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GLOSSARY
Absorptance/
Absorption factor
Ratio of the absorbed radiant or luminous flux to the incident flux.
Accommodation
Focal adjustment of the eye, generally spontaneous, made for the purpose
of obtaining maximum visual acuity at various distances.
Angle of tilt
Upward inclination of a luminaire from the horizontal.
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.
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).
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 with built in resistor 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(the term luminosity is obsolete)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.
Catenary arrangement
Arrangement of road-lighting luminaires suspended with their main beam
axes at right angles to the road axis.
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 TLS 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 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 temperature
Temperature of the black body that emits radiation of the same chromaticity
as the radiation considered. Unit Kelvin, K.
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.
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