher illuminance at certain specified positions, for instance those at which work is carried out.
longitudinal uniformity (of road surface luminance of a carriageway) (Ul)
Lowest of the ratios determined for each driving lane of the carriageway as the ratio of the lowest to the highest road surface luminance found in a line in the centre along the driving lane.
louvres
(1) Daylight: fixed or adjustable blades or baffles on windows to restrict daylight and/or preclude sunlight.
(2) Luminaires: fixed blades to restrict or reflect some portion of the light from the lamp or light source.
luminaire
Apparatus that distributes, filters or transforms the light transmitted from one or more lamps and which includes all the parts necessary for fixing and protecting the lamps (excepting the lamps themselves) and, where necessary, circuit auxiliaries together with the means for connecting them to the electrical supply.
luminaire code
Any combination of letters and numbers by which the luminaire type is identified.
luminaire maintenance factor (FLM)
Ratio of the light output ratio of a luminaire at a given time to the initial light output ratio.
Note: luminaire maintenance factor is sometimes signified by the abbreviation LMF.
luminaire luminous efficacy (h1)
Quotient of the luminous flux emitted by the luminaire by the power absorbed by the lamp and associated circuits of the luminaire (unit: lm·W–1).
luminaire standby energy consumption (WP, t)
Standby energy consumed in period t, by the luminaire emergency lighting charging circuit plus the standby control system controlling the luminaires when the lamps are not operating (unit: kW·h).
luminaire parasitic power (Ppi)
Input power consumed by the charging circuit of emergency lighting luminaires and the standby power for automatic controls in the luminaire when lamps are not operating (unit: W).
luminaire power (Pi)
Input power consumed by the lamp(s), control gear and control circuit in or associated with the luminaire, which includes any standby power when the luminaire is turned on (unit: W)
Note: the rated luminaire power (Pi) for a specific luminaire may be obtained from the luminaire manufacturer.
luminance (in a given direction, at a given point of a real or imaginary surface) (L)
The luminous intensity of the light emitted or reflected in a given direction from an element of the surface, divided by the area of the element projected in the same direction (unit: cd·m–2 = lm·m–2·sr–1).
luminance contrast
Photometric quantity intended to correlate with brightness contrast, usually defined by one of a number of equations which involve the luminances of the stimuli considered.
luminance meter
Instrument for measuring luminance.
luminous efficacy of a source (h)
Quotient of the luminous flux emitted by the power absorbed by the source (unit: lm·W–1).
luminous environment
Lighting considered in relation to its physiological and psychological effects.
luminous flux (F )
Quantity derived from radiant flux (radiant power) by evaluating the radiation according to the spectral sensitivity of the human eye (as defined by the CIE standard photometric observer). It is the light power emitted by a source or received by a surface (unit: lumen (lm)).
Note: the values used for the spectral sensitivity of the CIE standard photometric observer are those of the spectral luminous efficiency function V(l).
See also rated luminous flux and rated luminous flux.
luminous intensity (of a source, in a given direction) (Iv)
Luminous flux per unit solid angle in the direction in question, i.e. the luminous flux on a small surface, divided by the solid angle that the surface subtends at the source.
Note: luminous intensity may also be denoted using the symbol I.
luminous intensity classes
Classes for external luminaires where the maximum luminous intensity is restricted above certain angles from the downward vertical. Needed to restrict disability glare from installations where the threshold increment (fn) cannot be calculated. Also used to control obtrusive light.
Note: see BS EN 13201-2 (BSI, 2015).
maintained emergency luminaire
Luminaire in which emergency light sources are operating at all times when normal lighting or emergenc
lighting is required.
maintained illuminance (–Em)
Value below which the average illuminance on the specified area should not fall.
Note: it is the average illuminance at the time maintenance should be carried out.
maintained luminance (–Lm)
Value below which the average luminance on the specified area should not (unit: cd·m–2)
Note: it is the average luminance at the time maintenance should be carried out.
maintenance cycle
Repetition of lamp replacement, lamp/luminaire cleaning and room surface cleaning intervals.
maintenance factor
Ratio of illuminance produced by the lighting installation after a certain period to the illuminance produced by the installation when new.
Note: the maintenance factor takes into account light losses caused by dirt accumulation on luminaires and room surfaces (in interiors) or other relevant surfaces (in exteriors, where appropriate), and the decrease of the luminous flux of lamps.
See also lamp lumen maintenance factor, luminaire maintenance factor and room surface
maintenance factor.
mesopic vision
Vision intermediate between photopic and scotopic vision.
