Glossary of lighting design terms

A  B Ballast C  D  E  F  G  Glare  H  I  K  L Lamps M N O  P  R  S  T  U  V Vision W Z

 

A

Absorption
The light taken up within a surface or medium, affecting color and intensity of the light.

Accent Lighting
Directional lighting to emphasize a particular object or draw attention to a display item, landscape, or architectural feature.

Accent Luminaire
A type of luminaire that provide directional lighting to accent an object or an area within a space.

Accommodation
The process by which the eye changes focus from one distance to another.

Adaptation 
The process by which the human eye adjusts to a change in light level, when the visual system becomes accustomed to more or less light than it was exposed to during an immediately preceding period. Adaptation time increases with age.

Adjustable Head
An adjustable luminaire that allows for aiming at different angles to provide directional lighting.

Alternating Current (AC)
Flow of electricity which cycles or alternates direction many times per second. The number of cycles per second is referred to as frequency. Common residential frequency used in the United States is 60 Hertz (cycles per second).

Ambient Lighting
The general lighting present in an area - excluding task lighting and accent lighting but including general lighting and daylight streaming in. In theatre and photography it is the background or fill light in a space.

Ambient Temperature 
The surrounding temperature within an environment. Extremes can affect lamp performance; for example, fluorescent lamps do not like cold and LED’s do not like heat.

Amperes  (AMP or A)
The standard unit of measurement for electric current that is equal to one coulomb per second. It defines the quantity of electrons moving past a given point in a circuit during a specific period. In incandescent lamps, the current is related to voltage and power as follows: Watts (power) = Volts x Amps (current).  Amps = Watts Divided by Volts

Annual Energy Savings
The difference per year in kwh between the energy used comparing different lighting design solutions.

Annual Energy Use
The energy used per year in kwh by a lighting system.

Annual Lamp Replacement Costs
The cost per year of replacement lamps, excluding labor.

Annual Operating Cost Savings
The difference between the annual operating cost of different lighting solutions.

Annual Operating Cost
The cost per year of electricity and replacement lamps.

Anode 
The ‘positive’ terminal of a diode.

ANSI
American National Standards Institute, A consensus-based organization which coordinates voluntary standards for the physical, electrical and performance characteristics of lamps, ballasts, luminaires and other lighting and electrical equipment.

ANSI ballast type 
Ballast type used to operate lamp in accordance with ANSI standard.

ANSI codes 
These are 3-letter codes assigned by the American National Standards Institute. They provide a system of assuring mechanical and electrical interchangeability among similarly coded lamps from various manufacturers.

Aperture
An opening, usually in a recessed luminaire, through which light enters a space.

Application 
Refers to the particular use for a lamp or luminaire. 

Arc 
A general term for a high intensity electrical discharge occurring between two electrodes in a gaseous medium, usually accompanied by the generation of heat and the emission of light, HID lamps are arc lamps.

Arc Lamp 
A light source containing an arc. Also called a discharge lamp, or an arc discharge lamp.

Arc Length 
In High Intensity Discharge lamps: the distance between the electrode tips, which represents the physical length of the electrical discharge.

Arc Tube
A tube enclosed by the outer glass envelope of a HID lamp and made of clear quartz or ceramic that contains the arc stream.

ASHRAE
American Society of Heating, Refrigerating and Air-Conditioning Engineers. An International technical society organized to advance the arts and sciences of heating, ventilation, air-conditioning and refrigeration.  ASHRAE publishes a well recognized series of standards and guidelines relating to HVAC systems and issues.

Atmosphere 
Designates the type of gas or vacuum filling a volume or chamber of the lamp. This chamber might contain a filament or it might refer to the bulb which contains the arc tube.

 

B

Background Lighting
Illumination of the background to provide much needed separation between subject and background.

Backlighting
A light  source placed behind an actor, object, or scene to create a highlight that separates the subject from the background.

Baffle
An element that shields a light source from direct view at certain angles or that absorbs unwanted light. Can be rings, honeycomb, continuous, opaque, or translucent

Ballast
A current controlling device that is used with a fluorescent or high-intensity discharge lamp to provide the necessary circuit conditions (voltage, current, and wave form) for starting and operating the lamp. LED ballast is called a ‘driver.’

Ballast Class ‘A-F’ 
Sound ratings for ballasts measure the quietest  ‘A’ =20-24 decibels for residential and offices to ‘D’ = 37- 42 used in industrial areas, ‘F’ is 49 and higher.  Electronic ballasts are generally quieter than magnetic ballasts; however, mounting methods and loose parts in the fixture can contribute to hum. (see ballast noise ‘Hum’) 

Ballast Class ‘P’
Contains a thermal protective device which deactivates the ballast when the case reaches a certain critical temperature. The device resets automatically when the case temperature drops to a lower temperature.

Ballast  - Compact Fluorescent
The ballast in a two-piece compact fluorescent lamp product. It can have a medium screwbase with a socket, a GU-24 pin based socket. The ballast and lamp connect together using a socket-and-base design that ensures compatibility of lamps and ballasts. 2 or 4 pin fluorescent sockets require separate compact fluorescent ballast.

Ballast  - Constant Wattage (CW)
A premium type of HID ballast in which the primary and secondary coils are isolated. It is considered a high performance, high loss ballast featuring excellent output regulation.

Ballast  - Constant Wattage Autotransformer (CWA)
 A popular type of HID ballast in which the primary and secondary coils are electrically connected. Considered an appropriate balance between cost and performance.

Ballast Cycling
Undesirable condition under which the ballast turns lamps on and off (cycles) due to the overheating of the thermal switch inside the ballast. This may be due to incorrect lamps, improper voltage being supplied, high ambient temperature around the fixture, or the early stage of ballast failure.

Ballast Factor (BF)
Defined as the light output (in lumens) from a lamp operated by commercial ballast, as compared to laboratory standard reference ballast specified by ANSI. The ballast efficiency factor (BEF) is the ballast factor divided by the input power of the ballast. The higher the BEF (within the same lamp-ballast type) the more efficient the ballast.  Rated Lamp Lumens x Ballast Factor = Net Lumens. For example, a ballast with a ballast factor of 0.93 will result in the lamp's emitting 93% of its rated lumen output.  With fluorescent lamps, an electronic ballast may produce more light than the reference test ballast and have a ballast factor greater than one. A ballast with a lower BF results in less light output and also generally consumes less power.

Ballast - Electromagnetic or Magnetic
A magnetic core and coil to provide the voltage and current that are needed to start the lamp(s) and to maintain its operation. Also called ‘core and coil.’  Fluorescent lamps with magnetic ballasts flicker at a normally unnoticeable frequency of 100 or 120 hertz and this flickering can cause problems for some individuals with light sensitivity.

Ballast - Electromagnetic Interference (EMI) 
High frequency electronic ballasts and other electronic devices can produce a small amount of radio waves which can interfere with radio and TV. Federal mandated requirements must be met for EMI levels before an electronic device is considered FCC compliant. (FCC is the Federal Communications Commission.

Ballast - Electronic
A ballast used with discharge lamps that uses solid state electronic circuitry to provide the voltage and current that are needed to start the lamp(s) and to maintain its operation.  Fluorescent system efficiency is increased due to high frequency lamp operation.

Ballast  - Electronic Dimming
A variable output electronic fluorescent ballast.

Ballast - Energy Saving
A type of magnetic ballast designed so that the components operate more efficiently, cooler and longer than a ‘standard magnetic’ ballast. By US law, standard magnetic ballasts can no longer be manufactured.

Ballast  - High Power Factor (HPF)
A ballast with a 0.9 or higher rated power factor, which is achieved by using a capacitor.

Ballast  - Instant Start & Rapid Start
A fluorescent ballast with a circuit that ignites the lamp instantly with a very high starting voltage from the ballast, across the lamp with no preheating of the cathode. Instant start lamps have single-pin bases.

Ballast  - Low Power Factor (LPF)
Ballast with a power factor of 0.79 or less - also called normal power factor (NPF) ballast. LPF ballast requires about twice the line current of HPF ballast so fewer LPF ballasts can be installed on a circuit, which increases installation cost. See Power Factor.
Ballast  - Noise ‘Hum’
Sound made by operating Core & Coil assemblies in both electromagnetic and some electronic ballasts, generated by the vibration of laminations in the electromagnetic field that transforms the voltage and current used by discharge lamps. The sound made by high frequency electronic ballasts is lower than electromagnetic ballasts with some designs virtually inaudible. (See Ballast class ‘A-F’)

Ballast  - Normal Power Factor (NPF)
A ballast/lamp combination in which no components (e.g., capacitors) have been added to correct the power factor, making it normal (essentially low, typically 0.5 or 50%).

Ballast  - Preheat
A type of ballast/lamp circuit that uses a separate starter to heat up a fluorescent lamp before high voltage is applied to start the lamp

Ballast  - Programmed Rapid Start (PS)
A method of starting fluorescent lamps, associated with electronic ballasts, where low voltage is applied to the cathode prior to lamp ignition.

Ballast - Pulse Start
An HID ballast with a high voltage ignitor to start the lamp.

Ballast - PSMH
Pulse Start Metal Halide HID Lighting.

Ballast - Rapid Start
A fluorescent lamp ballast that utilizes continuous cathode heating, while the system is energized, to start and maintain lamp light output at efficient levels. Rapid start ballasts may be either electromagnetic, electronic or hybrid designs. Full-range fluorescent lamp dimming is only possible with rapid start systems. Rapid start lamps have bi-pin base.

