Click on a letter below to view the corresponding section of the glossary:
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- accent lighting
- directional lighting to emphasize a particular object or
to draw attention to a part of the field of view. See directional lighting.
- adaptation
- the process by which the retina of the eye becomes
accustomed to more or less light than it was exposed to during an immediately preceding
period. It results in a change in the sensitivity of the eye to light.
- baffle
- a single opaque or translucent element to shield a source
from direct view at certain angles, or to absorb unwanted light.
- ballast
- a device used with an electric-discharge lamp to obtain
the necessary circuit conditions (voltage, current and wave form) for starting and
operating; all fluorescent and HID light sources require a ballast for proper operation.
Dimming ballasts are special ballasts which when used together with a dimmer will vary the
light output of a lamp.
- ballast factor, BF
- the measured ability of a particular ballast to produce
light from the lamp(s) it powers; ballast factor is derived by dividing the lumen output
of a particular lamp/ballast combination by the lumen output of the same lamp(s) on a reference
ballast.
- beam angle
- the angle between the two directions for which the
intensity (candlepower) is 50% of the maximum intensity as measured in a plane through the
nominal beam centerline (center beam candlepower).
- beam spread
- (in any plane) the angle between the two directions in the
plane in which the candlepower is equal to a stated percent (usually ten percent) of the
maximum candlepower in the beam.
- brightness
- see luminance.
- candela, cd
- the unit of measure indicating the luminous intensity
(candlepower) of a light source in a specific direction; any given light source will have
many different intensities, depending upon the direction considered.
- candlepower distribution
- a curve that represents the variation in luminous
intensity (expressed in candelas) in a plane through the light center of a lamp
or luminaire; each lamp or lamp/luminaire combination has a unique set of candlepower
distributions that indicate how light will be spread.
- candlepower distribution curve
- a curve, generally polar, representing the variation of
luminous intensity of a lamp or luminaire in a plane through the light center.
- center beam candlepower, CBCP
- the intensity of light produced at the center of a
reflector lamp, expressed in candelas.
- chromaticity
- the aspect of color that includes consideration of its
dominant wavelength and purity.
- coefficient of utilization, CU
- the ratio of the luminous flux (lumens) from a
luminaire received on the work-plane to the lumens emitted by the luminaire's lamps alone.
- color rendering index, CRI
- the measure of a light source's ability to render the
color of objects "correctly," as compared with a reference source with
comparable color temperature; the scale peaks at 100.
- correlated color temperature, CCT
- a specification of the color appearance of a lamp relating
its color to that of a reference source heated to a particular temperature, measured in
degrees Kelvin (K); CCT generally measures the "warmth" or "coolness"
of light source appearance.
- current, I
- a measure of the flow of electricity, expressed in amperes
(A).
- cut-off angle
- (of a luminaire) the angle from the vertical at which a
reflector, louver or other shielding device cuts off direct visibility of a light source.
It is the complementary angle of the shielding angle.
- diffuse lighting
- light that is not predominantly incident from any
particular direction.
- diffuser
- a device to redirect of scatter the light from a source by
the process of diffuse transmission.
- direct glare
- glare resulting from high luminance or insufficiently
shielded light sources in the field of view, or from reflecting areas of high luminance.
It is usually associated with bright areas such as luminaires, ceilings and windows that
are outside the visual task or region being viewed.
- direct lighting
- lighting by luminaires distributing 90 to 100 percent of
the emitted light in the general direction of the surface to be illuminated. The term
usually refers to light emitted in a downward direction. (See accent lighting.)
- directional lighting
- illumination on the work-plane or on an object
predominantly from a single direction.
- disability glare
- glare resulting in reduced visual performance and
visibility. It is often accompanied by discomfort.
- discomfort glare
- glare producing discomfort. It does not necessarily
interfere with visual performance or visibility.
- efficacy
- efficiency of a light source expressed in lumens per watt
(LPW or lm/W).
- energy
- a measure of work done by an electrical system over a
given period of time, often expressed in kilowatt-hours (kWh).