Note: in mesopic vision, both the cones and the rods are active.
maintenance schedule
Set of instructions specifying maintenance cycle and servicing procedures.
mixed traffic
Traffic that consists of motor vehicles, cyclists, pedestrians etc.
motor traffic (motorized traffic)
Traffic that consists of motorized vehicles only.
nominal lamp wattage (Wlamp)
Approximate wattage used to designate or identify the lamp (unit: W).
non-daylight time usage (tN)
Annual operating hours during the non-daylight time (unit: h).
non-maintained emergency luminaire
Luminaire in which the emergency light sources are in operation only when the supply to the normal lighting fails.
obtrusive light
Spill light which because of quantitative, directional or spectral attributes in a given context gives rise to annoyance, discomfort, distraction or reduction in the ability to see essential information.
Note 1: in the case of outdoor sports lighting installations, obtrusive light is considered around the installation and not for spectators, referees or players within the sports area.
Note 2: in the case of large tertiary buildings with predominantly glazed facades, interior lighting may be considered as obtrusive light if it gives rise to annoyance, discomfort, distraction or a reduction in the ability to see essential information due to light spilling outside of the building structure.
occupancy dependency factor (Fo)
Factor indicating the proportion of time that a space is occupied, and lighting is required.
open area lighting (anti-panic lighting)
Part of emergency escape lighting provided to avoid panic and provide illumination allowing people to see their way to an escape route.
operating time (t)
Time period for energy consumption (unit: h).
See also annual operating time.
principal area (Aprinciple)
Actual playing area needed for the performance of a certain sport.
Note: usually this means the actual marked out ‘field’ area for that sport (for instance football), but in some cases this area comprises an extra playing area around the marked area (e.g. tennis, volleyball, table tennis). The dimensions of the particular area should be checked at the time when a lighting installation is being installed.
performance
See visual performance.
photoluminescence
Luminescence caused by absorption of optical radiation.
photometer
Instrument for measuring photometric quantities.
photometry
Measurement of quantities referring to radiation as evaluated according to a given spectral luminous efficiency function, e.g. V(l) or V′(l).
Note: photometry can also be defined as a measurement of quantities referring to radiation evaluated according to the spectral sensitivity of the human eye (as defined by the CIE standard photometric observer).
photopic vision
Vision by the normal eye when it is adapted to levels of luminance of at least several candelas per square metre.
Note: the cones are the principal active photoreceptors in photopic vision.
PIR (passive infrared)
Movement detector used as part of a presence or absence detection system.
Planckian radiator (blackbody)
Ideal thermal radiator that absorbs completely all incident radiation, whatever the wavelength, the direction of incidence or the polarization. This radiator has, for any wavelength and any direction, the maximum spectral concentration of radiance for a thermal radiator in thermal equilibrium at a given temperature.
practical emergency lamp flux
Lowest luminous flux of the lamp observed during the rated duration of the emergency mode (unit: lm).where the practical the initial lighting design lumens at 100 (lm) is modified by the emergency ballast lumen factor.
presence detection
The automatic detection of presence in a space in order to switch the luminaires on during space
occupancy.
radiant flux
See luminous flux.
rated luminous flux (of a type of lamp or luminaire)
Value of the initial luminous flux of a given type of lamp or luminaire declared by the manufacturer or the responsible vendor, the lamp or luminaire being operated under specified conditions (unit: lm).
Note 1: the initial luminous flux is the luminous flux of a lamp or luminaire after a short ageing period as specified in the relevant lamp standard.
Note 2: the rated luminous flux is sometimes marked on the lamp.
reference ballast
Special type ballast designed for the purpose of providing comparison standards for use in testing ballasts, for the selection of reference lamps and for testing regular production lamps under standardized conditions.
Note: it is essentially characterised by a stable voltage-to-current ratio, which is relatively uninfluenced by variations in current, temperature and magnetic surroundings.
reference surface
Surface on which illuminance is measured or specified.
reflectance (for incident radiation of given spectral composition, polarization and geometrical distribution) (r ) Ratio of the reflected radiant or luminous flux to the incident flux in the given conditions (unit: one).
Note 1: reflectance can also be defined as the ratio of the luminous flux reflected from a surface to the luminous flux incident on it.
Note 2: the reflectance generally depends on the direction and spectral distribution of the incident light and the surface finish.
reflections
See veiling reflections.
reflectometer
Instrument for measuring quantities pertaining to reflection.
Responsible Person
Delegated individual who is responsible for the provision and operation of appropriate emergency escape lighting.
rest mode
State of a self-contained emergency luminaire that has been intentionally extinguished while the normal supply is off and that, in the event of restoration of the normal supply, automatically reverts to normal mode.
rooflight
Daylight opening on the roof or on a horizontal surface of a building.
room surface maintenance factor (FRSM)
Ratio of the light reflected by the surfaces of a room after a certain period of use of the lighting installation to light reflected when the installation is considered conventionally as new.