Ballast - Regulation
The ability of a ballast to hold constant (or nearly constant) the output watts (light output) during fluctuations in the voltage feeding of the ballast. Normally specified as +/- percent change in output compared to +/- percent change in input.

Ballast - Starter or Ignitor
An electronic module or device used to assist in starting a discharge lamp, paired with or as part of the ballast, typically by providing a high-voltage surge to strike the arc.

Ballast - Terminal to Terminal Starting Lamp Voltage (VRMS) (Minimum or Maximum) 
The minimum or maximum allowed voltage allowed into lamp from ballast under varying conditions as specified.

Ballast - Thermal Factor
A factor used in lighting calculations that compensates for the change in light output of a fluorescent lamp due to a change in bulb wall temperature. It is applied when the lamp-ballast combination under consideration is different from that used in the photometric tests.

Ballast - Total Harmonic Distortion (THD) 
A measure of the distortion of an electrical wave form caused by ballasts and other inductive loads of the input current on alternating current (AC) power systems caused by higher order harmonics of the fundamental frequency (60Hz in North America). THD is expressed in percent and may refer to individual electrical loads (such as ballast) or a total electrical circuit or system in a building. ANSI C82.77 recommends THD not exceed 32% for individual commercial electronic ballasts, although some electrical utilities may require lower THDs on some systems. Excessive THDs on electrical systems can cause efficiency losses as well as overheating and deterioration of system components.

Ballast - Trigger Start
Type of ballast commonly used with 15-watt and 20-watt straight fluorescent lamps.

Ballast - Base or Socket 
The socket is the receptacle connecting the lamp to the electrical supply.  Types of bases include screw bases common for incandescent and HID lamps, and pin bases for linear fluorescent  and halogen lamps. LED arrays have wire connectors instead of sockets, although retrofit LED lamps are available with screw and pin sockets matching the lamps they are designed to replace.

Base Temperature (maximum) 
The maximum operating temperature permitted for the base in Celsius. Fixture manufacturers need to ensure that these conditions are satisfied in their fixture.

Batwing Distribution
Candlepower and distribution which serves to reduce glare and veiling reflections by having its maximum output in the 30° to 60° zone. (see direct/indirect)

Bayonet 
A style of bulb base which uses keyways instead of threads to connect the bulb to the fixture base. The bulb is locked in place by pushing it down and turning it clockwise.

Beam Angle or Beam Spread
The width of a light beam, expressed in degrees. The beam of light from a reflector-type lamp (PAR, R, ER, or MR) can be thought of as a cone. The beam spread is the angular width of the cone encompassing the central part of the beam out to the angle where the intensity is 50% of maximum. The beam angle sometimes called ‘beam spread’ is often part of the ordering code for the lamps. Example: The 50PAR30/HIR/NFL25 is a 50 watt PAR30 narrow flood lamp with a beam angle of 25 degrees. (See Lamp)

Bi-pin Base
A base with two pins that is used for some tungsten-halogen reflector lamps, low-voltage tungsten-halogen lamps, and fluorescent lamps.

Blackbody 
Used in calibrating color temperature.  An object that absorbs all radiation falling on it, at all wavelengths, is called a black body. When a black body is at a uniform temperature, its emission has a characteristic frequency distribution that depends on the temperature. Black-body radiation becomes a visible glow of light if the temperature of the object is high enough.
Black Light 
Also referred to as a UV light, ultraviolet light emitting mostly near UV (320 to 400 nm) and very little visible light.  Black lights are essential when UV light without visible light is needed, particularly in observing fluorescence, the colored glow that many substances emit when exposed to UV.  Black lights are employed for decorative and artistic lighting effects, for diagnostic and therapeutic uses in medicine, for the detection of substances tagged with fluorescent dyes, rock-hunting, for the curing of plastic resins and for attracting insects. Strong sources of long-wave ultraviolet light are used in tanning beds. Black light lamps are also used for the detection of counterfeit money. Most artificial ultraviolet sources are low power. Powerful ultraviolet sources present a hazard to eyes and skin; apparatus using these sources requires personal protective equipment.

Blacktop 
Whether or not the top of the miniature lamp has a blacktop coating. The coating is used to control unwanted brightness or glare.

Bollard
A short, thick post with a light at its top, primarily used outdoors for walkway and driveway entrance lighting. Can also function as a security barricade.

BUG System
Backlight, Uplight and Glare classification of outdoor lighting fixtures illustrated in the Model Lighting Ordinance (MLO) Released June 14, 201, developed byThe International Dark-Sky Association (IDA) and the Illuminating Engineering Society of North America (IESNA).

Bulb 
The outer envelope of a light source, usually quartz glass or other varieties of glass.
The outer glass bulb containing the light source. Layman’s term for ‘lamp’ or ‘light source.’

Bulb Material or Coating 
The type of glass (or quartz) used in the glass envelope surrounding the light source. The material can also have coatings applied to achieve particular performances.

Brightness
Subjective impression of light reaching the eye. This ambiguous term brightness does not correlate exactly with luminance, which is measured with an instrument.

 

C

Canadian Standards Association (CSA) 
An organization that writes standards and tests lighting equipment for performance as well as electrical and fire safety. Canadian provincial laws generally require that all products sold for consumer use in Canada must have CSA or equivalent approval.

Candela (cd) 
The measure of luminous intensity of a source in a given direction. The term has been retained from the early days of lighting when a standard candle of a fixed size and composition was defined as producing one candela in every direction. A plot of intensity versus direction is called a candela distribution curve and is often provided for reflectorized lamps and for luminaires with a lamp operating in them.

Candela Distribution
A curve, often on polar coordinates, illustrating the variation of luminous intensity of a lamp or luminaire in a plane through the light center.

Candle Lamp
A decorative incandescent lamp with a bulb shaped like a flame. The lamp designation is usually ‘IF’ or ‘C.’

Candlepower 
A measure of luminous intensity of a light source in a specific direction, measured in candelas.

Candlepower Distribution Curve 
A graphical presentation of the distribution of light intensity of a light source, shown as a curve, generally polar, in a plane through the light center, of a reflector lamp or luminaire.

Candlepower  Mean Spherical)
Initial mean spherical candlepower at the design voltage. Mean spherical candlepower is the generally accepted method of rating the total light output of miniature lamps. To convert this rating to lumens, multiply it by 12.57 (4 pi).

Cans
Common name for square or round recessed downlight luminaires. These are also called ‘high-hats.’ Also, a surface-mounted luminaire, usually a downlight, that has a cylindrical shape.

Cathode 
The ‘negative’ terminal of a diode.

Cathode Resistance 
Resistance of the cathode in a fluorescent lamp. It is measured ‘cold’ before the lamp is turned on (Rc) or ‘hot’ after the lamp is turned on (Rh). The ratio of the hot resistance to the cold resistance is also measured (Rh/Rc).

Cavity Ratio
A number indicating cavity proportions calculated from length, width and height used in ‘zonal cavity method’ to calculate the light level in a room.

CBM
Abbreviation for Certified Ballast Manufacturers Association.

CEC
Abbreviation for California Energy Commission.

Ceiling Mounted Luminaire
A luminaire mounted directly on the ceiling or other surface.

Center Beam Candlepower (CBCP)
The luminous intensity at the center of the beam of a blown or pressed reflector lamp (such a s a PAR lamp). Measured in candelas.

Central Controls
Lighting controls systems that control many luminaires from one or two locations.

Chandelier
A decorative, often branched, luminaire suspended from the ceiling.

Chip 
A very small square of semi-conducting material. Also known as a ‘die,’ it is the ‘active’ light-emitting component of an LED.

Chromaticity 
Measure to identify the color of a light source, or a secondary source like an illuminated object , typically expressed as (x,y) coordinates on a chromaticity chart.

Circadian rhythm

The circadian rhythm is an internal 24-hour clock that plays a critical role in when we fall asleep and when we wake up. When it is dark, your body produces more melatonin; when it is light, the production of melatonin drops. Being exposed to bright lights in the evening or too little light during the day can disrupt the body' s normal melatonin cycles. For example, jet lag and shift work can disrupt melatonin cycles. The retina of the eye contains classical photoreceptors (rods and cones), which are used for conventional vision. But the retina also contains specialized ganglion cells which are directly photosensitive, and project directly to the suprachiasmatic nucleus (SCN) where they help in the entrainment of this master circadian clock.

Circline Lamp
A fluorescent lamp bent in a circle so that the ends meet at the socket.

Coefficient of Utilization (CU)
The ratio of lumens from a luminaire received on the work plane to the lumens produced by the lamps alone.

Cold Cathode Lamp
An electric-discharge lamp whose mode of operation is that of a flow discharge.

Color
The part of the electromagnetic spectrum visible to humans is characterized by its wavelength (or frequency) and its intensity.  Wavelengths approximately from 390 nm to 750 nm are known as ‘visible light’.  We see ‘color’ when light strikes a surface and a particular color is reflected and all other colors are absorbed. The color we see is affected by the wavelengths emitted by a particular lamp, this is why ‘balanced’ light sources are desirable.  Color bias can affect the way we see colors.

Color Bin 
LEDs are sorted according to their wavelength or CIE coordinates into different groupings or ‘bins.’ (See Color Quality Scale - CQS)
Color Quality Scale (CQS)
A Color Quality Scale (CQS) is being developed at The National Institute of Standards and Technology (NIST) an agency of the U.S. Department of Commerce, with input from the lighting industry and the CIE (International Commission on Illumination) to address the problems of the CIE Color Rendering Index (CRI) for solid-state light sources and to meet the new needs of the lighting industry and consumers for communicating color quality of lighting products. The method for calculating the CQS is based on modifications to the method used for the CRI.

