lighting glossary

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 focusfrom 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 mirror-box 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 7.7-8.0 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.