lighting glossary
absorption
A process by which incident radiant flux is converted to another form of energy, usually
(and ultimately) heat.
accent light
Directional lighting designed to emphasize a particular object or to draw attention to a
part of the field of view.
accommodation
The process by which the eye changes focus from one distance to another.
adaptation
The process by which the eye becomes accustomed to varying quantities of light or to
light of a different color.
altitude
The vertical angular distance of a point in the sky above the horizon. Altitude is
measured positively from the horizon to the zenith, from 0 to 90 degrees.
ambient light
Electric and/or natural lighting throughout a space that produces uniform general
illumination.
artificial sky
An enclosure that simulates the luminance distribution of a real sky for the purpose of
testing physical daylighting models. See hemispherical dome artificial sky and mirrorbox artificial sky.
azimuth
The horizontal angular distance between the vertical plane containing a point in the sky
and true south.
ballast
A magnetic or electronic device used to control the starting and operation of discharge
lamps.
ballast factor
The ratio of lamp lumen output on a particular ballast as compared to that lamp's
(lamps') rated lumen output on a reference ballast under ANSI test conditions (free,
unmoving air at 25° C)
beam component
That component of flux received directly (or by specular reflection or transmission) from
a point source (such as the sun or small lamp). It is a direct component.
blinding glare
Glare that is so intense that, for an appreciable length of time after it has been removed,
no object can be seen.
bulb
The glass outer envelope component of a lamp.
candela (cd)
The SI unit of luminous intensity (formerly called the candle). One candela equals one
lumen per steradian-the luminous intensity, in a give direction, of a source that emits
monochromatic radiation at a frequency of 540E12 hertz and of which the radiant
intensity in that direction is 1/683 watts per steradian.
chromaticity
Refers to the dominant or complementary wavelength and purity aspects of the color
taken together, or of the aspects specified by the chromaticity coordinates of the color
taken together.
clerestory
That part of a building rising clear of the roofs or other parts, whose walls contain
windows for lighting the interior.
coefficient of utilization (CU)
The ratio of lumens from a luminaire received on the work plane to the total quantity of
lumens emitted by the lamps of that luminaire.
color rendering index
A measurement of the amount of color shift that objects undergo when lighted by a light
source as compared with the color of those same objects when seen under a reference
light source of comparable color temperature. CRI values generally range from 0 to 100.
color temperature
The absolute temperature of a blackbody radiator having a chromaticity equal to that of
the light source (see correlated color temperature).
cone
A retinal receptor that dominates the retinal response when the luminance level is high
and provides the basis for the perception of color.
contrast
The ratio of the luminance of an object to that of its immediate background.
cosine law
The law that the illuminance on any surface varies as the cosine of the angle of
incidence. (The angle of incidence is the angle between the normal to the surface and
the direction of the incident light.)
cut-off angle
The critical viewing angle beyond which a source can no longer be seen because of an
obstruction (such as a baffle or overhang).
dark adaptation
The process by which the retina becomes adapted to a luminance of less than 0.01
footlamberts.
daylight factor (DF)
The ratio of daylight illumination at a given point on a given plane due to the light
received directly or indirectly from a sky of assumed or known luminance distribution, to
the illumination on a horizontal plane due to an unobstructed hemisphere of this sky,
expressed as a percentage. Direct sunlight is excluded for both values of illumination.
The daylight factor is the sum of the sky component, the external reflected component,
and the internal reflected component. The interior plane is usually horizontal. If the sky
condition is the C.I.E. standard overcast condition, then the DF will remain constant,
regardless of absolute exterior illuminance. If used in conjunction with other than
standard overcast conditions, the sky conditions should be specified. The term is also
informally applied to the ratio of horizontal interior to exterior illuminance in the
fenestration plane; under clear sky conditions, the DF remains constant only if the
fenestration is completely diffusing (such as an ideal opalescent glass).
diffusing (surface)
Those surfaces and glazing that redistribute some of the incident flux by scattering in all
directions.
disability glare
Glare resulting in reduced visual performance and visibility . Often accompanied by
discomfort glare.