Color Rendering Index (CRI)
A scale of the effect of a light source on the color appearance of an object compared to its color appearance under a reference light source. Expressed on a scale of 1 to 100, where 100 indicates no color shift. A low CRI rating suggests that the colors of objects will appear unnatural under that particular light source. CRI differences among lamps are not usually significant (visible to the eye) unless the difference is more than 3-5 points.

Color Temperature
The color temperature is a specification of the color appearance of a light source, relating the color to a reference source heated to a particular temperature, measured by the thermal unit Kelvin (K). Yellowish-white ("warm") sources, like incandescent lamps, have lower color temperatures in the 2700K-3000K range; white and bluish-white ("cool") sources, such as cool white (4100K) and natural daylight (6000K), have higher color temperatures. The higher the color temperature the whiter, or bluer, the light will be.

Colorimetry
The measurement of color.

Compact Fluorescent Lamp (CFL) 
The general term applied to fluorescent lamps that are single-ended and that have smaller diameter tubes that are bent to form a compact shape. Some CFLs have integral ballasts and medium or candelabra screw bases for easy replacement of incandescent lamps. Also called PL, Twin-Tube, CFL, or BIAX lamps.

Cone Reflector
Parabolic reflector that directs light downward thereby eliminating brightness at high angles.

Contrast
The relationship between the luminance of an object and its background.

Control
A mechanism to turn lamps on and off, or dim lamps. Controls include switches, dimmers, timing devices, motion detectors, photo sensors, and central control systems.

Coolbeam 
(See DICHROIC REFLECTOR.)

Cool White 
A term loosely used to denote a color temperature of around 4100K. The Cool White (CW) designation is used specifically for T12 and other fluorescent lamps using halophosphors and having a CRI of 62.

Cornice or Soffit Luminaire
An architectural luminaire that directs light up or downward from the cornice or soffit between the wall and ceiling or floor.

Correlated Color Temperature (CCT):
The color temperature is a specification of the color appearance of a light source, relating the color to a reference source heated to a particular temperature, measured by the thermal unit Kelvin (K). Yellowish-white ("warm") sources, like incandescent lamps, have lower color temperatures in the 2700K-3000K range; white and bluish-white ("cool") sources, such as cool white (4100K) and natural daylight (6000K), have higher color temperatures. The higher the color temperature the whiter, or bluer, the light will be.

Cost of Light 
Usually refers to the cost of operating and maintaining a lighting system on an ongoing basis. The 88-8-4 rule states that (typically) 88% is the cost of electricity, 8% is labor and only 4% is the cost of lamps.

  • Cost of Energy =  Watts Used X Energy Rate / Divided by 1000  Hours of Operation
  • Energy Savings =  Watts Saved X Energy Rate / Divided by 1000  Hours of Operation

Cove Lighting
An architectural luminaire that directs light from sources that are mounted in a cove to the ceiling or upper wall. A cove is a ledge or shelf on the wall, or a recess in the wall.

Crest Factor (Max Current) 
The ratio of the peak lamp current to average lamp operating current (RMS). The lower the current crest factor is, the gentler the ballast is on the lamp.

Current Type (AC/DC) 
Whether the operational voltage is based on Alternating Current or Direct Current

Current
A flow of electric charge, measured in amperes or amps.

Cut Off Angle
The angle from a fixture's vertical axis at which a reflector, louver, or other shielding device cuts off direct visibility of a lamp. It is the complementary angle of the shielding angle.

 

D

Daylight
Light produced by solar radiation. Daylight includes direct sunlight, sunlight scattered by the atmosphere, and sunlight reflected from clouds or other surfaces.

Daylight Compensation
A dimming system controlled by a photocell that reduces the output of the lamps when daylight is present. As daylight levels increase, lamp intensity decreases. An energy-saving technique used in areas with significant daylight contribution.

Daylight Harvesting 
Lighting design for building interiors that makes use of daylight as a way of reducing energy consumption.

Daylight Lamp 
A lamp resembling the color of daylight, typically with a color temperature of 5500K to 6500K.

Daylight Sensor
A device which senses the amount of daylight in a room and controls the luminaire accordingly.

Dichroic Deflector (or Filter) 
A reflector (or filter) that reflects one region of the spectrum while allowing the other region(s) to pass through producing an intense color. 
Also a reflector lamp with a dichroic reflector designed as a "cool beam," where most of the heat has been removed from the beam by allowing it to pass through the reflector while the light has been reflected.

Die 
A very small square of semi-conducting material. Also known as a ‘chip,’ it is the ‘active’ light-emitting component of an LED.

Diffuse
Term describing an even distribution of light, refering to the scattering or softening of light. Diffuse lighting produces less-distinct shadows than directional lighting. Diffuse lighting provides light on the work plane or on an object that does not appear to come from any particular direction.

Diffuser
A translucent piece of glass or plastic sheet to redirect or scatter the light from a source in a fixture.

Dimmer
A device used to control the intensity of light emitted by a luminaire by controlling the voltage or current available to it, usually reducing the wattage consumed.

Dimmable 
Whether or not the lamp lumens can be varied while maintaining reliability -  without shortening lamp life or flickering.

Direct Light
A direct source of light which is cast downwards from a fixture to provide lighting with uniform levels of illumination. Open, louvered, and lensed fixtures can all be ‘direct’.

Direct / Indirect
A source of light in which light is cast both up and down from a fixture to provide a combination of direct and indirect illumination.

Directional Lighting
The lighting produced by luminaires that distribute all, or nearly all, of the light in one direction.

Directional Luminaire
A luminaire that provides directional lighting, including downlights, accent luminaires, and the like.

Dominant Wavelength 
A quantitative measure of the color of an light source as perceived by the human eye. It is usually measured in nanometers (a billionth of a meter).

Downlight
A type of ceiling luminaire, usually fully recessed, where most of the light is directed downward; however, may be adjustable or directional.

 

E

Efficacy (of a light source)
A measurement of how effective a light source is in converting electrical energy to lumens of visible light. This metric is used to compare light output to energy consumption, measured in lumens per watt.

Efficiency (of a luminaire)
The efficiency of a light source is the fraction of electrical energy converted to light. The efficiency of a luminaire or fixture is the percentage of the lamp lumens that actually comes out of the fixture.

Electrodeless Lamps 
Light sources where the discharge occurs in a chamber with no electrodes (no metal.) The energy for the discharge is supplied by radio frequency excitation, e.g., microwaves (For example: GE Genura).

Electroluminescent (EL)
A light source technology used in exit signs, nightlights, watches, and automotive instrument panel backlighting, that provides uniform brightness, long lamp life, while consuming very little energy.  Electroluminescent relies on phosphorescent materials which glow when exposed to a small electrical current.

Electrical Discharge 
A condition under which a gas becomes electrically conducting and becomes capable of transmitting current, usually accompanied by the emission of visible and other radiation. An electric spark in air is an example of an electrical discharge, as is a welder's arc and a lightning bolt.

Ellipsoidal Reflector Lamp (ER-lamp)
An incandescent lamp with an internal reflector that has a focal point a few inches in front of the lamp face. ER-lamps are particularly effective at increasing the efficacy of grooved-baffle recessed downlights or track heads to reduce the amount of light absorbed by the baffle trim.

Electromagnetic Interference (EMI)
High frequency interference (electrical noise) caused by electronic components or fluorescent lamps that interferes with the operation of electrical equipment. EMI is measured in micro-volts, and can be controlled by filters. Because EMI can interfere with communication devices, the Federal Communication Commission (FCC) has established limits for EMI.

Enclosed Fixture Rated Lamps
Lamps that are approved for burning in enclosed fixtures which have an acrylic lens or plate glass enclosure, as opposed to open fixtures.

Energy Policy Act (EPACT) 
Comprehensive energy legislation passed by the U.S. Congress in 1992. The lighting portion includes lamp labeling and minimum energy efficacy (lumens/watt) requirements for many commonly used incandescent and fluorescent lamp types. Federal Canadian legislation sets similar minimum energy efficacy requirements for incandescent reflector lamps and common linear fluorescent lamps.

Energy Policy Act (EPACT) Indicator 
Means this lamp is Federally regulated for Energy Efficiency (See Energy Policy Act).

Energy
The product of power (watts) and time (hours). Energy used for lighting can be saved either by reducing the amount of power required or by reducing the amount of time lighting is used.

Exterior Lighting
Lighting for the outside of a building, including decorative and functional lighting.

‘Eyeball’ Luminaire
A recessed luminaire with a partially recessed sphere that can be rotated to provide adjustable, directional lighting.

Eye Sensitivity 
A curve depicting the sensitivity of the human eye as a function of wavelength (or color). The peak of human eye sensitivity is in the yellow-green region of the spectrum. The normal curve refers to photopic vision or the response of the cones. (See Vision)

 

F

Facade Lighting
Floodlighting the exterior of a structure for security or for illuminating architectural features.

Federal Communications Commission (FCC) 
The U.S. Federal agency that regulates emissions in the radio frequency portion of the electromagnetic spectrum. Part 18 of the FCC rules specifies electromagnetic interference (EMI) from lighting devices operating at frequencies greater than 9 kilohertz (kHz). Typical electronically-ballasted compact fluorescent lamps operate in the 24 - 100 kHz frequency range.

Field Angle 
The angular dimension of the cone of light from reflectorized lamps (such as R and PAR types) encompassing the central part of the beam out to the angle where the intensity is 10% of maximum (See Beam Angle).

