Glossary

Glossary

A

Absorptance (formerly absorption factor).
Ratio of the absorbed radiant or luminous flux to the incident flux.
Absorption.
Conversion of radiant energy to a different form of energy by interaction with matter.
Accommodation.
Focal adjustment of the eye, generally spontaneous, made for the purpose of obtaining maximum visual acuity at various distances.
Acrylic
The generic term for a family of transparent plastics used in making lighting fixture lenses, diffusers and refractors.
Adaptation.
1. The process by which the state of the visual system is modified according to the luminances or the color stimuli presented to it..
2. The final state of the process.
Ambient
The surrounding environment of a device such as a fixture or ballast. It usually refers to temperature or sound conditions.
Ambient lighting
Lighting throughout an area that produces general illumination.
Angle of tilt
see tilt, angle of.
Arc discharge (in a gas or vapor).
Electric discharge characterized by a cathode fall that is small compared with that in a glow discharge.
Arc lamp.
A discharge lamp in which the light is emitted by an arc discharge or by its electrodes. Note.
The electrodes can be either of carbon (operating in air) or of metal (operating in a pressurized gas).
Arc tube.
An envelope, usually quartz or ceramic that contains the arc of a discharge light source.
Atmospheric transmissivity.
The ratio of the directly transmitted flux incident on a surface after passing through unit thickness of the atmosphere to the flux that would be incident on the same surface if the flux had passed through a vacuum. (similar to the extinction coefficient in astronomy)
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..
Average luminance.
‘luminance’ is a property of a geometric ray. Luminance as measured by conventional meters is averaged with respect to two independent variables — area and solid angle (steradian). Both must be defined for a complete description of a luminance measurement. Standard metric for measurement is candela per square meter (cd/m2) or footlambert (fl). Cd/m2 is preferred.
Average maximum candlepower
The average of the 10 highest readings in a floodlight beam.

B
Baffle.
An opaque or translucent element to shield a source from direct view at certain angles, to absorb or block unwanted light, or to reflect and redirect light. Note.
Helps with reducing impact of flag lighting and flood lighting of large areas.
Ballast factor.
The measure of how close actual light output from a lamp-ballast combination compared to the lamps’ rated light output is called the ballast factor (ansi, 1984).
Ballast.
A transformer device equipped with starting circuitry required by electric discharge light sources like fluorescent or hid lamps to regulate voltage and current supplied to the lamp during startup and manage voltage at constant levels to sustain an electrical arc passing through atomized vapors throughout the lamp’s operation.
Barn doors.
A set of adjustable flaps — usually two, four, or eight — which can be attached to the front of a luminaire (usually a fresnel spotlight) in order to partially control the shape and spread of the light beam. Note.
Works great for low impact flag lighting, however, care should be taken to protect the luminaire from tampering.
Beam angle.
The angle between the two directions for which the intensity is 50% of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the beam angle is generally given for two planes at 90°, typically the maximum and minimum angles. Typically reported in ‘type b” photometry.
Beam axis.
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 luminaire..
Beam efficiency.
The ratio of the flux emitted within the solid angle defined by the beam spread, to the bare lamp flux..
Beam lumens
The lumens contained within the beam spread of a floodlight.
Beam lumens of a projector.
The quantity of light contained in that part of the beam at which i = 1/2 imax (europe) or i = 1/10 imax (usa)..
Beam spread
The vertical and horizontal displacement of the beam in degrees, bounded by the angle at which 10% of maximum candlepower occurs. (maximum candlepower is the highest intensity in the beam.)
Black body(planckian radiator).
Thermal radiator that absorbs completely all incident radiation, whatever the wavelength, the direction of incidence or the polarization. This radiator has, for any wavelength, the maximum spectral concentration of radiant exitance at a given temperature..
Blended-light lamp.
Lamp containing in the same bulb a high-pressure mercury vapor discharge tube and an incandescent lamp filament connected in series. The bulb may be diffusing or coated with a fluorescent material. For example, the mll lamp..
Bracket(mast arm).
An attachment to a lamp post or pole from which a luminaire is suspended.
Brightness (the term luminosity is obsolete).
Attribute of visual sensation according to which an area appears to emit more or less light.
Note:
Brightness according to the definition is also an attribute of color. In british recommendations the term ‘brightness’ is now reserved to describe brightness of color; luminosity should be used in all other instances..

C

Candela
Unit of luminous intensity.
Candlepower distribution curve.
A curve showing the variation of luminous intensity of a lamp or luminaire with angle. Generally depicted as a polar curve.
Candlepower.
Luminous intensity expressed in candelas.
Catenary arrangement.
Arrangement of road-lighting luminaires suspended with their main beam axes at right angles to the road axis..
Cavity ratio (cr).
Geometric proportions of the ceiling, floor and room cavities.Room cavity ratio = 5h (room length + room width)
————————————
Room length x room width

Cavity.
An upper, lower or intermediate zone or region of a room designated as ceiling, floor or room cavity.
Central arrangement
see twin-central arrangement.
Chromaticity diagram.
Plane diagram showing the result of mixtures of color stimuli, each chromaticity being represented unambiguously by a single point on the diagram..
Cie standard photometric observer.
Ideal observer having a relative spectral responsivity curve that conforms to the v() or to the v’() functions for photopic and scotopic vision, respectively, and that complies with the summation law implied in the definition of luminous flux..
Coefficient of utilization (cu).
The percentage of light from a light fixture which reaches the seeing task. It is a function of the fixture, each having its own set of cu’s for a wide range of the following factors

