AC (Alternating Current)
Current which passes from the generator in one direction and then the other, alternately.
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.
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.

Angle of tilt see Tilt, angle of
Arc
Intense luminous discharge formed by the passage of electric current across a space between electrodes.
Arc discharge (in a gas or vapor)
Electric discharge characterized by a cathode fall that is small compared with that in a glow discharge.
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.

Ballast
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.
Ballast Hum
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.
Ballast Losses
Power which is supplied to a ballast but is not converted into lamp energy. Ballast loss is dissipated as heat.
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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 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 angle (in the plane through the beam axis) over which the luminous intensity drops to a stated percentage of its peak intensity.
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.
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Black body locus (Planckian locus)
The line in a chromaticity diagram representing the chromaticity of full (or Planckian) radiators of different temperatures.
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.
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.

CIE standard photometric observer
Ideal observer having a relative spectral responsivity curve that conforms to the V(lamda) or to the V'(lamda) functions for photopic and scotopic vision, respectively, and that complies with the summation law implied in the definition of luminous flux.
Capacitor
Device in ballast that stores electrical energy. Often used for power factor correction and lamp regulation (see "Power Factor").
Catenary arrangement
Arrangement of road-lighting luminaires suspended with their main beam axes at right angles to the road axis.
Catenary luminaire
Luminaire designed to be suspended from a cable with its main beam axis at right angles to the axis of the road.
Cathode see 'Electrode.'
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Central arrangement see Twin-central arrangement
Chromaticity
Color quality of a color stimulus definable by its chromaticity coordinates, or by its dominant (or complementary) wavelength and its excitation purity taken together.
Chromaticity diagram
Plane diagram showing the result of mixtures of color stimuli, each chromaticity being represented unambiguously by a single point on the diagram.
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.
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Color appearance
General expression for the color impression received when looking at a light source.
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 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.
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.
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Color triangle see Chromaticity diagram
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.
Cone
Photoreceptors in the retina containing lightsensitive pigments capable of initiating the process of photopic vision.
Contrast
Subjective assessment of the difference in appearance of two parts of a field of view seen simultaneously or successively.
(See also Luminance contrast)
Contrast threshold see Threshold 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.
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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
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)
Cove lighting
Lighting system comprising light sources shielded by a ledge or recess, and distributing light over the ceiling and upper wall.
Unit Kelvin K.
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Crest Factor (Lamp Current Crest Factor)
Ratio of peak lamp current to RMS or average lamp operating current.
Cross factor (CF)
A luminaire characteristic that indicates the suitability of the luminaire for use under wet and foggy weather conditions.
Unit Kelvin K.

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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.
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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.
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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
Glare that causes discomfort without necessarily impairing the vision of objects.
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.
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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.
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Dustproof luminaire
Luminaire so constructed that dust of specified nature and fineness cannot enter it when it is used in a dust-laden atmosphere.

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 Electrotechnical Commission (IEC).
Effective (road) width
The horizontal distance between the vertical through a luminaire center and the kerb farthest from that luminaire.
Efficacy see Luminous efficacy
Electric discharge in 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.

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Note
This results in the emission of electromagnetic radiation, which plays an essential part in all its applications in lighting.
Electrical efficiency
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.
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Electrode
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.
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.
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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.

Filament
Metal Tungsten wire coated with Barium Oxide that emits
electrons when voltage is applied.
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
Projector designed for floodlighting, usually capable of being pointed in any direction and of weatherproof construction.
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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.

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Note
This term is most commonly applied to lowpressure 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.
Full radiator see Planckian radiator

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 significant objects, or both, due to an unsuitable distribution or range of luminance or to extreme contrasts in space or time.
Glare control mark
Number denoting the degree to which discomfort glare is controlled.
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.

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Halide lamp see Metal halide lamp
Halogen lamp
Gas-filled lamp containing a tungsten filament and a small proportion of halogens.
Harmonic
An integral multiple of the fundamental frequency (50/60 Hz) that becomes a component of the current (see "Harmonic Distortion").
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 Wye 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.
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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.
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.

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

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.

Lamellae louvre
A louvre of which the main shielding elements are in the form of straight strips of opaque or translucent material.
Lamp Current Crest Factor see Crest Factor
Lamp Filament see Filament
Lamp Watts
Input power used to operate lamps.
Lamp mortality see Mortality rate
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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
Lens panel
A refracting panel consisting of one or more, usually cylindrical, fresnel lenses.
Light
Any radiation capable of causing a visual sensation direct i.e. Visible radiation
Light controller
That part of a luminaire designed to control the spatial distribution of the luminous flux of the lamp(s).

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Note
In practice a light controller will also act as a screen.
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Light distribution see Luminous intensity distribution
Light loss factor see Maintenance factor 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.
Light stimulus
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.
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.
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Local lighting
Lighting for a specific visual task, additional to and controlled separately from the general lighting.

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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.
Louvre
Screen made of translucent or opaque components and geometrically disposed to prevent lamps from being directly visible over a given angle.
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.
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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
Unit of measure for amount of light produced by a lamp once it is started. One lumen equals one foot candle per square foot.
Lumens Per Watt see Luminous efficacy
Luminaire
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.

Note
In road lighting the term 'lantern' is also sometimes used.
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.
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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.
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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.
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Luminous environment
Lighting considered in relation to its physiological and psychological effects.
Luminous flux
The quantity derived from radiant flux by evaluating the radiation according to its action upon the CIE standard photometric observer.
Unit lumen, lm.
Luminous flux
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.
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.

Note
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.

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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.
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Luminous intensity distribution
Distribution of the luminous intensities of a lamp or luminaire in all spatial directions.

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
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.

Note
The use of the term Depreciation factor as the reciprocal of maintenance factor is deprecated.
Mesopic vision
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.
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Mirror reflector see Specular 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.
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Mounting height
The distance between the reference plane and the plane of the luminaires.

Nominal voltage
voltage at which the lamp is intended to be used i.e. 6V, 12V, 24V for Automotive Lighting products.
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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 Xrays (approx = 1 nm) and the region of transition to radio waves (approx = 1 mm).
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Overall uniformity (Uo)
The ratio of minimum to the average luminance over the area of road considered.
Overhang
The horizontal distance between a vertical line passing through the luminaire center and the nearest kerb of the road.

QL induction lamp (system)
A lamp (system), based on the lowpressure 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.

Radiant efficiency
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
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.
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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.
Rainproof luminaire
Luminaire so constructed as to withstand the penetration of rain, and for use out of doors.
Rapid Start
Lamp starting method in which lamp filaments are preheated to facilitate lamp ignition.
Recessed luminaire
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.
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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
Device in which the phenomenon of reflection is used to alter the spatial distribution of the luminous flux from a source.
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.
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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 nonhomogeneous 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.
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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 lightsensitive 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.

Thermal radiation
1. Process of emission in which the radiant 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.
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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.
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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.
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V'(lamda) curve
Spectral luminous efficiency curve for scotopic vision.
V(lamda) curve
Spectral luminous efficiency curve for photopic 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
Luminaire so constructed that a specified vapor or gas cannot enter its enclosure.
Veiling luminance see Equivalent veiling luminance
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Visible radiation
Any radiation capable of causing a visual sensation direct.
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
The degree of visual satisfaction produced by the visual 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 yield may be monocular or binocular.
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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.

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.