| 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 colour stimuli presented to it. |
| |
| 2. The final state of the process |
| |
| Average illuminance (Eav) :
over a surface, Illuminance averaged over the specified
surface. |
| |
| Average luminance coefficient (Qo)
: 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. |
| |
| 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. |
| |
| Colour Rendering : general
expression for the effect of an illuminant on the
colour appearance of objects in conscious of subconscious
comparison with their colour appearance under a
reference illuminant. |
| |
| Colour rendering index (Rg) :
of a light source. Measure of the degree to which
the psycho-physical colours of objects illuminated
by the source conform to those of the same objects
illuminated by a reference illuminant for specified
condition. |
| |
| Colour space : Geometric representation
of colours in space, usually of three dimensions. |
|
| |
| Colour temperature : temperature
of the black body that emits radiation of the same
chromaticity as the radiation considered.Unit: Kelvin,
K. |
| |
| Direct Lighting : lighting
by means of luminaires with a light distribution
such that 90 to 100 percent of the emitted luminous
flux reaches the working plane direct, assuming
that this plane is unbounded. Disability glare :
Glare that impairs the vision of objects without
necessarily causing discomfort. |
| |
| Discomfort glare : glare that
causes discomfort without necessarily impairing
the vision of objects. |
| |
| 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. |
| |
| 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. |
| |
| Iso-intensity diagram : array
of iso-intensity curves. |
| |
| Iso-luminance curve (diagram) :
locus of points on a surface at which the luminance
is the same, for given positions of the observer
and of the source or sources in relation to the
surface. |
| |
| Isolux curve (diagram) : locus
of points on a surface where the illuminance has
the same value. |
| |
| 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. Lamp Mortality (Morality
rate) : the number of operating hours elapsed before
a certain percentage of the lamps fail. |
| |
| 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. |
| |
| 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. |
| |
| Localised
lighting : lighting designed to illuminate an
area with a higher illuminance at certain specified
positions, for instance those at which work is carried
out. |
| |
| Longitudinal
uniformity (U1) : the ratio of minimum to maximum
luminance along a line parallel to the road axis
through the observer's position |
| |
| Luminance (L) : in a given
direction, at a given point of 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 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 metre, cd/m2 |
| |
| Luminous
flux ((py), ((p) : the quantity derived from
radiant flux by evaluat/ng the radiation according
to its action upon the CIE standard photometric
observer. Unit: lumen, 1m. |
| |
| Luminous Intensity (ly, 1) :
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. |
| |
| 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. |
| |
| 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. |
| |
| Overall
uniformity (UQ) : 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
centre and the nearest kerb of the road. |
| |
| Peak intensity : the luminous
intensity of a luminaire in the direction of the
beam axis. |
| |
| 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. |
| |
| 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 = lxbh(l
xb) |
| |
| where : I 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 Service
illuminance: mean illuminance during one maintenance
cycle of an installation averaged over the relevant
area. |
| |
| 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. |
| |
| Threshold increment (Tl) :
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.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, characterised by values
greater than unity. |
| |
| 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 ofJthe utilised flux
to the luminous flux leaving the luminaires. |
| |
| Utilisation factor: ratio of
the utilised flux to the luminous flux emitted by
the lamps. |
