L
lamp a
generic term for a source created to produce optical radiation. By extension,
the term is also used to denote sources that radiate in regions of the spectrum
adjacent to the visible. Note Through
popular usage, a portable luminaire consisting of a lamp with shade, reflector,
enclosing globe, housing, or other accessories is also sometimes called a lamp.
In such cases, in order to distinguish between the assembled unit and the light
source within it, the latter is often called a bulb or tube, if it is electrically
powered. See also luminaire.
lamp
burnout factor the fractional loss of task illuminance due
to burned-out lamps left in place for long periods.
lamp
lumen depreciation (LLD) factor the fractional loss of lamp
lumens at rated operating conditions that progressively occurs during lamp
operation.
lamp
position factor The ratio of the luminous flux of a lamp at
a given operating position to the luminous flux when the lamp is operated in
the position at which the lamp lumens are rated.
lamp
post
a standard support provided with the necessary internal attachments for wiring
and the external attachments for the bracket and luminaire.
lamp
shielding angle, φ the angle between the plane of the
baffles or louver grid and the plane most
laser an acronym
for light amplification by stimulated emission of radiation. The laser produces
a highly monochromatic and coherent (spatially and temporally) beam of
radiation. A steady oscillation of nearly a single electromagnetic mode is
maintained in a volume of an active material bounded by highly reflecting
surfaces, called a resonator. The frequency of oscillation varies according to
the material used and the methods of initially exciting or pumping the
material.
lateral
width of a light distribution in roadway lighting, the lateral
angle between the reference line and the width line, measured in the cone of
maximum candlepower. This angular width includes the line of maximum
candlepower. See reference line and width line.
lens† a
glass or plastic element used in luminaires to change the direction and control
the distribution of light rays; also, the part of the eye that allows objects
at different distances to be focused onto the retina.
life
performance curve a curve that presents the variation of a
particular characteristic of a light source (such as luminous flux, intensity,
etc.) throughout the life of the source.
Note Life performance curves sometimes are called maintenance curves,
for example, lumen maintenance
life
test of lamps a test in which lamps are operated under
specified conditions for a specified length of time for the purpose of
obtaining information on lamp life. Measurements of photometric and electrical
characteristics can be made at specified intervals of time during this
test.
light
radiant energy that is capable of exciting the retina and
producing a visual sensation. The visible portion of the electromagnetic
spectrum extends from about 380 to 770 nm.
Note The subjective impression produced by stimulating the retina is
sometimes designated as light. Visual sensations are sometimes arbitrarily
defined as sensations of light, and in line with this concept, it is sometimes
said that light cannot exist until an eye has been stimulated. Electrical
stimulation of the retina or the visual cortex is described as producing
flashes of light. In illuminating engineering, however, light is a physical
entity--radiant energy weighted by the luminous efficiency function. It is a
physical stimulus that can be applied to the retina. See spectral luminous
efficacy of radiant flux and values of spectral luminous efficiency for
photopic vision.
light
adaptation the process by which the retina becomes adapted to a
luminance greater than about 3.4 cd/m2.
light-emitting
diode (LED) a p-n junction solid-state diode whose
radiated output is a function of its physical construction, material used, and
exciting current. The output can be in the IR or in the visible region.
light
loss factor (LLF) Formerly called maintenance factor. The
ratio of illuminance (or exitance or luminance) for a given area to the value
that would occur if lamps operated at their (initial) rated lumens and if no
system variation or depreciation had occurred. Components of this factor can be
either initial or maintained. Note The
light loss factor is used in lighting calculations as an allowance for lamp(s)
or luminaire(s) operating at other than rated conditions (initial) and for the
depreciation of lamps, light control elements, and room surfaces to values
below the initial or design conditions, so that a minimum desired level of
illuminance can be maintained in service. The light loss factor had formerly
been widely interpreted as the ratio of average illuminance in service to
initial illuminance.
light
meter A common name for an illuminance meter. See illuminance
(lux or footcandle) meter.
light
source color the color of the light emitted by a
source. Note The color of a point source
can be defined by its luminous intensity and chromaticity coordinates; the
color of an extended source can be defined by its luminance and chromaticity
coordinates. See color temperature, correlated color temperature, and perceived
light source color.
lighting
effectiveness factor (LEF) the ratio of equivalent sphere
illumination to measured or calculated task illuminance.
lightness (of
a perceived patch of surface color) the brightness of an area judged relative
to the brightness of a similarly illuminated area that appears to be white or
highly transmitting.
louver (or
louver grid) a series of baffles used to shield a source from view at certain
angles, to absorb or block unwanted light, or to reflect or redirect light. The
baffles are usually arranged in a geometric pattern.
louver shielding angle, θ
the angle between the horizontal plane of the baffles or louver grid and the
plane at which the louver conceals all objects above. See lamp shielding
angle. Note The planes usually are so
chosen that their intersection is parallel with the louvered blade.
louvered
ceiling a ceiling-area lighting system composed of a wall-to-wall installation
of multicell louvers shielding the light sources mounted above it. See luminous
ceiling.
low-bay
lighting interior lighting where the roof trusses or ceiling
height is approximately 7.6 m (25 ft) or less above the floor.
low-key
lighting a type of lighting that, applied to a scene, results in
a picture having gradations from middle gray to black with comparatively
limited areas of light grays and whites. See high-key lighting.
low-pressure
mercury lamp a discharge lamp (with or without a phosphor
coating) in which the partial pressure of the mercury vapor does not exceed 100
Pa during operation.