  • Fixture efficiency, distribution and mounting height .
  • Room proportions
  • Room-surface reflectances
Coffer.
Recessed panel or dome in ceiling..
Cold-start lamp(instant-start lamp, usa).
Discharge lamp designed to start without preheating of the electrodes, for example ‘tl’s fluorescent lamp..
Color appearance.
General expression for the color impression received when looking at a light source..
Color rendering index (cri).
A measure of the degree of color shift that objects undergo when illuminated by a lamp, compared with those same objects when illuminated by a reference source of comparable correlated color temperature (cct). A cri of 100 represents the maximum value similar to the light of the sun at midday. A lower cri value indicates that some colors may appear unnatural when illuminated by the lamp. Incandescent lamps have a cri above 95. The cool white fluorescent lamp has a cri of 62; fluorescent lamps containing rare-earth phosphors are available with cri values of 80 and above.
Color rendering.
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 space.
Geometric representation of colors in space, usually of three dimensions..
Color temperature.
Temperature of the black body that emits radiation of the same chromaticity as the radiation considered..
Unit kelvin, k..
Color triangle
see chromaticity diagram.
Cone.
Photoreceptors in the retina containing light¬sensitive pigments capable of initiating the process of photopic vision..
Contrast threshold
see threshold contrast.
Contrast.
Subjective assessment of the difference in appearance of two parts of a field of view seen simultaneously or successively..
(see also luminance contrast) .
Control.
Luminaire characteristic, determined by the value of the specific luminaire index (sli), that indicates the degree of glare control present. Luminaires are classified as being of limited, moderate, or tight control..
Cornice lighting.
Lighting system comprising light sources shielded by a panel parallel to the wall and attached to the ceiling, and distributing light over the wall..
Corrected specular factor (s1′) .
A factor representing the degree of specular reflection of a wet road surface..
Correlated color temperature (cct).
A specification of the apparent color of a light source relative to the color appearance of an ideal incandescent source held at a particular temperature and measured on the kelvin (k) scale. The cct rating for a lamp is a general indication of the warmth or coolness of its appearance. As cct increases, the appearance of the source shifts from reddish white toward bluish white; therefore, the higher the color temperature, the cooler the color appearance. Lamps with a cct rating below 3200 degrees kelvin are considered warm sources, whereas those with a cct above 4000 degrees kelvin are considered cool in appearance.
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
Cove lighting.
Lighting system comprising light sources shielded by a ledge or recess, and distributing light over the ceiling and upper wall..
Cross factor (cf).
A luminaire characteristic that indicates the suitability of the luminaire for use under wet and foggy weather conditions..
Cutoff angle (of a luminaire).
The angle measured up from nadir between the vertical axis and the first line of sight in which the bare source is not visible.

D
Depreciation factor (deprecated).
The reciprocal of the maintenance factor..
Diffuse reflection.
Diffusion by reflection in which, on the macroscopic scale, there is no regular reflection..
Diffuse transmission.
Transmission in which, on the macroscopic scale, there is no regular transmission..
Diffused lighting.
Lighting in which the light on the working plane or on an object is not incident predominantly from a particular direction..
Diffuser.
Device used to alter the spatial distribution of radiation and depending essentially on the phenomenon of diffusion..
Diffusion (scattering).
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..
Dimmer.
A device in the electrical circuit for varying the luminous flux from lamps in a lighting installation..
Direct flux.
On a surface. The luminous flux received by the surface direct from the luminaires of the installation..
Direct lighting.
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..
Direct ratio.
Of an interior lighting installation. The ratio of the direct flux on the working plane to the downward flux of the installation..
Directional lighting.
Lighting in which the light on the working plane or on an object is incident predominantly from a particular direction..
Disability glare.
Glare that impairs the vision of objects without necessarily causing discomfort..
Discharge lamp.
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..
Discomfort glare.
See Disability glare.
Dispersion.
1. Phenomenon of the change in velocity of propagation of monochromatic radiations in a medium as a function of the frequency of these radiations..
2. Property of a medium giving rise to this phenomenon..
Divergence of a beam.
The angular extent in a specified plane which contains all the radius vectors of the polar curve of luminous intensity having lengths greater than a specified fraction of the maximum..
Note.
Commonly-used values of the specified fraction are 1/10 and 1/2..
The divergence is then referred to as ‘one-tenth peak divergence’ or ‘one-half peak divergence’ respectively..
Downlight (downlighter).
Small luminaire concentrating the light, usually recessed in the ceiling..
Downward light output ratio
see upward light output ratio.
Drip-proof luminaire.
Luminaire so constructed that, when mounted in its designed position, it will withstand drops of water falling in a substantially vertical direction..
Driver stopping distance.
The total distance traveled while a vehicle is being brought to rest, measured from the position of the vehicle at the instant the driver has an opportunity to perceive that he should stop his vehicle..
Dustproof luminaire.
Luminaire so constructed that dust of specified nature and fineness cannot enter it when it is used in a dust-laden atmosphere..