discomfort glare
Glare producing discomfort. Does not necessarily interfere with visual performance or
visibility.
efficacy
A measure of the luminous efficiency of a radiant flux, expressed in lumens per watt as
the quotient of the total luminous flux by the total radiant flux. For daylighting, this is the
quotient of visible flux incident on a surface to radiant flux on that surface. For electric
sources, this is the quotient of the total luminous flux emitted by the total lamp power
input.
emissivity/emittance
The ratio of radiance (for directional emissivity) or radiant exitance (for hemispherical
emissivity) of an element of surface on a temperature radiator to that of a blackbody at
the same temperature. By Kirchoff's Law, for a given wavelength of the electromagnetic
spectrum, emissivity of a surface equals its absorptivity (and is the reciprocal of its
reflectivity).
exitance
The density of light reflecting from a surface at a point, measured in lumens per square
foot (formerly "footlamberts"). It is determined by multiplying the footcandles striking a
diffuse reflecting surface times the reflectance of that surface.
fenestration
Any opening or arrangement of openings (normally filled with glazing media) for the
admission of daylight, including any devices in the immediate proximity of the opening
that affect distribution (such as baffles, louvers, draperies, overhangs, light-shelves,
jambs, sills, and other light-diffusing materials).
fixture
Informal substitute term for luminaire.
fluorescent
A discharge lamp in which a phosphor coating transforms ultraviolet energy into visible
light.
flux
The time rate of flow. For example, volume per hour is the flux of a fluid.
footcandle
A standard measurement of illuminance, representing the amount of illuminance on a
surface one foot square on which there is a uniformly distributed flux of one lumen.
footlambert (fl)
A unit of luminance equal to 0.3183010 candela per square foot, or to the uniform
luminance of a perfectly diffusing surface emitting or reflecting light at a rate of one
lumen per square foot, or to the average luminance of a surface emitting or reflecting
light at that rate. An unobstructed sky of one footlambert uniform luminance contributes
one footcandle of illuminance on a horizontal plane.
fovea
A small region at the center of the retina, subtending about two degrees and forming the
site of the most distinct vision and greatest color discrimination.
glare
See direct glare, disability glare, discomfort glare, reflected glare.
glare index
A method of predicting the presence of discomfort glare due to daylighting. Factors
affecting the glare index include the size and relative position of fenestration, sky
luminance, and interior luminance. Most widely used in Europe, the glare index is similar
to the index of sensation and the discomfort glare rating, which are used in North
America for electric lighting applications.
hue
The attribute of a color that allows it to be classified as red, yellow, blue, and so on.
IALD
International Association of Lighting Designers
IESNA
Illuminating Engineering Society of North America
illuminance
The density of incident luminous flux on a surface; illuminance is the standard metric for
lighting levels, and is measured in lux (lx) or footcandles (fc).
incandescence
The emission of visible electromagnetic radiation due to the thermal excitation of atoms
or molecules.
indirect sources
Surfaces which, after being illuminated by other sources (direct sources such as the
sun, sky, or electric light, or other indirect sources), have measurable luminance and, in
turn, become sources themselves.
infrared radiation
Radiation with wavelengths too long to be perceived by the human eye (that is, longer
than 0.77 microns) and less than 1,000 microns. Room IR is infrared radiation in the 7.78.0 micron region and typical of that radiated from surfaces near room temperature.
inverse-square law
The law stating that the illuminance at a point on a surface varies directly with the
intensity of a point source, and inversely as the square of the distance between that
source and that surface.
irradiance (E)
The density of radiant flux incident on a surface.
isolux (isofootcandle) line
A line plotted on any appropriate set of coordinates to show all the points on a surface
where the daylight illuminance is the same. A series of such lines for various illuminance
values is called an isolux (isofootcandle) diagram.
lamp
An electrically energized source of light, commonly called a bulb or tube.
lamp lumen depreciation
The decrease over time of lamp lumen output, caused by bulb wall blackening,
phosphor exhaustion, filament depreciation and other factors.
light
Radiant energy that is capable of exciting the retina and producing a visual sensation.