Filament Design 
Filaments are designated by a letter combination in which C is a coiled wire filament, CC is a coiled wire that is itself wound into a larger coil, and SR is a straight ribbon filament. Numbers represent the type of filament-support arrangement.

Filament
A fine wire heated electrically to incandescence in an electric lamp.

Fitting
The part of a luminaire that holds the lamp and sometimes mounting hardware.

Fixture
The part of a luminaire not including the lamp, accessories.

Fixture Requirements 
Describes fixture requirements for HID lamps.

  • O = Open or Enclosed Fixtures
  • E = Enclosed Fixtures Only
  • S = Lamps operated in a vertical position (Base Up or Down) ±15º, can be used in an open fixture. Lamps burned in any other orientation must be used in "enclosed fixtures only." See additional details in the e-Catalog Help Menu under the HID category.

Flicker 
The periodic variation in light level caused by dimming or alternating current operation that can lead to strobe effects.

Floodlight 
A luminaire used to light a scene or object to a level much brighter than its surroundings. Usually floodlights are aimed at the object or area of interest.

Fluorescence
The ability of some materials, such as phosphors, to convert ultraviolet energy into visible light.

Fluorescent Lamp

See Lamps

Footcandle (fc) 
A unit of illuminance or light falling onto a surface. It stands for the light level on a surface one foot from a standard candle. One footcandle is equal to one lumen per square foot. One footcandle equals 10.76 lux.

Footcandle Meter
A light metering device that measures the illuminance falling onto a surface, calibrated in footcandles, also called an incidence meter.  Compared to a meter that measures reflected light.

Footlambert
Imperial English unit of luminance equal to 3.426 candela per square meter. The foot-lambert is used in the motion picture industry for measuring the luminance of images on a projection screen.

Forward Current 
Current through a diode in the direction of its greatest conduction.

Forward Voltage (VF) 
The voltage across a diode for a given forward current.

Frequency (nominal operations) 
The stated operating frequency in Hz of a discharge lamp.

Full Spectrum Lighting 
A marketing term, typically associated with light sources that are similar to some forms of natural daylight (5000K and above, 90+ CRI), but sometimes more broadly used for lamps that have a smooth and continuous color spectrum.

Four-Way Switch
One of three switches that controls the same luminaire or group of luminaires. The luminaire(s) may be turned on or off from any of the three switches. It is called a four-way switch because it contains four contact points: the luminaire and the three switches.

 

G

General Lighting - See ambient lighting.

Glare 
The effect of brightness or differences in brightness within the visual field sufficiently high to cause annoyance, discomfort or loss of visual performance.  (See Visual Comfort Probability -VCP) 

Glare – Disability Glare
If task performance is affected, it is called disability glare

Glare – Discomfort Glare
Visual discomfort caused by excessive brightness is called discomfort glare.

Glare – Direct Glare
Glare resulting from very bright sources of light in the field of view. It usually is associated with bright light from luminaires and windows. A direct glare source may affect occupant’s performance by reducing the apparent contrast of objects in the field of view, especially those near the source of light.

Glare – Indirect Glare
Glare produced from a reflective surface.

Glare – Reflected Glare
Glare resulting from bright reflections from polished or glossy surfaces in the field of view. Reflected glare usually is associated with reflections from within a visual task or areas in close proximity to the region being viewed. (See veiling reflections)

Globe
A spherical transparent or diffusing enclosure that is intended to protect a lamp, to diffuse its light, or to change the color of the light.

Globe Luminaire
A luminaire with a spherical diffuser, typically used for ambient lighting.

Grazing Light
Directional light, usually downward at an extreme angle, that emphasizes the texture of surfaces by creating contrast between highlights on raised portions and shadows beyond them. Heavily textured surfaces, such as stucco, and bricks are complemented by grazing light.

 

H

Halophosphates
The class of phosphors that commonly are used in fluorescent lamps. Halophosphates are limited in their ability to provide a high color rendering index without sacrificing light output. See also rare-earth phosphors.

Harmonic Distortion
A harmonic is a sinusoidal component of a periodic wave having a frequency that is a multiple of the fundamental frequency. Harmonic distortion from lighting equipment can interfere with other appliances and the operation of electric power networks. The total harmonic distortion (THD) is usually expressed as a percentage of the fundamental line current. THD for 4-foot fluorescent ballasts usually range from 20% to 40%. For compact fluorescent ballasts, THD levels greater than 50% are not uncommon.

High-Bay Lighting 
Lighting designed for (typically) industrial locations with a ceiling height of 20 feet and above.

Highlighting
See accent lighting.

Hot Spot
An area of higher illumination than that on the immediate surrounding area, often resulting from a lamp being placed close to a surface. Hot spots also can occur due to improper optical design of a luminaire.

Hollywood lights
A luminaire that uses a strip of multiple globe lamps mounted on one or more sides of a mirror. Traditionally used surrounding make up mirrors in Hollywood.

Hot Restart or Restrike Time – See Restrike
Human Factors
The study of the interaction of people and lighting.

 

I

IES & IESNA - Illuminating Engineering Society of North America
Abbreviation for Illuminating Engineering Society of North America. Professional society setting illumination standards that are widely referenced.  The IESNA recommends illuminance levels for a variety of lighting applications in which visual performance (for example, speed and accuracy) is important. These recommendations are a function of the visual task being performed, the adaptation level of the observer, and the age of the observer.

Illuminance
A photometric term that quantifies light, luminous flux,  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 1lux. For conversion purposes, 1 footcandle is equal to 10.76 lux.

Illuminance Meter 
A light metering device that measures the illuminance at a location calibrated either in footcandles or in lux. A Cosine-Corrected illuminance meter that measures the light level correctly irrespective of the angle the light is coming from.

Incentive
A reimbursement of a portion of the cost of a product. Incentives commonly are offered by electric utilities and manufacturers on some energy saving lighting products. Also known as rebates.

Incandescent Lamp – See Lamps

Incident light
The amount of light falling onto a surface. Light meters are defined by their ability to sense incident or reflected light.

Indirect Lighting
The method of lighting a space by directing the light from luminaires upward towards the ceiling. The light scattered off the ceiling produces a soft, diffuse illumination for the entire area. Also known as Uplighting.

Infrared Radiation 
Electromagnetic energy radiated in the wavelength range of about 770 to 1,000,000 nanometers. Energy in this range cannot be seen by the human eye, but can be sensed as heat by the skin.

Initial Cost
The original cost of equipment, lamps, and installation, exclusive of operating costs such as energy, maintenance, and lamp replacement.

Initial Lumens
The lumens produced by a lamp after an initial burn in period (usually 100 hours).

Input Power
The active power that is used by a lamp or lamp/ ballast combination, measured in watts.

Input Watts
The total wattage required by both the ballast and the lamp in a luminaire.

Intensity
See luminous intensity.

Interval Timer
A lighting control that automatically switches the luminaire off after a selected time interval. An interval timer can be either electronic or mechanical.

Inverse Square Law 
Formula stating that if you double the distance from the light source, the light level goes down by a factor of 4, if you triple the distance, it goes down by a factor of 9, and so on.

Isocandela Plot, Isolux Plot,  Isofootcandle Plot
A plot with lines connecting points of equal luminous intensity around a source.

 

K

Kelvin (K)
A numerical scale used to describe the color of light. Light with a lower Kelvin rating will have a more yellow tint, while light with a higher kelvin rating will have a more blue tint.  The Kelvin scale has its zero at –273°C (See color temperature)
Kilowatt (kW)
The measure of electrical power equal to 1000 watts.

Kilowatt-Hour (kWh):
Measure of electrical energy consumed; 1 kilowatt-hour is equal to 1000 watts
used for 1 hour. The standard measure of electrical energy and the typical billing unit used by electrical utilities for electricity use. A 100-watt lamp operated for 10 hours consumes 1000 watt-hours (100 x 10) or one kilowatt-hour. If the utility charges $.10/kWh, then the electricity cost for the 10 hours of operation would be 10 cents (1 x $.10).

 

L

Lamps
A manufactured light source.  For electric lamps, it includes the bulb, the base, and the internal structure that produces light, either a filament or an arc tube. Lamps are often referred to as  ‘light bulbs.’ The term lamp also is commonly used to refer to plug-in luminaires as in desk, floor, and table lamps.

Lamp – A-lamp
Common incandescent  pear-shaped ‘light bulb.’ An A-lamp can have a clear glass bulb, or a white coating, or an etched frost, or silver or mirror capped end.

Lamp – Lamp Current Crest Factor (LCCF)
The peak lamp current divided by the RMS (average) lamp current. Lamp manufacturers require <1.7 for best lamp life. An LCCF of 1.414 is a perfect sine wave.

Lamp Disposal
Refers to the proper recycling of lamps containing mercury or other hazardous materials.

Lamp – Energy Saving Lamp
A lower wattage lamp, generally producing fewer lumens.

Lamp – ER-lamp
An incandescent or retrofit fluorescent ellipsoidal reflector lamp.

Lamp – Flood
Used to refer to the beam pattern of a reflector lamp, or luminaire that disperses the light over a wide beam angle. (Flood as opposed to spot)

Lamp – Fluorescent Lamp
A discharge light source consisting of a tube filled with argon, along with krypton or other inert gas. When electrical current is applied, the resulting arc emits ultraviolet radiation (UV) that excites the phosphor materials, applied as a thin layer on the inside of a glass tube which makes up the structure of the lamp, causing them to radiate visible light.