E
Effective (road) width.
The horizontal distance between the vertical through a luminaire center and the kerb farthest from that luminaire..
Efficacy.
Efficacy is an efficiency ratio. The amount of luminous flux emitted by a light source in relation to the amount of electrical power consumed defines efficacy of the lamp. This ratio is expressed in terms of lumens per watt, or lm/w. The nominal efficacy of different lamp types varies enormously and this metric is used to determine energy efficiency of a light source.
Efficiency.
See luminaire efficiency
Electric dischargein a gas.
The passage of an electric current through gases and vapors by the production and movement of charge carriers under the influence of an electric field..
Note.
This results in the emission of electromagnetic radiation, which plays an essential part in all its applications in lighting..
Emergency lighting.
Lighting provided for use when the supply to the normal lighting fails..
Equivalent adaptation luminance.
That value of uniform luminance in front of an observer that would result in the same degree of perceptibility as with the actual prevailing non-uniform luminance distribution..
Equivalent sphere illumination (esi).
The amount of light in footcandles produced by a luminous sphere on a seeing task in the center of the sphere that will render the same “seeability” as the raw footcandles render the same task in the specific seeing environment under consideration.
Equivalent veiling luminance.
Luminance that has to be added, by superposition, to the luminance of both the adapting background and the object in order to make the luminance difference threshold in the absence of disability glare the same as that experienced in the presence of disability glare..
Escape lighting.
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..
Explosion-proof luminaire.
A luminaire which is completely enclosed and capable of withstanding an explosion of a specific gas or vapor that may occur within it and preventing the ignition of a specific gas or vapor surrounding the enclosure by sparks, flashes or explosion of the gas or vapor within. It must operate at such an external temperature that a surrounding flammable atmosphere will not be ignited.

F
Fixture .
The device which holds, protects and provides an optical system and power connections for the lamp(s). Fixture usually refers to interior lighting. See luminaire.
Flameproof luminaire see increased-safety luminaire.
Flashed area.
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..
Flicker.
Impression of fluctuating luminance or color..
Floodlight.
A projector designed for lighting a scene or object to a luminance considerably greater than its surroundings. It usually is capable of being pointed in any direction and is of weatherproof construction.
Fluorescence.
Photoluminescence that persists for an extremely short time after excitation..Fluorescent lamp. 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..
Note.
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..
Flux (luminous flux).
See lumen.
Footcandle (fc).
A quantitative unit for measuring illuminance.
The illumination on a surface one foot square on which there is a uniformly distributed flux of one lumen.
Footlambert.
The unit of luminance equal to one candle per square foot
Full radiator
see planckian radiator.

G
Gas-discharge lamps.
An electric lamp that produces light from gas atoms excited by an electric current.
General lighting.
Substantially uniform lighting of an area without provision for special local requirements..
General purpose floodlight (gp) .
A weatherproof unit so constructed that the housing forms the reflecting surface. The assembly is enclosed by a cover glass.
Glare control mark.
Number denoting the degree to which discomfort glare is controlled..
Glare.
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..
Glare, direct.
Glare resulting from high luminances or insufficiently shielded light sources in the field of view.
Glare, disability.
Glare resulting in reduced visual performance and visibility and often accompanied by discomfort.
Glare, discomfort.
Glare producing discomfort. It does not necessarily interfere with visual performance or visibility.
Glow discharge.
Electric discharge in which the secondary emission from the cathode is much greater than the thermionic emission..
Goniophotometer.
Photometer for measuring the directional light distribution characteristics of sources or luminaires..
Grid (lay-in).
A type of ceiling construction where the supporting members (inverted t’s) are exposed, and the ceiling tiles and lighting fixtures are laid-in on the flanges of the t’s.
Group relamping.
The method of replacing all lamps in an area at a designated point in the lamp’s life instead of individually as they fail.
Halide lamp see metal halide lamp.

H
Halogen lamp.
Gas-filled lamp containing a tungsten filament and a small proportion of halogens..
Hazardous location.
An area where ignitable vapors or dust may cause a fire or explosion created by energy emitted from lighting or other electrical equipment.
Heavy duty floodlight (hd).
A weatherproof unit having a substantially constructed metal housing into which is placed a separate and removable reflector. A weatherproof hinged door with cover glass encloses the assembly but provides an unobstructed light opening at least equal to the effective diameter of the reflector.
High bay.
Generally refers to industrial lighting where high mounting heights may be encountered. Many industrial hid-type fixtures are called high bays.
High energy discharge.
Better known as high intensity discharge (hid) lighting, which encompasses fluorescent, high pressure sodium (hps), metal halide (mh), mercury vapor (hgv) and low pressure sodium (lps) lamp types. All hid lighting requires a ballast transformer that regulates the operating and starting voltage and current supplying the lamp. Each type of hid lighting offers a different efficacy and effective lamp life, as well as color rendering properties.
High intensity discharge (hid).
The term that applies to a family of light sources consisting of mercury vapor, metal halide, and high pressure sodium lamps. Although low pressure sodium lamps are not hid sources, they often are included in the hid category.
High mast lighting.
Illumination of a large area by means of a group of luminaires which are designed to be mounted on the top of a high mast generally 60 feet or higher.
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..
Hot-start lamp
see preheat lamp.
Hue.
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..

