| 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 quality lightstabilized plastics used in making
fixture diffusers and lenses. |
| 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. |
| Air-Handling |
A
term applied to a recessed fixture which supplies or returns air to or
from a room. This is usually accomplished through slots along the sides/ends
of the fixture. |
| Ambient |
The
surrounding environment of a device such as a fixture or ballast. It usually
refers to temperature or sound conditions. |
| 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. |
| 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
Maximum Candlepower |
The
average of the 10 highest readings in a floodlight beam. |
| Ballast |
A
device which modifies incoming voltage and current to provide the circuit
conditions necessary to start and operate electric discharge lamps ( flourescent
and HID ). |
| Ballast |
Device
used with discharge lamps for stabilizing the current in the discharge. |
| 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
angle (in the plane through the beam axis) over which the luminous intensity
drops to a stated percentage of its peak intensity. |
| Beam
Spread |
The
vertical and horizontal displacement of the beam in degrees, bounded by
the angle at which 10% of maximum candlepower occurs. ( Maximum candle-power
is the highest intensity in the beam. ) |
| Black
body locus(Planckian locus) |
The
line in a chromaticity diagram representing the chromaticity of full (or
Planckian) radiators of different temperatures. |
| 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. |
| Brightness |
As
commonly applied, brightness ( or luminance ) is the intensity of the sensation
which results when viewing a surface or space which directs light into
the eyes. |
| 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. |
| Candela |
Unit
of luminous intensity. |
| Candlepower |
Luminous
intensity. |
| Candlepower
Distribution Curve |
A
curve showing the variation of luminous intensity of a lamp or luminaire
with angle. |
| 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. |
| Cavity |
An
upper, lower or intermediate zone or region of a room designated as ceiling,
floor or room cavity. |
| Cavity
Ratio (CR) |
Geometric
proportions of the ceiling, floor and room cavities. |
| 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. |
| 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 Beam Utilization (CBU) |
The
percentage of light from a 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: 1) Fixture efficiency, distribution and mounting
height 2) Room proportions 3) 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 |
General
expression for the effect of a light source on the color appearance of
objects when compared with their color appearance under a reference 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. |
| Color
triangle |
see
Chromaticity diagram |
| 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. |
| 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 |
| 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 between the vertical axis and the first line of sight at
which the bare source is not visible. |
| 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. |
| Diffuser |
The
shielding portion of a fixture, made of plastic or glass, through which
the light passes on its way to the seeing task. |
| 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 |
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. |
| 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. |
| Effective
(road) width |
The
horizontal distance between the vertical through a luminaire center and
the kerb farthest from that luminaire. |
| Efficacy |
see
Luminous efficacy |
| Efficacy |
The
ratio of light from lamp to the electrical power (watts) consumed. Usually
expressed in lumens per watt. |
| Efficiency |
The
ratio of the light leaving a luminaire to that emitted by the lamp, or
lamps, used therein. |
| 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 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. |
| Fixture |
The
device which holds, protects and provides an optical system and power connections
for the lamp(s). |
| 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. |
| 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 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. |
| Flux
(Luminous Flux) |
The
unit of light output. |
| Footcandle
(fc) |
A
quantitative unit for measuring illuminance. The illumination on a suface
one foot square on which there is a uniformly distirbuted flux on one lumen. |
| Footlambert |
The
unit of luminance equal to 1/Pi candle per sq. foot. |
| Full
radiator |
see
Planckian radiator |
| 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 cove glass. |
| Glare |
The
sensation produced by luminance within the visual field that is significantly
greater than the luminance to which the eyes are adapted. |
| 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. |
| Glare,
Direct |
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. |
| Halide
lamp |
see
Metal halide lamp |
| 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. |
| Heat
Extraction |
A
type of recessed fixture with provisions for the return of room air through
the fixture's lamp cavity, thus removing some of the heat generated by
the lamps ballasts. |
| Heavy
Duty Floodlight (HD) |
A
weatherproof unit having a substantially constructed metal housing into
which is placed a separate ant 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. |
| HID |
High
Intensity Discharge lighting, including mercury vapor, metal halide and
high pressure sodium light sources. Although low pressure sodium lamps
are not HID sources, they are often included in the HID category. |
| High
Bay |
Generally
refers to industrial lighting where high mounting heights may be encountered.
Many industrial HID type fixtures are called high bays. |
| High
Intensity Discharge (HID) |
High
Intensity Discharge lighting, including mercury vapor, metal halide and
high pressure sodium light sources. Although low pressure sodium lamps
are not HID sources, they are often 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. |
| Ignitor |
see
Starter |
| Illuminance |
The
density of luminous flux on a surface. Measured in footcandles or lux.
The former term for this quantity was illumination. |
| Illuminance
(E) |
At
a point on a surface. Quotient of the luminous flux incident on an element
of the surface containing the point, and the area of that element.
Unit
lux, lx.
Note
The
orientation of the surface may be defined, e.g. horizontal, vertical, hence
horizontal illuminance, vertical illuminance. |
| Illumination |
see
Lighting Incandescence
Emission
of optical radiation by the process of thermal radiation. |
| Incandescent
(electric) lamp |
Lamp
in which light is produced by means of an element heated to incandescence
by the passage of an electric current. |
| 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. |
| 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 that states that the illuminance at a point on a plane perpendicular
to the line joining the point and a source is inversely proportional to
the square of the distance between the source and the plane.
E = I/d2 |
| 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*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 |
| Iso-illuminance
curve |
Locus
of points on a surface where the illuminance has the same value. |
| 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
(Isofootcandle) 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. |
| Isolux
curve (diagram) |
Locus
of points on a surface where the illuminance has the same value. |
| 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 |
A
light source. |
| 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 |
| 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
(grid) |
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. |
| 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. |
| 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).