Lamp – Fluorescent Compact 2 pin, 4 pin, and Biax
Twin Tube , Triple Tube, and Quad (4) tube compact fluorescent lamps.  Two pin tend to blink on, for clean starting use 4 pin base.  Biax™ covers a range of single ended Compact Fluorescent lamps with High Lumen output and are available in 18, 24, 34, 36, 40, 55 and 80 watt versions. The single ended design is less than half the length of standard fluorescent lamps.

Lamp – Fluorescent Compact Self-Ballasted GU-24
GU24 bulbs have 2 pins which connect to the base with a twist-and-lock connection. Screw-in CFLs and incandescent bulbs cannot be used in GU24 fixtures. Title 24 compliant light source.

Lamp – Fluorescent Compact Self-Ballasted or Retrofit (CFL)
A compact fluorescent lamp with integral ballast that has a medium or candelabra screwbase that fits into the standard incandescent lamp socket.  Occasionally the lamp and ballast are separate pieces called

Lamp – Fluorescent Modular Compact Fluorescent Lamp
The replaceable lamp in a two-piece compact fluorescent lamp product. It is a single ended fluorescent lamp with a two- or four-pin base. When used with a modular compact fluorescent lamp ballast, the combination can replace an incandescent lamp.

Lamp – Fluorescent Linear
Any of the family of straight tubular fluorescent lamps. Lamps are available in 6-inch to 8-foot lengths, with the most-common length being 4 feet.

Lamp – Fluorescent High Output (HO)
A lamp fluorescent lamp designed to operate at higher currents (800 mA) and produce more light.

Lamp – Fluorescent Quad
Generally refers to a compact fluorescent lamp containing 4 U-shaped tubes, a double twin tube configuration.

Lamp – Fluorescent Specification Series (SP) Colors
Energy-efficient, all-purpose, tri-phosphor fluorescent lamp colors that provide good color rendering. The CRI for SP colors is 70 or above and varies by specific lamp type.

Lamp – Fluorescent Specification Series Deluxe (SPX) Colors
Energy-efficient, all-purpose, tri-phosphor fluorescent lamp colors that provide better color rendering than Specification Series (SP) colors. The CRI for SPX colors is 80 or above and varies by specific lamp type. All GE CFL products use SPX phosphors.

Lamp – Fluorescent T for Tubular
Lamp size = diameter measured in 1/8” increments. Each designation has a unique fitting at the end of the lamp and they are not interchangeable, they must be mated to the connectors and ballast.

  • T2 — 1/4" diameter fluorescent lamps.
  • T5 — 5/8" diameter fluorescent lamps.
  • T8 — 1" diameter fluorescent lamps.
  • T12 — 1-1/2" diameter fluorescent lamps.

Lamp – Twin-Tube
A single-ended fluorescent lamp with the tube bent into a very tight "U" or " H shape.
(See Compact Fluorescent Lamp)

Lamp – Fluorescent U-Shaped
A fluorescent lamp with the tube bent in the middle so that the ends fit into the same side of a luminaire.

Lamp – Fluorescent Very High Output (VHO)
A fluorescent lamp that operates at a "very high" current (1500 mA), producing more light output than a "high output" lamp (800 mA) or standard output lamp (430 mA).

Lamp – Fluorescent Warm White
Refers to a color temperature around 3000K, providing a yellowish-white light.

Lamp – Globe (G-lamp)
An incandescent lamp with a globeshaped bulb or a compact fluorescent lamp with a globe-shaped diffusing cover.

Lamp – Halogen Incandescent
An incandescent lamp whose filament is encapsulated in a quartz envelope filled with a halogen gas such as iodine, chlorine, bromine, and fluorine, that reacts with tungsten evaporated from the filament to redeposit it on the filament. They are sometimes referred to as quartz lamps because the capsule is made from quartz glass to allow the filaments to be operated at higher temperatures and have higher efficacies than common incandescent lamps. The ‘halogen cycle’ re-deposits the evaporated tungsten onto the filament or the hottest area, the reason why gloves are used to install this lamp, avoiding oils from hand creating a ‘hot spot’ on the bulb wall and causing the evaporated tungsten to adhere to the oil spot. Also called Quartz & Quartz Halogen.

Lamp – Halogen-IR (HIR)
A halogen lamp with an infrared-reflecting coating on the capsule that surrounds the filament. The coating redirects infrared energy onto the filament, which increases the temperature of the filament without additional input power, thereby increasing efficacy.

Lamp Height
Referenced by IEC as Dimension C. Also referred to as ‘Base Face to Top of Lamp’

Lamp – High-Intensity Discharge (HID)
A group of electric discharge lamps operating at relatively high pressures (compared to fluorescent lamps) within compact arc tubes which enclose various gases and metal salts. This group includes the lamp types known as mercury vapor, metal halide, and high-pressure sodium.

Lamp – High-Pressure Sodium (HPS)
HID light source in which radiation from sodium vapor under high pressure produces visible light. High-pressure sodium lamps are orangish in color appearance, take a few minutes to achieve full light output on lamp startup, and require several minutes to restart if power to the lamp is interrupted, even briefly. See also low-pressure sodium lamp

Lamp – Incandescent
A light source producing visible radiant energy by electrical resistance heating of a filament wire (usually of tungsten).

Lamp – Incandescent Three Level
Incandescent lamp having two filaments. Each can be operated separately or in combination with the other, which provides three different light outputs. A special socket is required to use the three levels of this lamp.

Lamp – Induction Lamp
Gases can be excited directly by radio-frequency or microwaves from a coil that creates induced electromagnetic fields. Induction lighting differs from a conventional discharge, which uses electrodes to carry current into the arc. Induction lamps have no electrodes inside the chamber and as a result, generally have longer life than standard lamps.

Lamp – Life
An average rated lamp life, in hours, indicating when 50% of a large group of lamps have failed, when operated at nominal lamp voltage and current.  For fluorescent lamps, the operating conditions include operation at nominal line voltage at 3 hours per start. For high intensity discharge lamps, the lamps are operated at 10 hours per start.
Lamp – Life Multiplier
A factor used to adjust the average rated lamp life to reflect the effects of hours per start and dimming of lamps.

Lamp – Lamp Lumen Depreciation Factor (LLD)
A factor that represents the reduction of lumen output over time. The factor is commonly used as a multiplier to the initial lumen rating in illuminance calculations, which compensates for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1.
(See also Lumen Depreciation and Mean Lumens).

Lamp – Low-Pressure Sodium
A low-pressure discharge lamp in which light is produced by radiation from sodium vapor.  Lengths range from 8.5” to 30.5”. Considered a monochromatic light source appearing orange in color (most colors are rendered as gray). Used in areas near astronomical observatories to keep the wavelengths below 480 nm to be filtered out of their photos.

Lamp – Low-Voltage Lamp
A lamp, typically compact halogen, that provides both intensity and good color rendition, operates at 6, 12 or 24 volts. A transformer must be used to convert the 120-volt line voltage to the lower voltage. Popular lamps are MR11, MR16, and PAR36.

Lamp – Life
See Rated Lamp Life.

Lamp – Maximum Overall Length (MOL)
The end-to-end measurement of a lamp, expressed in inches or millimeters.

Lamp – Medium Base
Usually refers to the screw base typically used in household incandescent lamps.

Lamp – Mercury Vapor Lamp (MV)
A type of high intensity discharge (HID) lamp in which most of the light is produced by radiation from mercury vapor. Emits a blue-green cast of light. Available in clear and phosphor-coated lamps. Considerable color shift can occur by end of life.

Lamp – Metal Halide Lamp (MH)
A type of high intensity discharge (HID) lamp in which most of the light is produced by radiation of a mixture of metallic vapor and additives of halides (e.g., sodium, thallium, scandium, indium and dysprosium) in the arc tube. Available in clear and phosphor-coated lamps.

Lamp – Mogul Base
A screw base used on larger lamps, 1000 watt incandescent and  many HID lamps.

Lamp – MR-Lamp (Multi-faceted Reflector)
MR-16, MR-11, MR-8 low voltage halogen reflector lamp that is used in lighting applications where precise beam control is required, such as accent lighting. Available in a wide range of beam spreads. Some MR lamps, such as projection lamps, are designed for line-voltage operation.

Lamp – PAR Lamp (Parabolic Aluminized Reflector)
PAR is an acronym for parabolic aluminized reflector. A PAR lamp, which may utilize either an incandescent filament, a halogen filament tube or a HID arc tube, is a precision pressed-glass reflector lamp. PAR lamps rely on both the internal reflector and prisms in the lens for control of the light beam.

Lamp – R-lamp
An incandescent filament or electric discharge lamp in which the sides of the outer blown-glass bulb are coated with a reflecting material so as to direct the light. The light-transmitting region may be clear, frosted, or patterned. See Also ER Lamps.

Lamp – Shape

  • A = Arbitrary
  • B = Bulbous or Torpedo
  • BR = Bulged Reflector
  • C = Christmas/Appliance/Nightlight
  • CA = Bent Tip
  • E, ED = Elliptical
  • ER = Elliptical Reflector
  • F, CA = Flame
  • G = Globe
  • MR = Mini Reflector
  • P, PS = Pear
  • PAR = Parabolic Reflector
  • R = Reflector
  • S = "S" Shape
  • T = Tubular

Lamp – Size
Lamp diameters at its maximum dimension are measured in 1/8” increments; T-8 fluorescent = 1” diameter and the common pear shaped incandescent A-19 = 2-3/8” diameter, PAR 36 = 4.5” diameter.