low-pressure
sodium (LPS) lamp a discharge lamp in which light is produced
by radiation from sodium vapor operating at a partial pressure of 0.1 to 1.5 Pa
(approximately 10−3 to 10−2 Torr).
lower
(passing) beams one or more beams directed low enough on the
left to avoid glare in the eyes of oncoming drivers and intended for use in
congested areas and on highways when meeting other vehicles within a distance
of 300 m (1000 ft). Formerly traffic beam.
lumen, lm
SI unit of luminous flux. Radiometrically, it is determined from the radiant
power as in luminous flux. Photometrically, it is the luminous flux emitted
within a unit solid angle (1 sr) by a point source having a uniform luminous
intensity of 1 cd.
lumen
depreciation† the decrease in lumen output that occurs as
a lamp is operated, until failure.
lumen
(or flux) method a lighting design procedure used for
predetermining the relation between the number and types of lamps or
luminaires, the room characteristics, and the average illuminance on the
workplane. It takes into account both direct and reflected flux.
luminaire
(light fixture) a complete lighting unit consisting of a lamp or lamps and
ballast(s) (when applicable) together with the parts designed to distribute the
light, to position and protect the lamps, and to connect the lamps to the power
supply.
luminaire
dirt depreciation (LDD) the fractional loss of task illuminance
due to luminaire dirt accumulation.
luminaire
efficiency the ratio of luminous flux (lumens) emitted by a
luminaire to that emitted by the lamp or lamps used therein.
luminaire
spacing criterion (SC)† a classification parameter for indoor
luminaires relating to the distribution of the direct illuminance component
produced on the work plane. The SC of a luminaire is an estimated maximum ratio
of spacing to mounting height above the work plane for a regular array of that
luminaire such that the work plane illuminance will be acceptably uniform. Note The SC is not a recommendation for the
spacing-to-mounting-height ratio for an installation. It is a characteristic
that assists in identifying appropriate luminaires when illuminance uniformity
is a design goal. The SC evolved but is distinctly different from an obsolete
luminaire parameter called the spacingto-mounting-height ratio. See Chapter 9,
Lighting Calculations, for the SC algorithm.
luminaire
surface depreciation factor the fractional loss of task
illuminance due to permanent deterioration of luminaire surfaces.
luminance, L
= d2φ/(dω dA cos θ) (in a direction and at a point of a real or imaginary
surface) the quotient of the luminous flux at an element of the surface
surrounding the point, and propagated in directions defined by an elementary
cone containing the given direction, by the product of the solid angle of the
cone and the area of the orthogonal projection of the element of the surface on
a plane perpendicular to the given direction. The luminous flux can be leaving,
passing through, and/or arriving at the surface. Formerly, photometric
brightness. By introducing the concept
of luminous intensity, luminance can be expressed as L = dI/(dA cos θ). Here,
luminance at a point on a surface in a direction is interpreted as the quotient
of luminous intensity in the given direction, produced by an element of the
surface surrounding the point, by the area of the orthogonal projection of the
element of surface on a plane, perpendicular to the given direction
luminance
contrast the relationship between the luminances of an object and
its immediate background. It is equal to (L1 − L2)/L1 or (L2 − L1)/L1 =
|∆L/L1|, where L1 and L2 are the luminances of the background and object,
respectively. The form of the equation must be specified. The ratio ∆L/L1 is
known as Weber's fraction. Note See note
under luminance. Because of the relationship among luminance, illuminance, and
reflectance, contrast often is expressed in terms of reflectance when only
reflecting surfaces are involved. Thus, contrast is equal to (ρ1 − ρ2)/ρ1, or (ρ2
− ρ1)/ρ1, where ρ1 and ρ2 are the reflectances of the background and object,
respectively. This method of computing contrast holds only for perfectly
diffusing surfaces; for other surfaces it is only an approximation unless the
angles of incidence and view are taken into consideration. See
reflectance.
luminance
difference the difference in luminance between two areas, such as
the detail of a visual task and its immediate background, in which case it is
quantitatively equal to the numerator in the formula for
luminance
factor, β the ratio of the luminance of a surface or medium
under specified conditions of incidence, observation, and light source, to the
luminance of a completely reflecting or transmitting, perfectly diffusing
surface or medium under the same conditions.
Note The reflectance or transmittance cannot exceed 1, but luminance
factor can have any value from 0 to values approaching infinity.
luminance
ratio the ratio between the luminances of any two areas in the
visual field.
luminance threshold the
minimum perceptible difference in luminance for a given state of adaptation of
the eye.
luminescence any
emission of light not ascribable directly to incandescence. See
electroluminescence, fluorescence, and phosphorescence.
luminous
ceiling a ceiling area lighting system comprising a continuous
surface of transmitting material of a diffusing or light-controlling character
with light sources mounted above it. See louvered ceiling.
luminous
density, w = dQ/dV quantity of light (luminous energy) per unit
volume.
luminous
efficacy of radiant flux the quotient of the total luminous flux
by the total radiant flux. It is expressed in lumens per watt.
luminous
efficacy of a source of light the quotient of the total luminous
flux emitted the total lamp power input. It is expressed in lumens per
watt. Note The term luminous efficiency
has in the past been extensively used for this concept.
luminous
efficiency† See spectral luminous efficiency of radiant flux.
luminous
energy† See quantity of light.
luminous
flux density at a surface, dΦ/dA the luminous flux per
unit area at a point on a surface. Note
This need not be a physical surface; it can also be a mathematical plane. See
also illuminance and luminous exitance.
lux, lx
the SI unit of illuminance. One lux is one lumen per square meter (lm/m2). See
the Appendix for conversion values.