Directional lighting to emphasize a particular object or to
draw attention to a part of the field of view. See directional
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.
A measure of electrical current. In incandescent lamps, the
current is relative to voltage and power as follows: Current
(Amps) = Power (Watts) / Voltage (Volts).
A single opaque or translucent element to shield a source
from direct view at certain angles, or to absorb unwanted
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.
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.
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
The angle (in any plane) 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.
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.
A term used for the luminous intensity of a light source.
The intensity in any one direction from the standard candle
is called one candela (formerly one candlepower; cp).
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.
A curve, generally polar, representing the variation of luminous
intensity of a lamp or luminaire in a plane through the light
beam candlepower, CBCP
The intensity of light produced at the center of a reflector
lamp, expressed in candelas.
The aspect of color that includes consideration of its dominant
wavelength and purity.
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
rendering index (CRI)
A measurement of the color shift an object undergoes when
illuminated by the light source, as compared to a reference
source at the same color temperature. Color rendering is measured
on an index from 0-100, with natural daylight and incandescent
lighting both equal to 100. Objects and people viewed under
lamps with a high color rendering index (CRI) appear more
true to life.
The chromaticity of an ideal "black body" when it
is heated to a specific temperature in Kelvin (K). It is the
measure of the color of light, not actual temperature.
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.
A measure of the flow of electricity, expressed in amperes
The angle (of a luminaire) 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
Light that is not predominantly incident from any particular
A device to redirect of scatter the light from a source by
the process of diffuse transmission.
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.
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
Illumination on the work-plane or on an object predominantly
from a single direction.
Glare resulting in reduced visual performance and visibility.
It is often accompanied by discomfort.
Glare producing discomfort. It does not necessarily interfere
with visual performance or visibility.
Efficiency of a light source expressed in lumens per watt
(LPW or lm/W).
A measure of work done by an electrical system over a given
period of time, often expressed in kilowatt-hours (kWh).
Reflections which enhance appearance described in such terms
as sparkling, glittering, etc.
Energy Saving service Company
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.
A low pressure mercury electric discharge lamp, tubular in
shape in which a fluorescent coating (phosphor) transforms
ultraviolet energy into visible light.
A unit used in measuring direct illumination. It is defined
as the illumination produced from a source of one candela
at a point on a surface of one foot away and perpendicular
to the source of light. A lumen per square foot.
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.
Lighting designed to provide a substantially uniform illuminance
throughout an area, exclusive of any provision for special
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.
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.
A unit of freqency equal to one cycle per second; see frequency.
intensity discharge (HID) lamps
A general group of lamps consisting of mercury, metal halide
and high pressure sodium lamps.
The lamp does not have to cool down before being turned back
The result of the use of light, illuminance, the density ofluminous
flux on a surface, is usually measured in footcandles.
A lamp in which light is produced by a filament heated to
incandescence by an electric current.
Lighting by luminaires distributing 90 to 100 percent of the
emitted light upward.
A circuit used to start specially designed fluorescent lamps
without the aid of a starter. The circuit utilizes higher
open circuit voltage than is required for the same length
preheat lamps, to strike the arc instantly.
This circuit is used today in slimline and cold cathode lamps.
Instant start 40-watt bipin lamps are made with a short-circuiting
device built into the base.
Ionization smoke detectors use an ionization chamber and a
source of ionizing radiation to detect smoke. Ionization smoke
detector is more common because it is inexpensive and better
at detecting the smaller amounts of smoke produced by flaming
Ionization smoke detectors feature a harmless radioactive
source within a dual detection chamber. Ionizaiton smoke detectors
repsond to invisible by-products of combustion. They operate
by sensing for a change in the electrical conductivitiy across
the detection chamber. The advantage of the ionization detector
is that the smoke can be invisible to the human eye, while
remaining very much visible to the ionization detector.
Ionization smoke detectors respond first to fast flaming fires.
A flaming fire devours combustibles extremely fast, spreads
rapidly, and generates considerable heat with little smoke.
A larger unit of power, a thousand watts (watts x 1000 kilowatts).
The measure of electrical usage from which electricity billing
is determined. For example, a 100-watt bulb operated for 1000
hours would consume 100 kilowatt hours (100 watt x 1000 hours
= 100 kWh). At a billing rate of $0.10/kWH, this bulb would
cost $10.00 (100 kWh x $0.10/kWh) to operate.
Manufactured light source; the 3 broad categores of electric
lamps are incandescent, fluorescent, and high intensity discharge
(HID). Also a generic term for a man-made source of light.
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.
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
(Leadership in Energy and Environmental Design)
LEED is a third party certification program and the nationally
accepted benchmark for the design, construction and operation
of high performance green buildings. LEED promotes a whole-building
approach to sustainability by recognizing performance in five
key areas of human and environmental health: sustainable site
development, water savings, energy efficiency, materials selection
and indoor environmental quality.
A glass or plastic element used in luminaires to change the
direction and control the distribution of light rays.
The term generally applied to the visible energy from a source.
