Average illuminance (Eav)
Over a surface, Illuminance averaged over the specified surface.
Average luminance coefficient (Q0)
A measure for the lightness of a road surface being defined as the value of the luminance coefficient q averaged over a specified solid angle of light incidence.
Device used with discharge lamps for stabilizing the current in the discharge.
Ballast Efficacy Factor (BEF)
Value used to evaluate various lighting systems based upon light output and power input. Mathematical formula: Ballast Efficacy Factor equals Ballast Factor (percent value) divided by ballast Input Power (watts).
Ballast Factor (BF)
Measure of light output from lamp operated by commercial ballast, as compared to a laboratory standard reference ballast. Ballast factor .90 means ballast produces 90% of light produced by ANSI C82.2 or IEC82 reference ballast operating same lamps.
Sound generated by the vibration of laminations in the electromagnetic field that transforms the current for discharge lamp use. Since electronic ballasts do not utilize large laminated coils, they operate with lower sound levels as compared to core & coil ballasts.
Power which is supplied to a ballast but is not converted into lamp energy. Ballast loss is dissipated as heat.
The direction in the center of the solid angle which is bounded by directions having luminous intensities of 90% of the maximum intensity of a luminaries.
The ratio of the flux emitted within the solid angle defined by the beam spread, to the bare lamp flux.
Device in ballast that stores electrical energy. Often used for power factor correction and lamp regulation (see “Power Factor”).
Arrangement of road-lighting luminaires suspended with their main beam axes at right angles to the road axis.
Luminaire designed to be suspended from a cable with its main beam axis at right angles to the axis of the road.
Discharge lamp designed to start without preheating of the electrodes, for example ‘TL’S fluorescent lamp.
General expression for the color impression received when looking at a light source.
General expression for the effect of an illuminant on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference illuminant.
Color rendering index (Ra) of a light source
Measure of the degree to which the psycho-physical colors of objects illuminated by the source conform to those of the same objects illuminated by a reference illuminant for specified conditions.
Temperature of the black body that emits radiation of the same chromaticity as the radiation considered. (Unit Kelvin, K)
Lighting system comprising light sources shielded by a panel parallel to the wall and attached to the ceiling, and distributing light over the wall.
Correlated color temperature
The temperature of the Planckian radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions. (Unit Kelvin, K)
Cosine law of incidence
The law that states that illuminance at a point on a plane is proportional to the cosine of the angle of light incidence (the angle between the direction of the incident light and the normal to the plane).
E = I/d2 cos(alpha)
Lighting system comprising light sources shielded by a ledge or recess, and distributing light over the ceiling and upper wall.
Lighting in which the light on the working plane or on an object is not incident predominantly from a particular direction.
Device used to alter the spatial distribution of radiation and depending essentially on the phenomenon of diffusion.
Change of the spatial distribution of a beam of radiation when it is deviated in many directions by a surface or by a medium, without change of frequency of its monochromatic components.
A device in the electrical circuit for varying the luminous flux from lamps in a lighting installation.
On a surface. The luminous flux received by the surface direct from the luminaires of the installation.
Lighting by means of luminaires with a light distribution such that 90 to 100 per cent of the emitted luminous flux reaches the working plane direct, assuming that this plane is unbounded.
Of an interior lighting installation. The ratio of the direct flux on the working plane to the downward flux of the installation.
Lighting in which the light on the working plane or on an object is incident predominantly from a particular direction.
Lamp in which the light is produced, directly or indirectly, by an electric discharge through a gas, a metal vapor, or a mixture of several gases and vapors.
Glare that causes discomfort without necessarily impairing the vision of objects.
Small luminaire concentrating the light, usually recessed in the ceiling.
Luminaire so constructed that dust of specified nature and fineness cannot enter it when it is used in a dust-laden atmosphere.
The input power consumption of the electronic ballast is transferred mostly to the lamp, while a small part is dissipated in the internal circuit of the ballast. The efficiency is defined as the PL/Pin ratio, where PL is the lamp power and Pin is the input power to the ballast. For example, if 10% of the total power is dissipated in heat, the ballast efficiency is 90%. In comparison with standard magnetic ballasts, electronic ballasts have a higher electrical efficiency. The electronic ballasts are more economic in terms of efficiency as they drive the lamp at a similar lumen level to magnetic circuits but with reduced power consumption.
Metal filaments that emit electrodes in a fluorescent lamp. Negatively charged free electrons emitted by one electrode are attracted to the positive electrode (anode), creating an electric current and arc between electrodes.