I
Iesna.
The illuminating engineering society of north america. Http://www.iesna.org
Ignitor
see starter
Illuminance (e).
Illuminance refers to luminous flux per unit area that arrives at a particular surface from one or more light sources. Illuminance is measured horizontally and vertically in units of lux (lx) or footcandles (fc). When the incident light is not parallel, which is normal, a cosine diffuser must be used as the measurement geometry for `visible light meter’ detector heads.
Illumination.
The lighting in an area, or the result of the use of light. Also, the density of light per unit of area, sometimes called “illuminance”; measured in footcandles or lux. The intensity of light is called “luminance” and is measured in candelas per square meter.
Incandescent (electric) lamp.
Lamp in which light is produced by means of an element heated to incandescence by the passage of an electric current..
Inclination, angle of
see tilt, angle of.
Increased safety luminaire.
Enclosed luminaire that satisfies the appropriate regulations for use in situations where there is risk of explosion..
Indirect flux.
On a surface. The luminous flux received by the surface from a lighting installation after reflection from other surfaces..
Indirect lighting.
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..
Infrared radiation.
Optical radiation for which the wavelengths are longer than those for visible radiation..
Initial light output.
A lamp’s light output in lumens after 100 hours of seasoning. Also referred to in lamp catalogs as initial lumens.
Intensity (i)
see luminous intensity.
Intensity distribution
see luminous intensity distribution.
Inter-reflection (or interflection).
General effect of the reflections of radiation between several reflecting surfaces..
Inverse square law.
The law stating that the illuminance “e” at a point on a surface varies directly with the intensity “i” of a point source and inversely as the square of the distance “d” between the source and the point. If the surface at the point is normal to the direction of the incident light the law is expressed by e=i/d .
Irradiance (ee; e) .
At a point of a surface. Quotient of the radiant flux incident on an element of the surface containing the point, and the area of that element..
Unit watts per square meter, w/m2
Isocandela line.
A line plotted on any appropriate set of coordinates to show all the points on a surface where the illuminance is the same. A series of such lines for various illuminance values is called an isolux (isofootcandle) diagram.
Iso-intensity curve.
Curve traced on an imaginary sphere with the source at its center and joining all the points corresponding to those directions in which the luminous intensity is the same, or a plane projection of this curve..
Iso-intensity diagram
array of iso-intensity curves..
Isoluminance curve (diagram).
Locus of points on a surface at which the luminance is the same, for given positions of the observer and of the source or sources in relation to the surface..
Isolux curve (diagram).
Locus of points on a surface where the illuminance has the same value..

J
Jet-proof luminaire.
Luminaire constructed to withstand a direct jet of water from any direction..

K
Kerb ratio.
The ratio of the average illuminance on the footpath of a specified width to the average illuminance on the same width of adjacent carriageway..
Kilowatt.
One thousand watts.
Kilowatt-hour (kwh).
A unit of energy equivalent to one kilowatt (1 kw) of power expended for one hour of time.

