Directional lighting to emphasize a particular object or to draw attention to a part of the field of view. See directional lighting.
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.
ANSI American National Standards Institute
The organization that develops voluntary guidelines and produces
standards for the electrical and other industries.
A completely sealed quartz or ceramic tube where the electrical
discharge (arc) occurs and light is generated.
A single opaque or translucent element to shield a source from direct
view at certain angles, or to absorb unwanted light.
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.
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)
(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.
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.
Candle Power 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.
CCT rating is based on operation at rated watts in the designated operating
position. For universal lamps, CCT is rated in the vertical position.
All other operating positions will lead to higher values of CCT.
Center 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.
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.
Light that is not predominantly incident from any particular direction.
A device to redirect of scatter the light from a source by the process of
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 lighting.)
Illumination on the work-plane or on an object predominantly from a
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.
Double – Ended
Lamps that have two bases opposite one another for series electrical
connection, mounting connection and heat dissipation.
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).
U.S. Department of Energy (DOE) designation for products meeting
certain energy efficiency and performance standards.
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 tungsten wire purposely positioned inside a lamp bulb that when
heated, electrically generates radiation in the visible, infrared and
A reflector lamp with a relatively wide beam pattern.
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.
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 local
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 frequency, 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 frequency 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.
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.
Lighting by luminaires distributing 90 to 100 percent of the emitted
The light output of an HID lamp at rated power on a reference ballast
after 100 hours of operation.
Manufactured light source; the 3 broad categories of electric lamps are
incandescent, fluorescent, and high intensity discharge (HID)
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
A glass or plastic element used in luminaires to change the direction and
control the distribution of light rays.
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
Radiant energy that is capable of producing a visual sensation.
Light Center Length (LCL)
The distance from the center of the visible arc discharge to the bottom
of the contact of the base.
Light Emitting Diode (LED)
Semiconductor diodes, electronic devices that permit current flow in
only one direction.The diode is formed by bringing two slightly different
materials together, forming a junction.When a forward voltage is
applied, current flow begins and energy is radiated in the form of light.
800.324.1496 + www.tcpi.com Glossary
n, Pronunciation: "i-n-vâ-shn"
a new idea, method, or device
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 on Centers - LOC
Distance between LED lights,TCP’s Holiday lighting products.
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
Lumen - Lm
A unit of luminous flux; the overall light output of a luminous source is
measured in 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 light output.
See lumen depreciation.
Lumens Per Watt - LPW
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.
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).
The relationship between the luminances of an object and its immediate
The ratio between the luminances of any two areas in the visual field.
The time rate of flow of light.
Lux - Lx
A unit of illuminance equal to 1 lumen per square meter.
Maximum Overall Length - MOL
The maximum allowable distance from the top of the glass bulb to the
end of the contact of the base.
A non-glossy dull surface as opposed to a shiny (specular) surface.
Light reflected from a matte surface is diffuse.
Lumen output of a light source after the source has been used. Mean
lumen values for fluorescent and HID lamps are typically measured at
40% of rated life.
Milli-Candela - MCD
The measurement of light output of an LED in thousandths of a candela.
Nanometer - NM
A unit of length equal to 10-9 meters; commonly used as a unit of
Open Rated Lamp - Mogul Base
Designed with an extended contact pin on the bottom of the base. Lamps
should be used with an extended contact pin on the bottom of the base.
Lamps should be used with an open fixture mogul socket (EX39) which
prevents electrical contact if a non open rated lamp is used.
Metal halide lamps are specified and designed to be operated in certain
positions such as a vertical or base-up.
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)
A measure of the effectiveness with which an electrical device converts
volt-amperes to watts; devices with power factors >0.90 are “high power
Pulse Start Lamp
Specially designed metal halide lamp that requires high voltage pulse for
starting. Has improved lumen maintenance.
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.
The number of operating hours at which 50% if most metal halide
lamps or 70% of some pulse start lamps initially started will still
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.
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
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.
Resistance - R
A measure of resistance to flow of current, expressed in ohms.
The time to re-ignite the arc of an HID lamp.
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.
Spectral 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.
A reflector lamp with a narrow beam pattern used to illuminate a specifically
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 flux.
Refers to TCP’s metal halide lamps with a protective quartz sleeve
around the arc tube. Suitable for use in open luminaires and reduces UV
by up to 90%.
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.
The location of objects or points in a space where the head and eyes are
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
Voltage - V
A measure of electrical potential, expressed in volts (V).
The amount of time from ignition of the lamp to 90% of full light output
of an HID lamp.
Watt - W
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.