Electromagnetic Ballast (Magnetic Ballast)
A ballast that uses a “Core & Coil” assembly to transform electrical current to start and operate fluorescent and high intensity discharge (HID) lamps.
Electronic (High Frequency) Ballast
A ballast that, with the help of electronic components, transforms current at high frequency to operate discharge lamps. Electronic ballasts typically operate at frequencies at or above 20,000 Hz.
Lighting provided for use when the supply to the normal lighting fails.
EMI (Electromagnetic Interference)
Electrical interference (“noise”) generated by electrical and electronic devices. Levels generated by high frequency electronic devices are subject to regulation by Federal Communications Commission (FCC) or International Electro-technical Commission (IEC).
That part of emergency lighting provided to ensure that an escape route can be effectively identified and used in the case of failure of the normal lighting system.
Metal Tungsten wire coated with Barium Oxide that emits electrons when voltage is applied.
Of a luminaire in a given direction. The area of the orthogonal projection of the luminous surface on a plane perpendicular to the specified direction.
Impression of fluctuating luminance or color.
Projector designed for floodlighting, usually capable of being pointed in any direction and of weatherproof construction.
Discharge lamp of the low-pressure mercury type in which most of the light is emitted by a layer of fluorescent material excited by the ultraviolet radiation from the discharge.
This term is most commonly applied to low pressure tubular fluorescent lamps, for example ‘TL’, ‘TL’D, ‘TL’E lamps.
Fluorescent mercury lamp
A high-pressure mercury lamp in which the light is produced partly by the mercury vapor and partly by a layer of fluorescent material on the inner surface of the outer bulb excited by the ultraviolet radiation of the discharge – for example: HPL-N, HPLR-N lamps.
Substantially uniform lighting of an area without provision for special local requirements.
Condition of vision in which there is discomfort or a reduction in the ability to see significant objects, or both, due to an unsuitable distribution or range of luminance or to extreme contrasts in space or time.
Gas-filled lamp containing a tungsten filament and a small proportion of halogens.
An integral multiple of the fundamental frequency (50/60 Hz) that becomes a component of the current (see “Harmonic Distortion”).
Distortion of an AC waveform caused by multiples of the fundamental frequency(harmonics). Odd triplet harmonics (thirds, ninths, etc.) may result in large currents on the neutral line in a four-wire three-phase system.
High Frequency Operation
Generally refers to the operation of electronic ballasts at frequencies between 20 and 60 kilohertz (kHz)-20,000 to 60,000 cycles per second.
High Power Factor Ballast
A ballast whose power factor is greater than 90%.
High-pressure mercury (vapor) lamp
Mercury vapor lamp, with or without a coating of phosphor, in which during operation the partial pressure of the vapor is of the order of 105 Pa – for example: HPL and HPL-N lamps.
High-pressure sodium (vapour) lamp
Sodium vapour lamp in which the partial pressure of the vapour during operation is of the order of 104 Pa – for example, SON and SON-T lamps.
Attribute of a visual sensation according to which an area appears to be similar to one of the perceived colors, red, yellow, green, and blue, or to a combination of two of them.
Illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. (Unit lux, lx)
The orientation of the surface may be defined, e.g. horizontal, vertical, hence horizontal illuminance, vertical illuminance.
Illumination see Lighting Incandescence
Emission of optical radiation by the process of thermal radiation.
Incandescent (electric) lamp
Lamp in which light is produced by means of an element heated to incandescence by the passage of an electric current.
Increased safety luminaire
Enclosed luminaire that satisfies the appropriate regulations for use in situations where there is risk of explosion.
On a surface. The luminous flux received by the surface from a lighting installation after reflection from other surfaces.
Lighting by means of luminaires with a light distribution such that not more than 10 per cent of the emitted luminous flux reaches the working plane direct, assuming that this plane is unbounded.
Optical radiation for which the wavelengths are longer than those for visible radiation.
Lumen output of a fluorescent lamp after it has been operating 100 hours.
Voltage provided to a ballast by a power line or power supply.
Power consumed by lamp ballast/system. The critical attribute along with Ballast Factor. Typically, lowering the input watts results in lowering the light input.
A transient condition, generally lasting 5-10 milliseconds that occurs during ballast start-up. Largely dependent upon ballast circuit design.
Lamp starting method in which lamps are started by high voltage input with no preheating of lamp filaments.
Inter-reflection (or interflection)
General effect of the reflections of radiation between several reflecting surfaces.
Input power used to operate lamps.
A refracting panel consisting of one or more, usually cylindrical, fresnel lenses.
Any radiation capable of causing a visual sensation direct i.e. Visible radiation.
That part of a luminaire designed to control the spatial distribution of the luminous flux of the lamp(s).