L
Lamellae louvre.
A louvre of which the main shielding elements are in the form of straight strips of opaque or translucent material..
Lamp life.
The median life span of a very large number of lamps (also known as the average rated life). Half of the lamps in a sample are likely to fail before the rated lamp life, and half are likely to survive beyond the rated lamp life. For discharge light sources, such as fluorescent and hid lamps, lamp life depends on the number of starts and the duration of the operating cycle each time the lamp is started.
Lamp lumen depreciation (lld).
A factor used in lighting calculations to account for the light loss that takes place in a lamp due to the gradual decay in lumen output over a designated period of burning time. The lld is contingent upon relamping schedules and the specific lamp involved.
Lamp mortality
see mortality rate..
Lamp.
A light source. Lamps used for outdoor lighting include hid, incandescent (including tungsten halogen), and fluorescent.
Lamp.
An artificial source of light (also a portable luminaire equipped with a cord and plug). Industry term commonly applied for a light bulb.
Landolt ring.
Two-dimensional ring with a gap, the width of the gap and the thickness of the ring each being equal to 1/5 of the ring’s outer diameter..
Lantern
see luminaire.
Lay-in.
see grid.
Led.
Light emitting diode, solid state light emitting phospher (luminous source).
Lens panel.
A refracting panel consisting of one or more, usually cylindrical, fresnel lenses..
Lens
The shielding or diffuser portion of a fixture, made of plastic or glass, through which the light passes on its way to the seeing task. (note.
Plastic lenses may be manufactured by the extrusion process or the injection molded process. Injection molded lenses are more expensive.)
Light controller.
That part of a luminaire designed to control the spatial distribution of the luminous flux of the lamp(s)..
Note.
In practice a light controller will also act as a screen..
Light distribution
see luminous intensity distribution.
Light loss factor (llf).
A percentage factor used in calculating the level of illumination that takes into account dirt accumulation on luminaire and room surfaces, lamp lumen depreciation, maintenance procedures and atmosphere conditions.
Light loss factor.
A factor used in calculating the level of illumination after a given period of time and under given conditions. It takes into account temp., dirt accumulations on the luminaire and room surfaces, lamp depreciation maintenance procedures and atmosphere conditions.
Light output.
The amount of light produced by a light source such as a lamp or a luminaire measured in lumens.
Light pollution.
Any negative or adverse effect caused as a result of unnaturally occurring light in the outdoor environment (i.e., skyglow, visible glare, light trespass, overlighting).
Light stimulus.
Visible radiation entering the eye and producing a sensation of light..
Light trespass.
A situation which occurs when, due to lack of adequate beam control, light from a source is distributed onto areas where the illumination is not wanted.
Light.
Any radiation capable of causing a visual sensation direct i.e. Visible radiation.
Lighting (or illumination).
Application of light to a scene, objects or their surroundings so that they may be seen..
Lighting distribution.
Luminaires are classified according to the manner in which they control or distribute the luminous flux.
Lightness.
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..
Local lighting.
Lighting for a specific visual task, additional to and controlled separately from the general lighting..
Note.
The term is often applied to lighting designed to illuminate a particularly small area, e.g. A desk top..
Localized 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 (ul) .
The ratio of minimum to maximum luminance along a line parallel to the road axis through the observer’s position..
Louver.
A series of baffles used to shield a source from view at certain angles or to absorb unwanted light.
Low pressure sodium lamp (lps).
An arc discharge lamp in which light is produced by radiation of sodium vapor at low pressure producing a single wavelength of visible energy, i.e. Yellow light. Lps lamps are monochromatic and have a color rendering index of 0. Due to the monochromatic spectral frequency the application of lps lamps for general outdoor lighting purposes are preferred near professional astronomical observatories.
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..
Lumen
The unit of light output. Light output is also referred to as light flux.
Lumen maintenance.
The ability of a lamp to retain its lumen output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (lld) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The lld factor is a dimensionless value between 0 and 1.
Lumens per watt (lu/w).
Lumen (light) output divided by lamp watts of input power consumed (i.e. Amount of light output for the electricity used); also commonly known as efficacy.
Luminaire dirt depreciation (ldd).
A factor used in lighting calculations to account for the light loss due to the accumulation of dirt on the luminaire. The ldd is contingent upon environment, cleaning schedules and the type of luminaire involved.
Luminaire efficiency.
Total lumen output from luminaire expressed as a ratio of total lamp lumens produced in relation to the luminous output of the luminaire. Commonly referred to as “fixture lumens” or “zonal fixture lumens.” luminaire efficiency is expressed as a decimal (real) number generally less than 1.0. For outdoor applications luminaire efficiency is rated in subcategories to describe the downward and upward efficiency. Overall luminaire efficiency considers both downward task light and wasted uplight.
Luminaire
The complete assembly of a lighting device consisting of an electric lamp (or lamps), lamp holder(s), lamp socket(s), reflector(s), lens, directional shielding and/or baffles designed to distribute the light output, to position and protect the lamps, and/or other components and accessories fixed permanently to the assembly, and electrical ballast transformer(s), capacitors, ignitors, and all internal circuitry connecting the lamps to the power supply. Also known as “light fixture”.
Luminance (l).
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/lux..
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..
Luminance factor() .
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 threshold.
Lowest luminance of a stimulus which enables it to be perceived..
Luminance yield factor.
Ratio of the average luminance (in cd/m2) to the average illuminance (in lux) of a road lighting installation..
Luminescence.
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..
Luminosity obsolete,
see brightness.
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..
Luminous efficacy ().
Of a source. Quotient of the luminous flux emitted and the power consumed..
Unit lumen per watt, lm/w..
Luminous environment.
Lighting considered in relation to its physiological and psychological effects..
Luminous flux (
Luminous flux is the light power of an illumination source like a lamp (bulb) or a light emitting diode (led) and is quantified in lumens (lm). Since illumination generated by these sources is not emitted in parallel beams of light it must be measured with a geometry independent of its spatial distribution.
Luminous intensity (i).
The part of luminous flux that radiates in a particular direction is measured in candela (cd). Luminous intensity is an important quality to define efficiency and quality of lighting equipment. Luminous intensity is measured using detectors with a defined field of view under point source conditions where the light source to detector distance is at least five times greater than the largest source dimension
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..
Note.
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..
Lux.
The metric unit of measurement of illuminance. The light on a surface of one meter square on which there is a uniformly distributed flux of one lumen. 10.76 lux equal 1 footcandle. Decalux = 10 lux.

M

Maintained footcandles.

Illuminance levels calculated by the application of an appropriate light loss factor to initial lamp lumens that accommodate the normal depreciating effects of operational use on a lighting system.
Maintained illuminance.
The average illuminance over the reference surface at the end of the complete maintenance cycle..
Note.
The maintained illuminance is the minimum value to which the illuminance is allowed to fall..
Maintenance factor.
A multiplier which is applied to account for aging of the lamp and for dirt build-up on the luminaire during the period for which the lamp is in place. Lamp aging and luminaire dirt build-up both reduce light output, the amount of reduction usually increasing with time. In common practice, maintenance factors are applied to initial footcandles to derive the minimum light level on the level on the area being illuminated. Light loss factor includes maintenance factor.
Mean light output.
Light output typically evaluated at 40% of rated lamp life for high pressure sodium and 50% of rated life for metal halide hid lamps. In combination with initial light output, mean light output may be used to estimate lamp lumen depreciation. In most lamp catalogs the mean lumen output is affiliated with continuous hours of use.
Mercury vapor lamp (hgv).
A high intensity discharge (hid) lamp in which the major portion of the light is produced by radiation from atomized mercury particles. Mercury vapor provides reasonably good color rendering of earth tones but is the least efficient of hid lamp types.
Mesopic vision.
Vision intermediate between photopic and scotopic vision..
Metal halide lamp (mh).
A high intensity discharge lamp in which the major portion of the light is produced by radiation mercury and other elements in the arc tube. Includes clear and phosphor-coated types. Mh lamps are available in probe-start, pulse-start (psmh), or ceramic (cmh or mhc) varieties. Lumen maintenance and color stability is best with the ceramic metal halide lamp. Generally available with correlated color temperatures of 3000 k or 4000 k.
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..
Metercandle (lux, lx).
The metric equivalent of footcandle.
Lux = lumens / square meters
1 footcandle = 10.76 lux
1 lux = 0.0929 footcandles
Mirror reflector seespecular reflector.
Mixed reflection.
Partly regular and partly diffuse reflection..
Mixed transmission.
Partly regular and partly diffuse transmission..
Monochromatic radiation.
Radiation characterized by a single frequency..
In practice, radiation of a very small range of frequencies that can be described by stating a single frequency..
Mortality rate.
The number of operating hours elapsed before a certain percentage of the lamps fail.
Mounting height.
The vertical distance between the luminaire and the surface to be lighted. It includes both the pole length and the base (above grade) to which the pole is affixed.