Lamp – Spot 
A reflector lamp with a tight beam of light, typically around 10 degrees or less.

Lamp – Tungsten-Halogen
See Lamp - halogen incandescent.

Lamp – Width 
Referenced by IEC as Dimension A.

Lamp – Quartz-Halogen
See lamp - halogen incandescent.

Lamp – Reduced-Wattage
A lamp that is of slightly lower wattage than the lamp it is intended to replace. A reduced-wattage lamp also provides less light.

Lamp – Restrike, Restart time
See Restrike

Lamp – Screw-In 
(See CFL).

Lamp – Seal Temperature (Maximum) 
The maximum operating temperature of the seal of the lamp in Celsius.

Lamp – Service Life (of a lamp)
The total time that passes, including time that the lamp is on and time that it is off, before the lamp must be replaced.

Lamp – Source Size 
For Projection lamps, this is defined as the dimensions of the rectangular area, centered on the lamp axis, within which all luminous parts of the filament lie, when viewed perpendicular to the axis of the filament coil or to the plane of C-13 and C-13D filaments.

Lamp - Type

  • Filament lamps - Incandescent, Halogen, Halogen-IR.
  • Discharge Lamps - Fluorescent, HID (High Intensity Discharge).
  • HID Lamps -Mercury, HPS (High Pressure Sodium), MH (Metal Halide) and CMH (Ceramic Metal Halide).
  • Low Pressure Sodium (LPS)
  • LED Lamps - Light Emitting Diodes (LEDs), Organic LEDs (OLED), Polymer Light-Emitting Diodes (PLED)
  • Induction Lamps -
  • Neon Lamps -

Lamp – Wall Temperature (Maximum Bulb)
The maximum operating bulb wall temperature in Celsius.

Lamp – Warm Up Time to 90% 
The time it takes for a High Intensity Discharge lamp to reach 90% of light output after being turned on. Compact fluorescent lamps can take 7 minutes to achieve full brightness.  Also see Restrike time or hot restrike time.

Lay In Troffer
A fixture; usually a 2' x 4' or 2’ x 2’fixture that sets or "lays" into a suspended ceiling grid.

Leadframe 
A metallic frame used for mounting and connecting LED chips. The leadframe functions as the electrical leads of the device

LED Light Emitting Diode
A semiconductor light source, diode, that radiates in the visible region of the spectrum.

LED Display Viewing Angle 
The LED industry defines viewing angle as the full angle at which brightness is half of the brightness from dead center. The image may seem garbled, poorly saturated, of poor contrast, blurry or too faint outside the stated viewing angle range.

Lens
A glass or acrylic plastic element used in luminaires to control, the distribution of light and or to provide protection. Lenses can be flat and fitted into the aperture, or cup-shaped or spherical to fit over a lamp. Luminaires often have lenses in addition to reflectors.

Life Cycle Cost
The total costs associated with purchasing, operating, and maintaining a system over the life of that system.

Light
Radiant energy that is capable of producing a visual sensation. The visible portion of the electromagnetic spectrum, that can be sensed or seen by the human eye, extends from about 380 to 770 nanometers. Visible light from a light source is measured in lumens.

Light Center Length (LCL) 
The distance between the center of the filament, or arc tube, and a reference plane - usually the bottom of the lamp base.

Light Distribution
The pattern of light that is produced by a lamp or a luminaire, or the patterns of light created in a room.

Light Loss Factor (LLF)
Factors used to calculate maintained light levels, illuminance, after a given period of time including reflector degradation, dirt, lamp depreciation, temperature and voltage fluctuations. LLFs are divided into two categories, recoverable and non-recoverable, examples are lamp lumen depreciation and luminaire surface depreciation.

Light Meter
See Illuminance Meter

Light Output
Luminous flux, measured in lumens. The light output rating of a lamp is a measure of its total integrated light output.

Light Pollution 
Light that is directed to areas where it is not needed. For example, light pollution directed or reflected into the sky creates a "dome" of wasted light and makes it difficult to see stars above cities. Also see Light Trespass.

Light Source – see Lamp
The object that produces the light. For electric lighting, a lamp; for daylighting, the sun.

Light Trespass (Spill Light) 
Light that is not aimed properly or shielded effectively can spill out into areas where it is not required, distracting drivers, pedestrians and neighbors. It can be annoying and occasionally disabling.

Lighting Design
The planned application of lighting systems to an indoor or outdoor space.

Lighting System
The set of equipment that is used to produce light, including a luminaire and control.

Lighting Technique
A way to light a space to achieve a desired effect.

Louver
A series of baffles or reflectors that is used to shield a light source from view at certain angles, absorb unwanted light, or reflect light. Also see Parabolic Reflector.

Low-Bay Lighting
Lighting used in industrial applications where the ceiling height is 20 feet or less. Common in big box retail and industrial settings. Also see High Bay

Low Voltage Switch
A relay (magnetically-operated switch) that allows local and remote control of lights, including centralized time clock or computer control.

Lumen
A measure of the luminous flux or quantity of light emitted by a source. For example, a dinner candle provides about 12 lumens. A 60-watt Soft White incandescent lamp provides about 840 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 a lumen maintenance curve) depicting the pattern of decreasing light output.

Lumens Per Watt (LPW)
The number of lumens produced by a light source for each watt of electrical power supplied to the light source. See Efficacy.

Lumen Maintenance
The deterioration in the amount of light that is emitted from a lamp over time. A lamp with a good lumen maintenance will emit a consistent amount of light over its lifetime, emitting as much as 90% of its original capability at the end of its lifespan. A lamp with poor lumen maintenance will lose as much as 50% of its ability to emit light over time.

Luminaire
A complete lighting unit consisting of a lamp or lamps, housing, ballast, sockets and any other necessary components to control light distribution and to connect the lamps to the power supply. Also called a light fixture.

Luminaire Efficiency
The ratio of total lumen output of a luminaire and the lumen output of the lamps, expressed as a percentage. For example, if two luminaires use the same lamps, more light will be emitted from the fixture with the higher efficiency.

Luminance
A measure of  surface brightness when an observer is looking in the direction of the surface. It is measured in candelas per square meter (or per square foot) It is expressed as footlamberts (English units) or candelas per square meter (Metric units).

Lumen Maintenance 
A measure of how well a lamp maintains its light output over time. It may be expressed numerically or as a graph of light output vs. time.

Luminance Ratio
The contrast ratio between the luminances of any two areas in the visual field, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing.  Also called contrast or brightness ratio.

Luminous Ceiling
A dropped or recessed ceiling containing lamps above translucent panels, providing bright, diffuse lighting.

Luminous Flux
Measured in lumens, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

Luminous Intensity 
A measure of the visibility of a light source generally expressed in candelas. It is defined as luminous flux per unit solid angle (steradian) in a given direction

Luminous intensity distribution data
Curve, generally plotted on polar or rectilinear coordinates, which represents variation in luminous intensity (in candelas) from a bare lamp or from a luminaire. Distribution data can also be presented in tabular format.

Lux
Standard international unit of illuminance equal to 1 lumen per square meter. One lux equals 0.0929 footcandles.


M

Maintained Illuminance
Refers to light levels of a space at other than initial or rated conditions. This terms considers light loss factors such as lamp lumen depreciation, luminaire dirt depreciation, and room surface dirt depreciation. See Light Loss Factor 

Matte Surface
A surface from which the reflection is predominantly diffuse.

Mean Lumens 
The average light output of a lamp over its rated life. Based on the shape of the lumen depreciation curve. Fluorescent and metal halide lamps , mean lumens are measured at 40% of rated lamp life . For mercury, high-pressure sodium and incandescent lamps, mean lumen ratings refer to lumens at 50% of rated lamp life.  LED mean lumens are not standardized yet, IES is developing standards in 2012. (See Lumen Maintenance).

Mesopic
See Vision
Motion Detector
Also called an occupancy sensor. A device that detects the movement of people, animals, and objects using a passive infrared and / or ultrasonic sensor. Motion detectors are used to control other devices, such as alarm systems and luminaires, so that these devices are activated when motion is detected. Some motion detectors offer manual on and/ or manual off override capabilities. See also passive infrared and ultrasonic.

Motion Detector Factor
A factor that is used in the economic analyses to adjust the hours of lighting use to account for a motion detector that turns off lamps when no motion is detected.

Monochromatic Light 
Light with only one wavelength (i.e., color) present.

Mounting Height 
Distance from the bottom of the fixture to either the finished floor or work plane, depending on usage. ADA requires specific minimum mounting heights for fixtures over 4” deep. Motion detectors also have a recommended mounting height. (Specify ‘to the bottom of the fixture’ because mounting hardware can be located anywhere on the luminarie.

Multiple-level Switching
A switching technique wherein the individual lamps, or groups of lamps, in a luminaire are switched independently to achieve multiple light outputs. Sometimes called bi-level switching.

 

N

Nadir
A reference direction directly below a luminaire, or "straight down" (0 degree angle).

Nanometer 
A unit of wavelength equal to one billionth of a meter, used in defining colors of light.

NEMA
Abbreviation for National Electrical Manufacturers Association.

NIST
Abbreviation for National Institute of Standards and Technology.

NATL
Nationally Approved Testing Laboratory

 

O

Occupancy Sensor
Also called a motion detector. A device that detects the movement of people, animals, and objects using a passive infrared and / or ultrasonic sensor. Motion detectors are used to control other devices, such as alarm systems and luminaires, so that these devices are activated when motion is detected. Some motion detectors offer manual on and/ or manual off override capabilities. See also passive infrared and ultrasonic.