In practice a light controller will also act as a screen.
Light output ratio Of a luminaire.
The ratio of the total flux of the luminaire, measured under specified practical conditions, to the sum of the individual luminous fluxes of the lamps operating outside the luminaire under specified conditions.
Visible radiation entering the eye and producing a sensation of light.
Lighting (or illumination)
Application of light to a scene, objects or their surroundings so that they may be seen.
Attribute of visual sensation in accordance with which a body seems to transmit or reflect diffusely a greater or smaller fraction of the incident light.
Lighting for a specific visual task, additional to and controlled separately from the general lighting.
The term is often applied to lighting designed to illuminate a particularly small area, e.g. a desk top.
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 (Ul)
The ratio of minimum to maximum luminance along a line parallel to the road axis through the observer’s position.
Low-pressure mercury (vapor) lamp
Mercury vapor lamp, with or without a coating of phosphor, in which during operation the partial pressure of the vapor does not exceed 100 Pa – for example a ‘TL’ lamp.
Low-pressure sodium (vapor) lamp
Sodium vapor lamp in which the partial pressure of the vapor during operation does not exceed 5 Pa – for example a SOX lamp.
Unit of measure for amount of light produced by a lamp once it is started. One lumen equals one foot candle per square foot.
Apparatus that distributes, filters or transforms the light given by a lamp or lamps and which includes all the items necessary for fixing and protecting these lamps and for connecting them to the supply circuit.
In road lighting the term ‘lantern’ is also sometimes used.
In a given direction, at a given point of a real or imaginary surface.
Quotient of the luminous flux transmitted by an elementary beam passing through the given point and propagating in the solid angle containing the given direction, and the product of the solid angle, the area of a section of that beam containing the given point, and the angle between the normal to that section and the direction of the beam. (Unit candela per square meter, cd/m2)
Luminance coefficient (q)
The ratio, for a specified direction of observation and direction of light incidence, between the luminance on an element of a surface and the illuminance on it. Unit candela per square meter per lux, cd/m2/lx)
Luminance contrast (C)
Between two parts of a visual field. The relative luminance difference of those parts in accordance with the formula: C = L1 – L2/L2 where the size of the two parts differs greatly and where:
• L1 = luminance of the smallest part (the object),
• L2 = luminance of the greatest part (the background).
Luminance distribution curve
Curve representing the luminance of a luminaire in a vertical plane as a function of the angle from the nadir.
At a point, of a non-self-radiating body, in a given direction, under specified conditions of illumination.
Ratio of the luminance of the body to that of a perfect reflecting or transmitting diffuser identically illuminated.
Luminance yield factor
Ratio of the average luminance (in cd/m2) to the average illuminance (in lux) of a road lighting installation.
Emission (by atoms, molecules or ions in a material) of optical radiation which, for certain wavelengths or restricted regions of the spectrum, is in excess of the radiation due to thermal emission from the material at the same temperature, as a result of these particles being excited by energy other than thermal agitation.
Luminous (perceived) color
Color perceived as belonging to an area that appears to be emitting light as a primary light source, or that appears to be specularly reflecting such light.
Of a source. Quotient of the luminous flux emitted and the power consumed.
Lighting considered in relation to its physiological and psychological effects.
Luminous flux is a photoelectric measurement to measure the intensity of luminous sensation vs. the power of the stimulus. As the light has a polychromatic nature and the human eye does not have a constant sensitivity to different colours, it is not possible to find a direct correspondence between luminous flux and power. In other words, it is not possible to express luminous flux in watts. The unit of luminous flux is called the “lumen” (Im). The “lumen” is the power of the luminous source weighted as function of the spectral distribution in respect to the sensitivity of human eye.
The quantity derived from radiant flux by evaluating the radiation according to its action upon the CIE standard photometric observer. (Unit lumen, lm)
Luminous intensity (Iv, I)
Of a source in a given direction. Quotient of the luminous flux leaving the source, propagated in an element of solid angle containing the given direction, and the element of solid angle. (Unit candela, cd)
The luminous intensity of luminaires is normally given either in a Luminous intensity diagram or in an Isocandela diagram.
Luminous intensity diagram (table)
Luminous intensity shown in the form of a polar diagram or table, in terms of candela per 1000 lumens of lamp flux. The diagram (table) for non-symmetrical light distributions gives the light distribution of a luminaire in at least two planes:
1. In a vertical plane through the longitudinal axis of the luminaire.
2. In a plane at right angles to that axis.
The luminous intensity diagram (table) can be used:
a) To provide a rough idea of the light distribution of the luminaire.
b) For the calculation of illuminance values at a point.
c) For the calculation of the luminance distribution of the luminaire.