N
Nadir.
An imaginary line extending vertically down, directly below the photometric center of a luminaire or lamp; designated as 0° vertical angle. Nadir is the lowest point on the surface of a sphere.
Note: watts always relates to the amount of energy consumed and never relates to the amount of light output.

O
Object (perceived) color.
Color perceived as belonging to an object either self-luminous or non-self-luminous..
Opaque medium.
Medium that transmits no radiation in the spectral range of interest..
Opposite arrangement.
A road-lighting arrangement in which the luminaires are placed on either side of the carriageway(s) opposite to one another..
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..
Note.
For luminaires using incandescent lamps only, the optical light output ratio and the light output ratio are in practice the same..
Optical radiation.
Electromagnetic radiation at wavelengths between the region of transition to x¬rays ( 1nm) and the region of transition to radio waves ( 1 mm)..
Optical system.
The lampcavity or environment (including diffusing media) designed as part of the fixture for the purpose of controlling the light output.
Overall uniformity (uo) .
The ratio of minimum to the average luminance over the area of road considered..
Overhang.
In roadway lighting, the distance between a vertical line passing through the luminaire and the curb or edge of the roadway.

P
Parabolic.
The term applied to certain low brightness louver and reflector shapes as derived from the geometric shape (curve) called a parabola where, if a light source is placed at the focal point of the parabola, the resultant emitted light will be redirected parallel to the parabola’s geometric axis.
Peak intensity.
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..
Perceived color.
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..
Phosphorescence.
Photoluminescence that persists for an appreciable time after excitation
Photocell.
An light sensing device used to control the electrical power distributed to luminaires in response to detected ambient light levels.
Photoluminescence.
Luminescence caused by absorption of optical radiation..
Photometry.
Measurement of quantities referring to radiation as evaluated according to a given luminous efficiency function, e.g. V() or v’()..
Photoperiod.
Natural or artificial cycle of light and darkness alternation to which living organisms may be exposed..
Photopic vision.
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..
Planckian locus.
The locus of points in a chromaticity diagram that represents chromaticities of the radiation of planckian radiators at different temperatures..
Planckian radiator.
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 at a given temperature..
Plenum.
That space between the structural ceiling slab and the finished ceiling. This space may contain air ducts, electrical wiring, etc. It’s the area that conceals the housing part of a recessed fixture.
Point source.
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..
Polycarbonate.
High quality light stabilized plastic used in making diffusers, lenses and refractors. Very tough and highly resistant to damage from impact of stone, pellets and in some instances, low calibre bullets. Very resistant to uv deterioration.
Position factor.
The light output of the lamp in a certain position divided by the light output of the lamp in the base-up position.
Power factor.
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..
Projector.
Luminaire using reflection and/or refraction to increase the luminous intensity within a limited solid angle..
Pulse-start metal halide lamp.
Metal halide lamps that operate in systems incorporating a high voltage ignitor to high pressure lamp systems. Pulse-start systems provide significantly higher efficiency and lower light loss factors than traditional metal halide systems.

Q
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..

R
Radiant efficiency (e).
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.
Radiant flux see radiant power.
Radiant power, (e, ).
Power emitted, transferred, or received in the form of radiation..
Unit watt, w..
Radiation.
1. Emission or transfer of energy in the form of electromagnetic waves or particles..
2. These electromagnetic waves or particles..
Rainproof luminaire.
Luminaire so constructed as to withstand the penetration of rain, and for use out of doors..
Recessed luminaire.
Luminaire mounted above the ceiling or behind a wall or other surface so that any visible projection is insignificant..
Recessed.
The term for a fixture mounted in a ceiling opening so that the housing of the fixture is hidden from view. The fixture’s lens/door assembly may be slightly protruding, flush or slightly regressed relative to the ceiling surface.
Reduced luminance coefficient (r).
The product of the luminaire coefficient (q) and cos3, where is the angle of light incidence..
Reference surface.
Surface on which illuminance is measured or specified..
Reflectance (formerly reflection factor).
Ratio of the reflected radiant or luminous flux to the incident flux..
Reflection.
Return of radiation by a surface or medium without change of frequency of its monochromatic components..
Reflector lamp.
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..
Reflector
device in which the phenomenon of reflection is used to alter the spatial distribution of the luminous flux from a source..
Refracting lens.
A curved or prismatic lens assembly that bends the emission angle of light rays in a desired direction from a luminaire.
Refracting louvre.
A louvre in which the screening depends essentially on the phenomenon of refraction..
Note.
A refracting louvre simultaneously serves as a light controller..
Refraction.
Change in the direction of propagation of radiation determined by change in the velocity of propagation in passing through an optically non¬homogeneous medium, or in passing from one medium to another..
Refractor.
Device in which the phenomenon of refraction is used to alter the spatial distribution of the luminous flux from a source..
Regular (or direct) transmission.
Transmission, without diffusion, in accordance with the laws of geometrical optics..
Regular (or specular) reflection.
Reflection, without diffusion, in accordance with the laws of geometrical optics; as in a mirror..
Relative luminance difference
see luminance contrast.
Retina.
Membrane at the back of the eye, which is sensitive to light stimuli and containing photoreceptors (cones and rods) and nerve cells that transmit the stimulation to the optic nerve..
Rod.
Photoreceptors in the retina containing light¬sensitive pigments capable of initiating the process of scotopic vision..
Rods probably play no part in color stimulus discrimination..
Room index (k).
Code number, representative of the geometry of a room, used in calculation of the utilization factor or the utilance..
Note 1.
Unless otherwise indicated, the room index is given by the formula .
K = (l x b)/h(l + b) .
Where.
L is the length of the room, b is the width and h is the distance of the luminaires above the working plane..
Note 2.
The ‘ceiling cavity index’ is calculated from the same formula except that h is the distance from ceiling to luminaires..
Rotationally symmetrical luminous intensity distribution.
Of a source. Distribution of luminous intensity that may be represented by rotating about an axis a polar luminous intensity curve in a plane containing that axis..