Open Circuit Voltage (OCV) 
Open Circuit Voltage measured across the socket the lamp screws into, with the ballast powered on. It is dangerous to stick a voltmeter into such a socket without precise knowledge of the ballast because exceedingly high voltages could be present.

Open Fixture Rated lamps
Lamps that are approved for burning in open fixtures (as opposed to enclosed fixtures which have an acrylic lens or plate glass enclosure).

Operating Cost - See Annual Operating Cost.

Operating Position or Burn Position 
Mercury and High Pressure Sodium lamps may be operated in any burn position and will still maintain their rated performance specifications. Metal Halide and Low Pressure Sodium lamps, however, are optimized for performance in specific burn positions, or may be restricted to certain burn positions for safety reasons.

  • U = Universal burning position
  • HBU = Horizontal -15º to Base Up
  • HBD = Horizontal +15º to Base Down
  • HOR = Horizontal ±15º
  • H45 = Horizontal to -45º only
  • VBU = Vertical Base Up ±15º
  • VBD = Vertical Base Down ±15º 

If no special burn position is noted, the burn position is universal.

Optics
A term referring to the components of a light fixture (such as reflectors, refractors, lenses, louvers) or to the light emitting or light-controlling performance of a fixture.

 

P

Parabolic Reflector
A reflector with a parabolic shape that usually is used to concentrate the light in the direction parallel to the axis of the reflector. The location of the light source relative to the reflector is crucial to the design of the reflector.

Parabolic Luminaire
A popular type lighting fixture that has a louver composed of aluminum baffles curved in a parabolic shape. The resultant light distribution produced by this shape provides reduced glare, better light control, and is considered to have greater aesthetic appeal.

Paracube
A metallic coated plastic louver made up of small squares. Often used to replace the lens in an installed troffer to enhance its appearance. The paracube is visually comfortable, but the luminaire efficiency is lowered. Also used in rooms with computer screens because of their glare-reducing qualities.

Passive Infrared (Type of Motion Detector)
Passive infrared motion detectors sense the motion of infrared energy (heat) within a space. A detector is located behind an infrared-transmitting lens, which is usually vertically segmented with multiple smaller lenses etched within each segment. This lens design results in horizontal and vertical "fan" pattern detection zones. When a passive infrared sensor detects motion from one zone to another, it activates whatever device it controls (usually an alarm system or one or more luminaires). See also motion detector.

Peak Wavelength 
The maximum wavelength of an LED.

Pendant Luminaire
A luminaire hung from a ceiling by supports. Also called a suspended luminaire, includes chandeliers.

Phosphors & Rare Earth Phosphors
Chemical compounds that are used to coat the inside of fluorescent, HID, and LED bubs, designed to absorb short wavelength ultraviolet radiation and to transform and emit it as visible light. Different phosphors radiate light at various wavelengths, combining them determines the color of the light. Rare-earth phosphors are used to achieve higher efficacy and better color rendering than can be achieved with halophosphates. Rare earth phosphors each produce light in a very narrow wavelength band. Although collectively they are more efficacious than halophosphates, they are more expensive. Thus, to reduce manufacturing costs, lamps often are coated first with halophosphates and then with a thin layer of rare-earth phosphors. RE designates a lamp containing rare-earth phosphors.

Photometry 
The measurement of light and related quantities.

Photocell
A light sensing device used to control luminaires and dimmers in response to detected light levels.

Photodynamic Therapy
A medical procedure used in treating some cancers and eye problems using a drug that is activated by light.

Photometric Report
A photometric report is a set of printed data describing the light distribution, efficiency, and zonal lumen output of a luminaire. This report is generated from laboratory testing combined with computer drafting of the data.

Photo Sensor
A device that converts light to electrical current. Based on the amount of incident light, a photosensor can switch a lamp on or off or regulate a lamp's light output to maintain a preset light level.

Plenum
The space between the ceiling and the floor or roof above.

Point Source
A type of lighting source where the beam distribution can be controlled by reflector design, for example MR-16.

Portable - Plug-In 
A lighting fixture with a cord, designed to be moved.

Power Factor (PF) 
The ratio of AC volts x amps through a device to AC wattage of the device. The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit, and is a dimensionless number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time.  Power factor is sometimes expressed as a percent. Incandescent lamps have power factors close to 1.0 because they are simple "resistive" loads. The power factor of a fluorescent and HID lamp system is determined by the ballast used. "High" power factor usually means a rating of 0.9 or greater. In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.

Power Reduction Factor
A factor that accounts for the reduction in power that is drawn by lamps when they are dimmed to a specified level (expressed as a fraction of full power), or when they are operated by multiple-level switching.

Pull-Cord
A string or chain that is attached to a switch that is mounted in a luminaire. Pull cords typically are used to control individual ceiling mounted luminaires, as in an attic or basement.

 

R

Radiation 
A general term for the release of energy in a "wave" or "ray" form. All light is radiant energy or radiation, as is heat, UV, microwaves, radio waves, etc.

Radio Frequency Interference (RFI)
Interference to the radio frequency band caused by other high frequency equipment or devices in the immediate area that can affect the operation of other electrical devices. Some fluorescent lighting systems generate RFI.

Rare Earth Phosphors – See Phosphors

Rated Lamp Life
For most lamp types (LEDs not included), rated lamp life is the length of time of a statistically large sample between first use and the point when 50% of the lamps have died. It is possible to define ‘useful life’ of a lamp based on practical considerations involving lumen depreciation and color shift (See Mean Lumens ).

Receptacle
An electrical outlet.

Recessed Luminaire
A luminaire that is mounted above the ceiling (or behind a wall or other surface) with the opening of the luminaire flush with the surface.

Reflectance
The ratio of light reflected from a surface to the light incident on the surface. Reflectances are often used for lighting calculations. The reflectance of a dark carpet is around 20%, and a clean white wall is roughly 50% to 60%.

Reflector
A surface of mirrored glass, painted metal, polished metal, or metalized plastic that is shaped to project the beam from a light source in a particular direction. Reflectors may be an integral part of a lamp or they may be part of the luminaire.

Refractor
A device used to redirect the light output from a source, primarily by bending the waves of light with a glass or plastic lens.

Relay Switch
A device that switches an electrical load on or off based on small changes in current or voltage. Examples: low voltage relay and solid state relay.

Relative Light Output (RLO)
The ratio of light output between a fixture's potential light output at optimum ambient temperatures and actual light output at actual ambient temperatures. For example, if a fixture at its optimal temperature of 75°F produces 10,000 Lumens and 8,000 Lumens 50°F, that fixture's RLO at 50°F is 8,000 Lumens ÷ 10,000 Lumens, or 80%.

Retrofit
Refers to upgrading a fixture, room, or building by installing new parts or equipment.

Re-strike Time
Refers to the restarting of a previously operating lamp shortly after turnoff.  Metal halide lamps typically require a minimum of 4-15 minutes to restart after turn-off. The lamp will only restart after the arc tube has cooled a sufficient amount; therefore, some luminaires are equipped with halogen lamps as temporary emergency light in case of intermittent power failure. Also known as ‘hot re-strike time.’ Warm up time is how long it takes to achieve 90% of full brightness.

Reverse Voltage (VR) 
Voltage across the diode for a given reverse current.

ROOM CAVITY RATIO (RCR): A ratio of room dimensions used to quantify how light will interact with room surfaces. A factor used in illuminance calculations.

Room Cavity Ratio (RCR) 
A ratio of room dimensions used to quantify how light will interact with room surfaces. A factor used in illuminance calculations. A cubical room will have an RCR of 10; the flatter the room, the lower the RCR.

 

S

Sconce
A decorative or functional wall mounted luminaire.

Self Luminous Exit Sign
An illumination technology using phosphor-coated glass tubes filled with radioactive tritium gas. The exit sign uses no electricity and thus does not need to be hardwired.

Semi-Specular
Term describing the light reflection characteristics of a material. Some light is reflected directionally, with some amount of scatter. A reflective but somewhat diffuse surface.

Shade
A device on a luminaire that is used to prevent glare (by hiding the light source from direct view), control light distribution, and sometimes diffuse (and perhaps color) the light emitted.

Shielding Angle
The angle measured from the ceiling or horizontal plane to the line of sight where the bare lamp in a luminaire becomes visible. Higher shielding angles reduce direct glare. It is the complementary angle of the cutoff angle.

Simple Payback
A term used to define the time required to save enough in operating costs by using one design compared to another.

Single-pole Switch
Single-location on-off switch that controls one luminaire, or group of luminaires.

Skylight
A clear or translucent panel set into a roof to admit daylight into a building.

Socket
The fitting on a luminaire that electrically connects the luminaire to the lamp.

Soffit Luminaire
An architectural luminaire that directs light up or downward from the cornice or soffit between the wall and ceiling or floor.

Spacing to Mounting Height Ratio 
Ratio of fixture spacing (distance apart) to mounting height above the work plane, path, or roadway; sometimes called spacing criterion

Specification Grade Luminaire
A luminaire that is produced with high-quality construction and materials compared to lower quality commodity-grade luminaire.

Specular & Specular Reflection
This word describes the finish of the material used in some louvers and reflectors.
A highly polished, mirrored, or shiny, surface, as opposed to diffuse, from which the reflection is predominantly directional. The angle of reflection is equal to the angle of incidence.

Spectral Power Distribution (SPD) 
A graph of the radiant power emitted by a light source as a function of wavelength. SPDs provide a visual profile or "fingerprint" of the color characteristics of the source throughout the visible part of the spectrum.