Light is usually measured in lumens or candlepower. When light
strikes a surface it is either absorbed, reflected or transmitted.
Light is said to travel in straight lines.
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.
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.
The unit that expresses the total quantity of light given
off by a source, regardless of direction. A lumen is defined
as the amount of light falling on a surface of one square
foot, every point of which is one foot away from a source
of one candlepower. A uniform source of one candlepower placed
in a sphere emits 12.57 lumens, or mean spherical candela
equals 12.57 lumens.
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
See lumen depreciation.
per watt (LPW)
A measure of the efficacy of a light source in terms of the
light produced for the power consumed. For example, a 100-watt
lamp producing 1750 lumens gives 17.5 lumens per watt.
A light fixture; the complete lighting unit, including lamp,
reflector, ballast, socket, wiring, diffuser, and housing.
The ratio of luminous flux (lumens) emitted by a luminaire
to that emitted by the lamp or lamps used therein.
(L) or brightness
The light emitted, transmitted or reflected from a unit area
of the source of surface is its brightness. It is usually
expressed in candles per square inch or Lamberts or foot Lamberts.
The relationship between the luminances of an object and its
The ratio between the luminances of any two areas in the visual
The time rate of flow of light.
A unit of illuminance equal to 1 lumen per square meter.
A non-glossy dull surface as opposed to a shiny (specular)
surface. Light reflected from a matte surface is diffuse.
The effect of using highly directional light to create form
through shadows and highlights.
A unit of length equal to 10-9 meters; commonly used as a
unit of wavelength.
The lamp must cool down before being turned back on.
Photoelectric smoke detectors look for the presence of visible
by-products of combustion in the detection chamber. When a
sufficient density of visible combustibles fill the detection
chamber, the photoelectric smoke detector sounds an alarm
Photoelectric smoke detectors respond first to slow smoldering
fires. A smoldering fire generates large amounts of thick,
black smoke with little heat and may smolder for hours before
bursting into flames.
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
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.
A circuit used in fluorescent lamps where the electrodes are
heated or warmed to a glow stage, by an auxiliary switch or
starter (can be a glow switch, thermal type or mechanical
device like a push button) before the lamps are lighted. This
system was used on the original fluorescent lamps and is still
in use today.
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.
The product of the luminous flux by the time it is maintained.
It is the time integral of luminous flux.
A circuit designed to start lamps by continuously heating
or preheating the electrodes. This circuit is a modern version
of the trigger start system and requires lamps designed for
this circuit. In the rapid start two-lamp circuit, one end
of each lamp is connected to a separate starting winding.
The other end of each lamp is connected to a common winding.
Except for slimline lamps, all modern fixtures using 40-watt
and higher lamps are equipped with Rapid Start ballasts.
The operating life (hours) at which 50% of the lamps are still
operating. Where a plus (+) is used in starting the life,
survival rate is 67% at the started time.
A ballast specially constructed to have certain prescribed
characteristics for use in testing electric-discharge lamps
and other ballasts.
The percentage of light reflected back from a surface, the
difference having been absorbed or transmitted by the surface.
Glare resulting form specular reflections of high luminance
in polished or glossy surfaces in the field of view.
The process by which flux leaves a surface or medium from
the incident side.
A device used to redirect the light by the process of reflection.
The process by which the direction of a ray of light changes
as it passes obliquely from one medium to another.
A device used to redirect the luminous flux from a source,
primarily by the process of refraction.
A measure of resistance to flow of current, expressed in ohms.
A general term to include all devices used to block, diffuse
or redirect light rays, including baffles, louvers, shades,
diffusers and lenses.
The complementary angle of the cut-off angle of a luminaire.
Smoke detectors consist of two basic parts: a sensor to sense
the smoke and a very loud electronic horn to wake people up.
Smoke detectors can run off of a 9-volt battery or 120-volt
house current. The two most common types of smoke detectors
used today are photoelectric smoke detectors
and ionization smoke detectors.
power distribution, SPD
A curve illustrating the distribution of power produced by
the lamp, at each wavelength across the spectrum.
A shiny, highly polished surface which reflects light at an
angle equal to that of the incident light.
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.
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.
The ratio of the flux transmitted by a medium to the incident
A circuit used to eliminate the starter and start the preheat
lamp almost instantly. In this circuit, each electrode is
connected to a separate winding in the ballasts so that the
electrode is continuously heated. This circuit is primarily
used on 20-watt and lower wattage fluorescent lamps today.
Reflections which partially or totally obscure the details
to be seen by reducing the contrast.
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
The location of objects or points in a space where the head
and eyes are kept fixed.
All portions of the visual field except the 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.
(V or E)
A measurement of electromotive force or the pressure of electricity.
This is analogous to the pressure in a water line, i.e., pounds
per square inch. The voltage of a circuit is the electrical
pressure it gives. In an incandescent lamp "voltage"
designates the supply voltage to which the lamps should be
connected. In other lamp types, it may refer to "operating
voltage" of a lighted arc discharge lamp.
Unit used to measure power consumption of a lamp. A unit of
electrical power equal to 1 joule per second.
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.