S
Safety lighting.
That part of emergency lighting provided to ensure the safety of people involved in a potentially hazardous process..
Saturation.
Attribute of visual sensation that permits a judgement to be made of the proportion of pure chromatic color in the total sensation..
Scotopic vision.
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..
Screen.
That part of a luminaire designed to prevent the lamps from being directly visible over a given range of angles..
Note.
In practice a screen will also act as a light controller..
Service illuminance.
Mean illuminance during one maintenance cycle of an installation averaged over the relevant area..
Setback.
The distance that the center of the luminaire is behind the area to be lighted by that luminaire.
Shielding angle (of a luminaire).
The angle between a horizontal line through the light center and the line of sight at which the bare source first becomes visible.
Single-sided arrangement.
A road-lighting arrangement in which the luminaires are placed on one side only of a carriageway..
Solid angle ().
The angle subtended at the center of a sphere by an area on its surface numerically equal to the square of the radius..
Unit steradian, sr..
Spacing to mounting height ratio .
The ratio of the distance between luminaire centers to the mounting height above a reference plane, usually the working plane.
Spacing.
In roadway lighting, the distance between successive light units measured along the center line of the street.
Spanwire arrangement.
A road-lighting arrangement in which the luminaires are suspended above the carriageway(s) on transverse wires..
Specific luminaire index (sli).
A quantity that indicates the glare control facility of a luminaire..
Spectral energy distribution.
Of a radiation. Description of the spectral character of a radiation by the relative spectral distribution of some radiometric quantity (radiant flux (power), radiant intensity, etc.)..
Spectral light distribution.
Of a radiation. Description of the spectral character of a radiation by the relative spectral distribution of some photometric quantity (luminous flux, luminous intensity, etc.)..
Spectral luminous efficiency curve.
Curve that gives the relative sensitivity (v) of the cie standard photometric observer for monochromatic radiation in dependence of the wavelength.
For photopic vision – v() curve .
For scotopic vision – v’() curve .
Spectrum locus.
Locus, in a chromaticity diagram or in a tristimulus space, of points that represent monochromatic stimuli..
Specular angle.
That angle between the perpendicular to a surface and the reflected ray. It is numerically equal to the angle of incidence.
Specular factor (s1 and s2) .
A factor representing the degree of specular reflection of a road surface..
Specular reflection
see regular reflection..
Specular reflector.
That part of a luminaire designed to reflect the luminous flux of the lamps in required directions by means of specular reflection..
Specular surface.
Shiny or glossy surfaces (including mirror and polished metals) that reflect incident light, providing a relatively narrow beam pattern.
Speed of perception.
The reciprocal of the minimum exposure time of an object required for it to be detected..
Spill light.
Of a floodlight. The scattered light falling outside the beam..
Spill shield
see louvre..
Spotlight.
A (small) projector giving concentrated light of usually not more than 20 divergence..
Spread.
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..
Staggered arrangement.
A road-lighting arrangement in which the luminaires are placed alternately on either side of the carriageway..
Stand-by lighting.
That part of emergency lighting that enables normal activities to continue substantially unchanged..
Starter.
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..
Starting device.
Electrical apparatus that provides the conditions required for starting a discharge..
Steradian (sr) .
Si unit of solid angle..
Stopping distance
see driver stopping distance.
Styrene (polystyrene).
The generic term for a family of plastics used in the making of fixture diffusers and lenses. Tends to yellow in time due to the effect of uv radiation from fluorescent, hid, and induction lamps.
Surface mounted.
Any fixture mounted directly on a wall or ceiling is surface mounted.
Suspension or pendant mounted.
Any fixture hung from a ceiling by supports (chains, hangers, etc.) Is suspension or pendant mounted.
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..
Symmetrical luminous intensity distribution.
Of a source. Distribution of luminous intensity having an axis of symmetry or at least one plane of symmetry..