The visible portion of the electromagnetic spectrum (light) extends from about 0.38 to
0.77 microns.
light loss factor (LLF)
A factor used in calculating the illuminance after a given period of time and under given
conditions. It takes into account temperature and voltage variations, lamp depreciation
(of electric luminaries), dirt accumulation on luminaire and room surfaces, maintenance
procedures and atmosphere conditions. Formerly called maintenance factor.
light shelf
A horizontal shelf positioned (usually above eye level) to reflect daylight onto the ceiling
and to shield direct flare from the sky.
lumen
The quantity of luminous flux emitted within a unit solid angle (one steradian) by a point
source with one candella intensity in all directions.
lumen method
A method of estimating the interior illuminance due to window daylighting at three
locations within a room. Based on empirical studies, the use of this method is primarily
limited to North America.
luminaire
A complete lighting unit, consisting of a lamp or lamps together with the components
required to distribute the light, position the lamps, and connect the lamps to a power
supply. Often referred to as a "fixture."
luminaire dirt depreciation
A multiplier used in lighting calculations to account for the reduction in illuminance
produced by the accumulation of dirt on a luminaire.
luminance
The luminous intensity of a surface in a given direction per unit area of that surface as
viewed from that direction; often incorrectly referred to as "brightness."
lux (lx)
The SI unit of illuminance equal to one lumen per square meter.
matte surface
Surface from which the reflection is predominantly diffuse, with or without a negligible
specular component.
monitor
A raised section of roof that includes a vertically (or near-vertically) glazed aperture for
the purpose of daylighting illumination.
near infrared (solar infrared)
The region of the electromagnetic spectrum between 0.77 to 1.4 microns. Most of the
infrared solar radiation falls into this region. This near infrared (or solar IR) region is
transmitted, absorbed, and reflected in a similar manner to visible light by most glazing
and nonmetallic building materials.
overcast sky
A sky luminance distribution three times brighter near (C.I.E. Standard Condition) the
zenith than at the horizon, as defined by a formula proposed by Moon and Spencer in
1942 and adopted by the Commission International de l'Eclairge in 1955
overhang
A horizontal building projection, usually above a window, for the purpose of shading.
photocell
A device that measures the amount of incident light present in a space.
peripheral vision
The seeing of objects displaced from the primary line of site and outside of the central
visual field.
point method
A method of estimating the illuminance at various locations in a building using
photometric data.
pupil
The opening in the iris of the eye that admits light.
radiant energy (radiation)
Energy traveling in the form of electromagnetic waves. Measured in units of energy such
as joules, ergs, or kilowatt-hours.
refraction
The process by which the direction of light changes as it passes obliquely from one
medium to another in which its speed is different.
reflectance
The ratio of reflected flux to incident flux.
retina
A light-sensitive membrane lining the posterior part of the inside of the eye.
rods
Retinal receptors that respond to low levels of luminance but cannot distinguish hues.
Not present in the center of the fovea region.
room cavity ratio
In lighting calculations, a measure of room proportion as determined by dimensions of
length, width, and height.
skylight
A relatively horizontal glazed roof aperture for the admission of daylight.
specular angle
The angle of mirror reflection (angle of incidence equals angle of reflectance).
specular reflection
The process by which incident light is redirected at the specular (mirror) angle.
specular transmission
The process by which incident flux passes through a surface or medium without
scattering.
task light
Light that is directed to a specific surface or area to provide illumination for visual tasks.
ultraviolet radiation (uv)
Any radiant energy within the wavelength range of 0.001 to 0.38 microns.
veiling reflection
Specular reflection superimposed upon diffuse reflection from an object that partially or
totally obscures the details to be seen by reducing the contrast. Controlled by
distributing the source over a larger area, relocating the source out of the reflected field
of view, changing the task surface specular reflectance or tilt, or relocating the observer.
visual acuity
A measure of the ability to distinguish fine details.
visual comfort probability (VCP)
The rating of a lighting system expressed as a percentage of the people who, when
viewing from a specified location and in a specified direction, will be expected to find it
comfortable in terms of discomfort glare.