Luminous intensity distribution
Distribution of the luminous intensities of a lamp or luminaire in all spatial directions.
The average illuminance over the reference surface at the end of the complete maintenance cycle.
The maintained illuminance is the minimum value to which the illuminance is allowed to fall.
Ratio of the average illuminance on the working plane after a specified period of use of a lighting installation to the average illuminance obtained under the same conditions for a new installation.
The use of the term Depreciation factor as the reciprocal of maintenance factor is depreciated.
Vision intermediate between photopic and scotopic vision.
Metal halide lamp
Discharge lamp in which the major portion of the light is produced by the radiation from a mixture of a metallic vapor (for example, mercury) and the products of the dissociation of halides (for example, halides of thallium, indium or sodium) – for example: HPI-T lamps.
Metal vapor lamp
Discharge lamp such as the ‘mercury (vapor) lamp’ and the ‘sodium (vapor) lamp’ in which the light is mainly produced in a metallic vapor.
The number of operating hours elapsed before a certain percentage of the lamps fail.
The distance between the reference plane and the plane of the luminaires.
voltage at which the lamp is intended to be used i.e. 6V, 12V, 24V for Automotive Lighting products.
Object (perceived) color
Color perceived as belonging to an object either self-luminous or non-self-luminous.
Medium that transmits no radiation in the spectral range of interest.
Optical light output ratio
Of a luminaire. The ratio of the total flux of the luminaire, measured under specified practical conditions, to the sum of the individual luminous fluxes of the lamps when inside the luminaire.
For luminaires using incandescent lamps only, the optical light output ratio and the light output ratio are in practice the same.
Electromagnetic radiation at wavelengths between the region of transition to Xrays (approx = 1 nm) and the region of transition to radio waves (approx = 1 mm).
Overall uniformity (Uo)
The ratio of minimum to the average luminance over the area of road considered.
The horizontal distance between a vertical line passing through the luminaire center and the nearest kerb of the road.
Parts Per Million (PPM)
1 PPM means one defect lamp for every 1 million lamps produced (1% = 10,000PPM)
The luminous intensity of a luminaire in the direction of the beam axis.
Pendent (pendant) luminaire
Luminaire provided with a cord, chain, tube, etc. which enables it to be suspended from a ceiling or other support.
Aspect of visual perception by which an observer may distinguish between two fields of view of the same size, shape and structure such as may be caused by differences in the spectral composition of the radiation concerned in the observation.
Natural or artificial cycle of light and darkness alternation to which living organisms may be exposed.
Vision when the eye is adapted to levels of luminance of at least several candela per square meter.
Vision mediated essentially or exclusively by cones.
Source of radiation the dimensions of which are small enough, compared with the distance between the source and the irradiated surface, for them to be neglected in calculations and measurements.
In an electric circuit. The ratio of the power in watts to the product of the r.m.s. values of voltage and current.
For sinusoidal waveforms, it is equal to the cosine of the angle of phase difference between voltage and current.
Preheat (or rapid-start) lamp
Hot cathode lamp designed to start with preheating of the electrodes – for example: ‘TL’M ‘TL’RS lamp.
QL induction lamp (system)
A lamp (system), based on the low pressure mercury discharge principle, but devoid of electrodes, in which the ionization of the gas within a discharge vessel is brought about by the induction of a high frequency electromagnetic field.
Of a source of radiation. Ratio of the radiant flux (power) emitted to the power consumed.
Radiant energy (Qe, Q)
Energy emitted, transferred, or received in the form of radiation. (Unit joule, J = W.s.)
Power emitted, transferred, or received in the form of radiation. (Unit watt, W)
1. Emission or transfer of energy in the form of electromagnetic waves or particles.
2. These electromagnetic waves or particles.
Radio Frequency Interference (RFI)
Form of electromagnetic interference that can be radiated through the air. Subject to FCC or IEC regulations. Should be a consideration when selecting a ballast that will be used in proximity to radio telemetry equipment, including radios, TV’s, cellular phones, portable phones, medical devices, etc.
Luminaire so constructed as to withstand the penetration of rain, and for use out of doors.
Lamp starting method in which lamp filaments are preheated to facilitate lamp ignition.
Luminaire mounted above the ceiling or behind a wall or other surface so that any visible projection is insignificant.
Reduced luminance coefficient (r)
The product of the luminaire coefficient (q) and cos3y where y is the angle of light incidence.
Reflectance (formerly Reflection factor)
Ratio of the reflected radiant or luminous flux to the incident flux.