- enhancing reflections
- reflections which enhance appearance described in such
terms as sparkling, glittering, etc.
- ESCO
- Energy Saving service Company
-
- fenestration
- any opening or arrangement of openings or windows
(normally filled with media for light control) for the admission of daylight or for the
transmission of electric lighting from one room to another room.
- fixture
- see luminaire.
- fluorescent lamp
- a low pressure mercury electric discharge lamp, tubular in
shape in which a fluorescent coating (phosphor) transforms ultraviolet energy into visible
light.
- footcandle, fc
- a unit of illuminance equal to 1 lumen per square foot.
- frequency
- the number of times per second that an alternating current
system reverses from positive to negative and back to positive, expressed in cycles per
second or hertz, Hz.
- general lighting
- lighting designed to provide a substantially uniform
illuminance throughout an area, exclusive of any provision for special local requirements.
- glare
- excessive brightness that may be caused by either direct
or indirect viewing of a light source; any brightness or brightness relationship that
annoys, distracts or reduces visibility.
- harmonic
- an electrical frequency that is an integer multiple of the
fundamental frequency; for example, if 60 Hz is the fundamental freqency, then 120 Hz is
the second harmonic and 180 Hz is the third harmonic; some electronic devices, such as
ballasts or power supplies, can cause harmonic distortion, directly affecting
power quality.
- Hertz, Hz
- a unit of freqency equal to one cycle per second; see frequency.
- high intensity discharge (HID)
lamps
- a general group of lamps consisting of mercury, metal
halide and high pressure sodium lamps.
- illuminance
- light arriving at a surface, expressed in lumens per unit
area; 1 lumen per square foot equals 1 footcandle, while 1 lumen per square meter
equals 1 lux.
- incandescent filament lamp
- a lamp in which light is produced by a filament heated to
incandescence by an electric current.
- indirect lighting
- lighting by luminaires distributing 90 to 100 percent of
the emitted light upward.
- lamp
- manufactured light source; the 3 broad categores of
electric lamps are incandescent, fluorescent, and high intensity discharge (HID).
- lamp life
- an average rating, in hours, indicating when 50% of a
large group of lamps have failed, when operated at nominal lamp voltage and current;
manufacturers use 3 hours per start for fluorescent lamps and 10 hours per start for HID
lamps when performing lamp life testing procedures; every lamp type has a unique mortality
curve that depicts its average rated life.
- lens
- a glass or plastic element used in luminaires to change
the direction and control the distribution of light rays.
- lamp
- a generic term for a man-made source of light.
- lamp lumen depreciation factor,
LLD
- the multiplier to be used in illumination calculations to
relate the initial rated output of light sources to the anticipated minimum rated output
based on the relamping program to be used.
- level of illumination
- see illuminance.
- light
- radiant energy that is capable of producing a visual
sensation.
- light loss factor, LLF
- a factor used in calculating illuminance after a given
period of time and under given conditions. It takes into account temperature and voltage
variations, dirt accumulation on luminaire and room surfaces, lamp depreciation,
maintenance procedures and atmosphere conditions. Formerly called maintenance factor.
- light source
- see lamp.
- louver
- a series of baffles used to shield a source from view at
certain angles or to absorb unwanted light. The baffles are usually arranged in a
geometric pattern.
- lumen, lm
- a unit of luminous flux; the overall light output of a
luminous source is measured in lumens.
- lumen depreciation
- the decrease in lumen output of a light source over time;
every lamp type has a unique lumen depreciation curve (sometimes called lumen
maintenance curve) depicting the pattern of decreasing light output.
- lumen maintenance
- see lumen depreciation.
- luminaire
- a light fixture; the complete lighting unit, including
lamp, reflector, ballast, socket, wiring, diffuser, and housing.
- luminaire efficiency
- the ratio of luminous flux (lumens) emitted by a luminaire
to that emitted by the lamp or lamps used therein.
- luminance, L
- light reflected in a particular direction; the photometric
quantity most closely associated with brightness perception, measured in units of luminous
intensity (candelas) per unit area (square feet or square meters).