Note
In practice
a light controller will also act as a screen. |
| Light
distribution |
see
Luminous intensity distribution |
| 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
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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| Luminaire |
A
complete lighting fixture including one or more lamps and a means for connection
to a power source. Many luminaires also include one or more ballasts and
elements to position and protect lamps and distribute their light. |
| Luminaire
Dirt Depreciation (LDD) |
A
factor used in lighting calculations to account for the dlight 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 |
The
ratio of the light leaving a luminaire to that emitted by the lamp, or
lamps, used therein. |
| 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
(Photometric Brightness). |
The
luminous intensity of any surface in a given direction per unit area of
that surface as viewed from that direction. Measured in footlamberts or
C/ in*in. All visible objects have some luminance. |
| 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,
seeBrightness |
| 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 |
The
time rate of flow of light. |
| Luminous
flux (?v), (?) |
The
quantity derived from radiant flux by evaluating the radiation according
to its action upon the CIE standard photometric observer.
Unit
lumen, lm. |
| Luminous
intensity (Iv, I) |
Of
a source in a given direction. Quotient of the luminous flux leaving the
source, propagated in an element of solid angle containing the given direction,
and the element of solid angle.
Unit
candela, cd.
Note
The
luminous intensity of luminaires is normally given either in a Luminous
intensity diagram or in anIsocandela 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.
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. |
| 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 a 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 area being illuminated. Light loss factor includes maintenance
factor. |
| 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. |
| Metercandle
(lux, lx) |
The
metric equivalent of foot-candle. Lx = 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. |
| Mounting
height |
The
distance between the reference plane and the plane of the luminaires. |
| 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
(?? 1nm) and the region of transition to radio waves (?? 1 mm). |
| Optical
System |
The
lamp cavity or environment (including diffusing media) designed as part
of the fixture for the purose of controlling the light output. |
| 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. |
| Overhang |
In
roadway lighting, the distance between a vertical line passing through
the luminaire and the curb or edge of the roadway. |
| Parabolic |
The
term applied to certain low brightness louver and reflector shapes as derived
from the geometric shape (curve) called 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. |
| 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 which
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. |
| Polychlorinated
Biphenly (PCB) |
A
dielectric substance used in ballast capacitors. |
| 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. |
| 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 highfrequency electromagnetic
field. |
| 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 |
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. |
| Recessed
luminaire |
Luminaire
mounted above the ceiling or behind a wall or other surface so that any
visible projection is insignificant. |
| Reduced
luminance coefficient ® |
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 |
Device
in which the phenomenon of reflection is used to alter the spatial distribution
of the luminous flux from a source. |
| Reflector |
A
device used to direct the light from a source by the process of reflection. |
| 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. |
| 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. |
| Refraction |
The
process by which the direction of a ray of light changes as it passes obliquely
from on 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. |
| Refractor |
A
device used to redirect the luminous flux from a source, primarily by the
process of refraction. |
| 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 |
seeLuminance
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. |
| 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 |
The
distance between the centers of two successive luminaires in an installation. |
| Spacing |
The
distance between successive lights from the center of one light to the
center of the next subsequent light. |
| Spacing
to Mounting Height Ratio |
The
ratio of the distance between luminaire centers to the mounting height
above a reference plane, usually the work-plane. |
| 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
distribution |
a.
Of a photometric quantity luminous flux, luminous intensity, etc. The spectral
concentration of the photometric quantity as a function of wavelength.
b. Of
a radiometric quantity radiant flux (power), radiant intensity, etc. The
spectral concentration of the radiometric quantity as a function of wavelength.
Note
Commonly
the relative spectral distribution is used, viz. the spectral concentration
of the photometric or radiometric quantity measured in terms of an arbitrary
value of this quantity. |
| 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 numberically 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 |
seeRegular
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 meatals) 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
Light |
Lumens
distributed by the luminaire which are outside the beam spread. |
| Spill
shield |
seeLouvre. |
| 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 |
seeDriver
stopping distance |
| Styrene
(polystyrene) |
The
generic term for a family of plastics used in the making of fixture diffusers
and lenses. These materials tend to yellow in time due to the effect of
ultraviolet raiation from fluorescent and HID lamps. |
| Surface
Mounted |
Any
fixture mounted directly on a ceiling is surface mounted. |
| Suspension
or Pendant Mounted |
Any
fixture hung from a ceiling by supports (chains, hangars, stems, 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. |
| 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. |
| 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
recessed lighting fixture. |
| 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. |
| Ultraviolet
radiation |
Optical
radiation for which the wavelengths are shorter than those for visible
radiation. |
| Uniform-chromaticity-scale
(UCS) diagram |
A
twodimensional 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. |
| 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. |
| 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'(?)
curve |
Spectral
luminous efficiency curve for scotopic vision. |
| V(?)
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 |
A
Luminaire designed and approved for installation in damp or wet locations.
Also described as "enclosed and gasketed." |
| Vapor-tight
luminaire |
Luminaire
so constructed that a specified vapor or gas cannot enter its enclosure. |
| Veiling
luminance |
seeEquivalent
veiling luminance |
| 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
Comfort Probability (VCP) |
An
empirical comfort rating system for fixtures as measured in various proportioned
rooms at different mounting heights. The VCP number represents the number
of people, out of a hundred, who would be comfortable in the specified
situation when seated in the most undesirable location. |
| 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. |
| 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. |
| 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. |
| 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. |