T
Thermal radiation.
1. Process of emission in which the radint energy originates in the thermal agitation of the particles of matter (atoms, molecules, ions)..
2. The radiation emitted by that process..
Threshold contrast.
The minimum perceptible contrast for a given state of adaptation of the eye..
Threshold increment (ti) .
Number indicating the degree to which disability glare is controlled..
Throw.
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..
Translucent medium.
Medium that transmits visible radiation largely by diffuse transmission, so that objects are not seen distinctly through it..
Transmission.
Passage of radiation through a medium without change of frequency of its monochromatic components..
Transmittance (formerly transmission factor).
Ratio of the transmitted radiant or luminous flux to the incident flux..
Transparent medium.
Medium in which the transmission is mainly regular and which usually has a high regular transmittance in the spectral range of interest..
Troffer.
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..
Twin-central arrangement.
A road-lighting arrangement in which the luminaires are placed along the central reserve of a dual carriageway on t-shaped masts..

U
Ultraviolet radiation.
Optical radiation for which the wavelengths are shorter than those for visible radiation..
Underwriters laboratories.
Commonly referred to as “ul”. An independent organization whose responsibilities are establishing standards and rigorous testing of electrical and other consumer products. When products pass these tests, they can be labeled advertised as being “ul listed.”
Uniform-chromaticity-scale (ucs) diagram.
A two¬dimensional diagram in which the coordinates are defined with the intention of making equal distances represent as nearly as possible equal steps of color discrimination for color stimuli of the same luminance throughout the diagram..
Uniformity ratio of illuminance.
On a given plane. A measure of the variation of illuminance over the plane expressed as either.
1. The ratio of the minimum to the maximum illuminance..
2. The ratio of the minimum to the average illuminance..
Note.
In some countries, the reciprocal of these ratios is used, characterized by values greater than unity..
Uniformity.
A measure describing the variation of illuminance (or luminance) over a given plane expressed as the ratio of either the maximum to minimum illuminance or the average to minimum illuminance.
Unit power density (upd).
A measure of efficiency commonly applied as a means to evaluate the electrical power consumed by the rated lamp input power of a lighting system intending to illuminate a specific area. This metric is commonly expressed as watts per square or linear foot. Upd is the same as lighting power density (lpd) used in some energy codes and standards.
Upper [lower] (luminous) flux.
Of a source. The luminous flux emitted above [below] a horizontal plane passing through the source..
Note.
It is essential that the plane be specified in every case..
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..
Upward wasted light ratio (uwlr).
The ratio of total direct uplight emitting at and above the horizontal plane divided by all downward cast light over a given defined illuminated area.
Utilance (u) .
Ratio of the utilized flux to the luminous flux leaving the luminaires..
Utilisation factor.
Ratio of the utilized flux to the luminous flux emitted by the lamps..
Utilized flux.
Luminous flux received on the reference surface under consideration..

V
V() curve.
Spectral luminous efficiency curve for photopic vision..
V’() curve.
Spectral luminous efficiency curve for scotopic vision..
Valance (or pelmet) lighting.
Lighting system comprising light sources shielded by a panel parallel to the wall at the top of a window..
Vapor-tight luminaire.
A luminaire designed and approved for installation in damp or wet locations. Also described as “enclosed and gasketed.”
Veiling luminance seeequivalent veiling luminance.
Visible light meter.
A hand held portable photometer used to measure illuminance in lux or footcandles (or both). Often referred to as an ‘illuminance meter’ or a ‘light meter.’
Visible radiation.
Any radiation capable of causing a visual sensationdirect..
Visual acuity; sharpness of vision.
1. Qualitatively .
Capacity for seeing distinctly objects very close together..
2. Quantitatively.
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..
Visual angle.
The angle subtended by an object or detail at the point of observation it is usually measured in minutes of arc..
Visual comfort probability (vcp).
An empirical comfort rating of a lighting system expressed as a percentage of people who, when viewing from a specified location and in a specified direction, will be expected to find it acceptable in terms of discomfort glare. Visual comfort probability is related to discomfort glare rating (dgr).
Visual comfort.
The degree of visual satisfaction produced by the visul environment..
Visual field.
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 ield may be monocular or binocular..
Visual guidance.
The totality of measures taken to give a road user an unambiguous and immediately recognizable picture of the course of the road ahead..
Visual performance.
The quantitative assessment of the visual system in the performance of a visual task.
Visual reliability of a motorist.
The ability of a motorist to continuously select and process, more or less subconsciously, that part of the visual information presented to him that is necessary for the safe control of his vehicle..
Visual system.
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..

W
Warm-up time.
The time it takes for a lamp to produce 90% of its initial light output when it is started, unless otherwise indicated. Most hid lamps reach normal operating temperature within 1/2 hour depending on ambient temperature due to weather conditions.
Watertight luminaire.
Luminaire constructed to withstand the penetration of water when immersed to a specified depth, but not intended for permanent use under water..
Note.
The term ‘submersible luminaire’ applies to a luminaire constructed to withstand indefinitely submersion in water to a specified depth..
Watt (w).
The si unit for measuring electric power. It defines the power or energy consumed by an electrical device.
Wavelength ().
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)..
Unit meter, m..
Work (or working) plane.
Reference surface defined as the plane at which work is usually done..
Work plane.
The plane at which level work is usually performed, and at which the illumination is specified and measured. Unless it is otherwise specified, this plane is assumed to be a horizontal plane 2.5 ft above the floor.
Wraparound (wrap).
A surface or suspension mounted fixture with a one-piece plastic lens which encloses the lamp compartment on both sides and across the bottom literally wrapping the lamps.

Z
Zonal cavity.
The latest and most advanced method used by the lighting industry in the determination of coefficients of utilization for various fixtures.
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.