Spectrum – See Visual Spectrum

Starting Temperature (Minimum) 
The minimum ambient temperature at which a lamp will start reliably.

Stroboscopic Effect or Strobe Light
Condition where rotating machinery or other rapidly moving objects appear to be standing still due to the alternating current supplied to light sources. Also blinking lights used in the entertainment industry for dramatic stop action effects. Strobe lighting can trigger seizures in photosensitive epilepsy, post warning signs and control frequency of strobe to recommended levels.
Sunburn 
Skin reddening and inflammation caused by overexposure to sun and sources containing UV-B and/or UV-C.

Surface Mounted Luminaire
A luminaire mounted directly on the ceiling or other surface.

Suspended Luminaire
A luminaire hung from a ceiling by supports. Also called a pendant luminaire.

System 
A term referring to the lamp and ballast combination, and sometimes to the entire lighting delivery system including the fixture, the optics, the particular layout and the lighting controls.

Switch
A device that turns a lamp or lamps on or off by completing or interrupting the power supplied to the lamp(s). See also single-pole switch, three-way switch, and four-way switch.

 

T

Tandem Wiring
A wiring option in which a ballasts is shared by two or more luminaires. This reduces labor, materials, and energy costs.

Task Lighting 
Lighting that is directed to a specific surface or area. Task lighting provides supplemental illumination for visual tasks.
TCLP Test 
The Toxicity Characteristic Leaching Procedure (TCLP) test, specified in the Resource Conservation and Recovery Act (RCRA) of 1990, is used to characterize fluorescent lamp waste as hazardous or nonhazardous waste. The TCLP test measures the ability of the mercury and/or lead in a lamp to leach from a landfill into groundwater

Thermal Characteristics
The manner in which a luminaire manages heat, either dissipating heat or retaining it.

Three Way Switch
One of two switches that control the same luminaire or group of luminaires. The luminaire(s) may be turned on or off from either of the two switches. Sometimes called master and slave when dimmer is included.

Timer
A lighting control that automatically switches the luminaire off after a selected time interval. An timer can be either electronic or mechanical. Also known as Interval Timer

Tint 
Tinting an LED can be accomplished by adding a dye to the encapsulant similar in color to the light emitted. This allows identification of the LED color in the OFF state, as well as offering color filtering of direct sunlight. Tinting may also reduce the intensity of the device.
Torchiere
A tall, often above eye level, portable indirect floor lamp sending all or nearly all of its light upward.

Track Head
An adjustable luminaire that connects to the track in a track lighting system.

Track Lighting
A lighting system with an electrically fed linear track that accepts one or more track heads. The track heads can be easily relocated along the track. Available as straight, flexible, or cable systems.

Trim
Baffles, cones, rims, and other treatments for apertures of downlights. Trim is usually the part of the luminaire that is visible from below the ceiling.

Trim Ring
A plastic or metal ring used to cover and seal the edges of holes that are cut into ceilings to install recessed luminaires. Available in multiple finish and colors.

Triphosphor
See rare-earth phosphors.

Troffer
A recessed luminaire that is installed in the plenum with the opening flush with the ceiling. Typically rectangular or square in shape, as in a 2-foot by 4-foot luminaire.

 

U

Uniform Code Council (UCC) 
The 12 digit case code derived from the last 12 digits of the 14 digit SCC code on GE's case content label.

Underwriters Laboratories (UL) 
Independent, not-for-profit product safety testing and certification organization. Testing products for public safety.  One of the Nationally Recognized Testing Laboratories (NRTLs)

Uniform Product Code (UPC) 
The 12 digit code on the saleable unit that is used for scanning at the register.

Utrasonic (Type of Motion Detector)
Ultrasonic motion detectors emit high-frequency sound waves (too high for the human ear to hear), which are reflected by objects and room surfaces to a receiver located in the detector. The reflected waves set up a static wave pattern; any disturbance in this pattern alters the frequency of the reflected wave, which is detected by the receiver. The receiver then activates whatever device the detector controls (usually an alarm system or one or more luminaires). See also motion detector.

Ultraviolet Radiation (UV)
Radiant energy in the range of about 100-380 nanometers (nm). For practical applications, the UV band is broken down further as follows:

  • Ozone-producing - 180-220 nm
  • Bactericidal (germicidal) - 220-300 nm
  • Erythemal (skin reddening) - 280-320 nm
  • ‘Black’ light - 320-400 nm

The International Commission on Illumination (CIE) defines the UV band as UV-A (315-400 nm); UV-B (280-315 nm) and UV-C (100-280 nm).

Undercabinet Lighting
Luminaires mounted on the underside of cabinets to provide task lighting, typically in a kitchen.

Uplighting
The method of lighting a space by directing the light from luminaires upward towards the ceiling. The light scattered off the ceiling produces a soft, diffuse illumination for the entire area. Also known as Indirect Lighting

 

V

Valance Lighting 
Lighting from light sources on a wall typically above eye level, shielded by horizontal panels. The light may be upward or downward directed. Often recessed into boxes housing with window covering materials.

Vandal Resistant Luminaire
Fixtures with rugged housings, break-resistant type shielding, and tamper-proof screws. Used in areas prone to vandalism such as campuses and jails.

Vanity Light
A wall-mounted luminaire located next to a mirror. See also ‘Hollywood’ lights.

Vision – Cone Vision
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. Cones have a sensitivity peaking in the yellow and corresponding to the eye response curve

Vision – Foveal Vision 
While the normal field of vision for each eye is about 135 degrees vertically and about 160 degrees horizontally, only the fovea has the ability to perceive and send clear, sharply focused visual images to the brain. This foveal field of vision represents a small conical area of between 1 and 3 degrees. To fully appreciate how small a one-degree field is, and to demonstrate foveal field, take a quarter from your pocket and tape it to a flat piece of glass, such as a window. Now back off 4 1/2 feet from the mounted quarter and close one eye. This region is populated almost entirely with cones, while the peripheral region has increasing numbers of rods.

Vision – Mesopic
Typically referring to nighttime outdoor lighting conditions, the region between Photopic and Scotopic vision

Vision – Rod Vision
Known as Night Vision the rods of the eye absorb light.  Rods require less light to function than cones, they are therefore the primary source of visual information at night. The threshold for rod vision is moonlight, where the human eye cannot distinguish color. 

Vision – Photopic 
Vision for which the cones in the eye are responsible; typically at high brightness and in the foveal or central region.  A type of light measured in lumens that is generally detected by common light meters and accounts for part of the human eye’s perception of brightness.

Vision – Scotopic 
Vision where the rods of the retina are exclusively responsible for seeing, typically like the light levels in the countryside on a moonless, starlit night

Vision – Scotopic / Photopic (S/P) Ratio 
This measurement accounts for the fact that of the two light sensors in the retina, rods are more sensitive to blue light (scotopic vision) and cones to yellow light (photopic vision). The scotopic/photopic (S/P) ratio is an attempt to capture the relative strengths of these two responses. S/P is calculated as the ration of scotopic lumens to photopic lumens for the light source on an ANSI reference ballast. Cooler sources (higher color temperatures lamps) tend to have higher values of the S/P ratio compared to warm sources.
The ratio of scotopic to photopic lumens produced by a light source. An appropriate S/P ratio will provide for a more comfortable atmosphere and better perceived brightness.
Vision – Scotopic Lumens
A type of light that is not generally detected by common light meters but which accounts for part of the human eye’s perception of brightness.

Visual Comfort Probability (VCP) 
A rating system for evaluating direct discomfort glare. This method is a subjective evaluation of visual comfort, when viewing from a specific location and in a specified direction, expressed as the percent of occupants of a space who will be bothered by direct glare. VCP allows for several factors: luminaire luminances at different angles of view, luminaire size, room size, luminaire mounting height, illuminance, and room surface reflectivity. VCP tables are often provided as part of photometric reports.

Visual Perception
Is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision

Visual Spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm.

Visual Task 
The task associated with seeing; objects and details that must be seen to perform an activity.

Volt 
The standard unit of measurement for electrical potential. It defines the "force" or "pressure" of electricity.

Voltage 
A measurement of the electromotive force in an electrical circuit or device, expressed in volts. Voltage can be thought of as being analogous to the pressure in a waterline. Standard household voltage in United States measures 120 volts, except for heavy duty appliances at 240 volts. Low voltage for outdoor and some indoor applications are 12 or 24 volts. Commercial voltages are significantly higher.

 

W

Wall Washing
A technique that lights a wall fairly evenly from top to bottom with light grazing the surface without spilling or wasting light away from the wall into the room.

Watt (W)
The unit for measuring electrical power. It defines the rate of energy consumption by an electrical device when it is in operation. The energy cost of operating an electrical device is calculated as its wattage times the hours of use. (See Kilowatt Hour).

Watt-Hour
Unit of electric energy. One watthour is the amount of energy consumed at the rate of 1 watt during a 1-hour period. (See Kilowatt Hour).

Wattage
The active electrical power consumed by a device.

Wavelength
The distance between two neighboring crests of a traveling wave. Wavelengths of light are measured in nanometers (1 nm = 1 billionth of a meter, or 1 x meters) The wavelength of visible light is between 400 and 700 nanometers.

Work Plane 
The level at which work is done and at which illuminance is specified and measured. For office applications, this is typically a horizontal plane 30 inches above the floor (desk height).

 

Z

Zenith
An imaginary point directly above a particular location, as in directly above the luminaire (180 angle).

 

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