Note: room surface maintenance factor is sometimes signified by the abbreviation RSMF.
safety sign
Sign that gives a general safety message, obtained by a combination of colour and geometric shape and which, by the addition of a graphic symbol or text, gives a particular safety message.
scene setting
A software function or manually via a scene setting switch in order to select the available lighting scenes in a space.
scene setting operation time (ts)
Operating hours of the scene setting controls (unit: h).
scotopic observer
Vision by the normal eye when it is adapted to levels of luminance less than some hundredths of a candela per square metre.
Note: the rods are the principal active photoreceptors in scotopic vision.
self-contained emergency luminaire
Luminaire providing maintained or non-maintained emergency lighting in which the battery, lamp,
control unit and the test and monitoring facilities, where provided, are contained within the luminaire or adjacent to it, within 1 m cable length.
semi-cylindrical illuminance (at a point) (Esz)
Total luminous flux falling on the curved surface of a very small semi-cylinder located at the specified point, divided by the curved surface area of the semi-cylinder (unit: lx).
Note: the axis of the semi-cylinder is taken to be vertical unless stated otherwise. The direction of the curved surface should be specified.
semi-direct lighting
Lighting by means of luminaires having a distribution of luminous intensity such that the fraction of the emitted luminous flux directly reaching the working plane, assumed to be of infinite extent, is 60% to 90%.
semi-indirect lighting
Lighting by means of luminaires having a distribution of luminous intensity such that the fraction of the emitted luminous flux directly reaching the working plane, assumed to be of infinite extent, is 10% to 40%.
shielding angle
The angle between the horizontal plane and the first line of sight at which the luminous parts of the lamps in the luminaire are directly visible (unit: degrees).
Note: the complementary angle to the shielding angle is the cut-off angle.
Skylight
Part of sky radiation capable of causing a visual sensation.
Note: when dealing with actinic effects of optical radiations, this term is commonly used for radiations extending beyond the visible region of the spectrum.
slave luminaire
See centrally supplied emergency luminaire.
source (light source)
Object that produces light or other radiant flux.
Note: the term light source indicates the source is essentially intended for illuminating and signalling purposes.
solar radiation
Electromagnetic radiation from the sun.
See also direct solar radiation and global solar radiation.
spacing (in an installation)
Distance between the light centres of adjacent luminaires of the installation.
spacing to height ratio
Ratio of spacing to the height of the geometric centres of the luminaires above the reference plane.
Note: for indoor lighting the reference plane is usually the horizontal working plane; for exterior lighting the reference plane is usually the ground.
spectral luminous efficiency
See luminous flux.
spherical illuminance (at a point) (Eo)
Total luminous flux falling on the whole surface of a very small sphere located at the specified point
divided by the surface area of the sphere (unit: lx).
spill light (stray light)
Light emitted by a lighting installation which falls outside the boundaries of the area for which the lighting installation is designed.
Spotlighting
Lighting designed to increase considerably the illuminance of a limited area or of an object relative to the surroundings, with minimum diffused lighting.
stroboscopic effect
Apparent change of motion and/or appearance of a moving object when the object is illuminated by a light of varying intensity.
Note: to obtain apparent immobilisation or constant change of movement, it is necessary that both the object movement and the light intensity variation are periodic, and some specific relation between the object movement and light variation frequencies exists. The effect is only observable if the amplitude of the light variation is above certain limits. The motion of the object can be rotational or translational.
standby lighting
That part of emergency lighting provided to enable normal activities to continue substantially unchanged.
standby energy consumption
See luminaire standby energy consumption.
standby power
See luminaire standby power.
standby power of the controls (with the lamps off) (Pci)
Standby input power to the control system in the luminaires during the period with the lamps not operating (unit: W).
stray light
See spill light.
sunlight
Part of direct solar radiation capable of causing a visual sensation.
Note: when dealing with actinic effects of optical radiations, this term is commonly used for radiations extending beyond the visible region of the spectrum.
surrounding area (immediate surrounding area)
Strip surrounding the task area within the field of vision.
Note: in exterior applications this strip should have a width of at least 2 m.
survival factor
See lamp survival factor.
task area
Partial are in the work place in which the visual task is carried out.