Return of radiation by a surface or medium without change of frequency of its monochromatic components.
Device in which the phenomenon of reflection is used to alter the spatial distribution of the luminous flux from a source.
Lamp in which part of the bulb is coated with a reflecting material, either diffuse or specular, so as to control the light, for example: HPL-R, MLR, and ‘TL’F lamps.
That part of emergency lighting provided to ensure the safety of people involved in a potentially hazardous process.
Attribute of visual sensation that permits a judgement to be made of the proportion of pure chromatic color in the total sensation.
Vision when the eye is adapted to levels of luminance below some hundredths of a candela per square meter; the rods are considered to be the principal active elements under these conditions.
The spectrum appears uncolored.
That part of a luminaire designed to prevent the lamps from being directly visible over a given range of angles.
In practice a screen will also act as a light controller.
Mean illuminance during one maintenance cycle of an installation averaged over the relevant area.
A road-lighting arrangement in which the luminaires are suspended above the carriageway(s) on transverse wires.
A (small) projector giving concentrated light of usually not more than 20 degrees divergence.
Quantity of a luminaire to indicate the extent to which the light is ‘spread out’ across the road.
Luminaires are classified as being of narrow, average or broad spread.
That part of emergency lighting that enables normal activities to continue substantially unchanged.
Device for starting a discharge lamp (in particular a fluorescent lamp) that provides for the necessary preheating of the electrodes and/or causes a voltage surge in combination with the series ballast.
Electrical apparatus that provides the conditions required for starting a discharge.
The starting time is the time the ballast needs to light and stabilize the lamp. According to this value it is possible to make a distinction between warm (preheated) and cold (not pre-heated) start designs. Cold start designs have the advantage of very quickly starting the lamp. They are suitable for installations with long operating hours and few switching cycles such as shops. Warm start electronic ballasts take longer to start the lamp (up to 2 seconds) but are more suitable for frequently switched applications.
Switch-start fluorescent lamp
Fluorescent lamp suitable for operation with a circuit requiring a starter for the preheating of the electrodes, for example ‘TL’D type.
Characteristic of a luminaire that indicates the extent to which the light is ‘thrown’ in the lengthwise direction of the road. Luminaires are classified as being of short, intermediate or long throw.
Tilt, angle of
Upward inclination of a luminaire from the horizontal.
Medium that transmits visible radiation largely by diffuse transmission, so that objects are not seen distinctly through it.
A long, recessed luminaire usually installed with the opening flush with the ceiling.
Tubular fluorescent lamp
Gas-filled lamp containing halogens or halogen compounds, the filament being of tungsten.
Optical radiation for which the wavelengths are shorter than those for visible radiation.
Upward [downward] light output ratio (luminaire efficiency)
The ratio of the flux emitted above [below] a horizontal plane passing through the luminaire to the total bare lamp flux.
Ratio of the utilized flux to the luminous flux emitted by the lamps.
Luminous flux received on the reference surface under consideration.
Valance (or pelmet) lighting
Lighting system comprising light sources shielded by a panel parallel to the wall at the top of a window.
Luminaire so constructed that a specified vapor or gas cannot enter its enclosure.
Any radiation capable of causing a visual sensation direct.
Visual acuity; sharpness of vision
Capacity for seeing distinctly objects very close together.
Reciprocal of the value (generally in minutes of arc) of the angular separation of two neighboring objects (points or lines) which the eye can just perceive as being separate.
The angle subtended by an object or detail at the point of observation it is usually measured in minutes of arc.
The degree of visual satisfaction produced by the visual environment.
Of the eye or eyes. The angular extent of the space in which an object can be perceived when the eye(s) regard(s) an object directly ahead. The yield may be monocular or binocular.
The totality of measures taken to give a road user an unambiguous and immediately recognizable picture of the course of the road ahead.
The quantitative assessment of the visual system in the performance of a visual task.
The group of structures comprising the eye, the optic nerve and certain parts of the brain, which transforms the light stimulus into a complex of nerve excitations, whose subjective correlate is visual perception.
Luminaire constructed to withstand the penetration of water when immersed to a specified depth, but not intended for permanent use under water.
The term ‘submersible luminaire’ applies to a luminaire constructed to withstand indefinitely submersion in water to a specified depth.
Distance in the direction of propagation of a periodic wave between two successive points at which the phase is the same (at the same time).
Work (or working) plane
Reference surface defined as the plane at which work is usually done.
Zonal luminous flux diagram
The graphical representation of the luminous flux distribution of a luminaire or a lamp in which the luminous flux emitted within a cone is plotted against the half-apex angle of that cone.