- luminance contrast
- the relationship between the luminances of an object and
its immediate background.
- luminance ratio
- the ratio between the luminances of any two areas in the
visual field.
- luminous flux
- the time rate of flow of light.
- lux, lx
- a unit of illuminance equal to 1 lumen per square meter.
- matte surface
- a non-glossy dull surface as opposed to a shiny (specular)
surface. Light reflected from a matte surface is diffuse.
- modeling
- the effect of using highly directional light to create
form through shadows and highlights.
- nanometer, nm
- a unit of length equal to 10-9 meters; commonly
used as a unit of wavelength.
- power
- the rate at which energy is taken from an electrical
system or dissipated by a load, expressed in watts (W); power that is generated
by a utility is typically expressed in volt-amperes (V-A).
- power factor
- a measure of the effectiveness with which an electrical
device converts volt-amperes to watts; devices with power factors >0.90 are "high
power factor" devices.
- quality of
lighting
- pertains to the distribution of luminance in a visual
environment. The term is used in a positive sense and implies that all luminances
contribute favorably to visual performance, visual comfort, ease of seeing, safety and
esthetics for the specific visual tasks involved.
- quantity of light
- the product of the luminous flux by the time it is
maintained. It is the time integral of luminous flux.
- reference ballast
- a ballast specially constructed to have certain prescribed
characteristics for use in testing electric-discharge lamps and other ballasts.
- reflectance, rho
- the percentage of light reflected back from a surface, the
difference having been absorbed or transmitted by the surface.
- reflected glare
- glare resulting form specular reflections of high
luminance in polished or glossy surfaces in the field of view.
- reflection
- the process by which flux leaves a surface or medium from
the incident side.
- reflector
- a device used to redirect the light by the process of
reflection.
- refraction
- the process by which the direction of a ray of light
changes as it passes obliquely from one medium to another.
- refractor
- a device used to redirect the luminous flux from a source,
primarily by the process of refraction.
- resistance, R
- a measure of resistance to flow of current, expressed in ohms.
- shielding
- a general term to include all devices used to block,
diffuse or redirect light rays, including baffles, louvers, shades, diffusers and lenses.
- shielding angle
- The complementary angle of the cut-off angle of a
luminaire.
- spectral power distribution, SPD
- a curve illustrating the distribution of power produced by
the lamp, at each wavelength across the spectrum.
- specular surface
- a shiny, highly polished surface which reflects light at
an angle equal to that of the incident light.
- supplementary lighting
- lighting used to provide an additional quantity and
quality of illumination that cannot be readily obtained by a general lighting system and
that supplements the general lighting level usually for specific task requirements.
- transmission
- the process by which incident flux leaves a surface or
medium on a side other than the incident side, the characteristics of many materials such
as glass, plastics and textiles.
- TCLP
- Toxicity
Characteristic Leaching Procedure -
EPA test used to identify hazardous waste.
- transmittance
- the ratio of the flux transmitted by a medium to the
incident flux.
- veiling
reflections
- reflections which partially or totally obscure the details
to be seen by reducing the contrast.
- visual comfort probability, VCP
- a discomfort glare calculation that predicts the percent
of observers positioned in the least favorable part of the room who would be expected to
judge a lighting condition to be comfortable. VCP rates the luminaire in its environment,
taking into account such factors as illuminance level, room dimensions and reflectances,
luminaire type, size and light distribution, number and location of luminaires, and
observer location and line of sight. The higher the VCP the more comfortable the lighting
environment.
- visual field
- the location of objects or points in a space where the
head and eyes are kept fixed.
- visual surround
- all portions of the visual field except the visual task.
- visual task
- those details and objects which must be seen for the
performance of a given activity, including the immediate background of the details or
objects.
- voltage, E
- a measure of electrical potential, expressed in volts
(V).
- watt, W
- a unit of electrical power equal to 1 joule per second.
- work plane
- the plane at which work usually is done, and on which the
illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a
horizontal plane 0.76 meters (30 inches) above the floor.
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Excerpted from the IES Lighting Handbook,
Illuminating Engineering Society of North America.
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