Note: for places where the size and/or location of the task area are unknown, the area where the task may occur is the task area.
total energy used for lighting (Wt)
Energy consumed in period t by the luminaires in a room or zone when the lamps are operating, plus the standby loads when the lamps are not operating (unit: kW·h).
total installed charging power of the emergency lighting luminaires in the room or zone (Pem)
Input charging power of all emergency lighting luminaires (unit: W).
total installed lighting power in the room or zone (Pn)
Power of all luminaires (unit: W).
total installed parasitic power of the controls in the room or zone (Ppc)
Input power of all control systems in luminaires when the lamps are not operating (unit: W).
traffic lane
Strip of carriageway intended to accommodate a single line of moving vehicles.
transmittance (for incident radiation of given spectral composition, polarization and geometrical
distribution) (t ) Ratio of the transmitted radiant or luminous flux to the incident flux in the given conditions.
unified glare rating limit (RUGL)
Upper limit of glare by the CIE Unified Glare Rating system.
uniformity (luminance, illuminance) (Uo)
Ratio of minimum illuminance (luminance) to average illuminance (luminance) on (of) a surface.
upward flux ratio
Ratio between the flux from all considered luminaires above the horizontal plane passing through the luminaires in their installed position on site plus their flux reflected by the ground and the minimal irreducible flux reflected towards the sky by the sole reference surface.
Note: upward flux ratio is sometimes signified by the abbreviation UFR.
upward light output ratio (of a luminaire) (RULO)
Ratio of the upward flux of the luminaire, measured under specified practical conditions with its own lamps and equipment, to the sum of the individual luminous fluxes of the same lamps when operated outside the luminaire with the same equipment, under specified conditions.
Note 1: upward light output ratio is sometimes signified by the abbreviation ULOR.
Note 2: the luminaire attitude should be declared so that appropriate corrections to the ULOR can be made if in application the installed attitude is different.
upward light ratio (RUL)
Proportion of the total luminaire flux that is emitted above the horizontal by all luminaires to the total luminaire flux from all luminaires in an installation, when the luminaires are mounted in their installed attitudes.
useful data
Lamp and luminaire data beneficial to the designers and users in the planning and operation of lighting installations.
utilance (of an installation, for a reference surface) (U)
Ratio of the luminous flux received by the reference surface to the sum of the individual total fluxes of the luminaires of the installation.
utilization factor (of an installation, for a reference surface) (FU)
Ratio of the luminous flux received by the reference surface to the sum of the individual luminous fluxes of the lamps of the installation.
V(l) correction
Correction of the spectral responsivity of a detector to match the photopic spectral sensitivity of the human eye.
veiling reflections
Specular reflections that appear on the object viewed and that partially or wholly obscure the details by reducing contrast.
visual acuity (visual resolution)
(1) Qualitatively: capacity for seeing distinctly fine details that have very small angular separation.
(2) Quantitatively: any of a number of measures of spatial discrimination such as the reciprocal of the value of the angular separation in minutes of arc of two neighbouring objects (points or lines or other specified stimuli) which the observer can just perceive to be separate.
visual comfort
Subjective condition of visual wellbeing induced by the visual environment.
visual field (field of vision)
Extent of space in which objects are visible to an eye at a given position and direction of view.
Note: in the horizontal plane meridian the field of vision extends to nearly 190° with both eyes open, the area seen binocularly is about 120°, and the area seen by one eye only is about 154°. The extent of the field of vision tends to diminish with age.
visual performance
Performance of the visual system as measured for instance by the speed and accuracy with which a visual task is performed.
visual task
Visual elements of the activity being undertaken.
Note: the main visual elements are the size of the structure, its luminance, its contrast against the background and its duration.
window
Daylight opening on a vertical or nearly vertical area of a room envelope.
workplace
Place intended to house workstations on the premises of the undertaking and/or establishment and any other place within the area of undertaking and/or establishment to which the worker has access in the course of his/her employment.
Work plane (working plane)
Reference surface defined as the plane at which work is normally done.
Workstation
Combination and spatial arrangement of work equipment, surrounded by the work environment under the conditions imposed by the work tasks.
References
BSI (2015) BS EN 13201-2: 2015: Road lighting. Performance requirements (London: British Standards Institution)
BSI (2018) BS EN 12665: 2018: Light and lighting. Basic terms and criteria for specifying lighting requirements (London: British Standards Institution)
CIE (2004) CIE 15: 2004: Colorimetry (Vienna: International Commission on Illumination)
IEC (online) IEC 60050: International Electrotechnical Vocabulary [online] (Geneva: International
Electrotechnical Commission (http://www.electropedia.org) (accessed August 2018)
SLL (2004) SLL Code for Lighting (London: Society of Light and Lighting)
And if you’ve just read all the way from top to bottom of this Glossary then either you wrote the thing originally (Chapeau! PaulRuffles) or youreally need to get your own copy of The SLL Lighting Handbook.
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