ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. compatible ballasts - An abbreviated list of common ballasts that will provide the necessary circuitry for a photosensor to operate correctly. Other ballasts may also be compatible; contact the photosensor manufacturer for details. continuous dimming - Control of a light source's intensity to practically any value within a given operating range. capacitor - A device used in electric circuitry to temporarily store electrical charge in the form of an electrostatic field. In lighting, a capacitor is used to smooth out alternating current from the power supply. time delay range - For motion sensors, the range of time that may be set for the interval between the last detected motion and the turning off of the lamps. lamp - A radiant light source. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) frequency - The number of cycles completed by a periodic wave in a given unit of time. Frequency is commonly reported in cycles per second, or hertz (Hz). electromagnetic interference (EMI) - The interference of unwanted electromagnetic signals with desirable signals. Electromagnetic interference may be transmitted in two ways: radiated through space or conducted by wiring. The Federal Communications Commission (FCC) sets electromagnetic interference limits on radio frequency (RF) lighting devices in FCC Part 18. electronic ballast - A ballast that uses electronic components instead of a magnetic core and coil to operate fluorescent lamps. Electronic ballasts operate lamps at 20 to 60 kHz, which results in reduced flicker and noise and increased efficacy compared with ballasts that operate lamps at 60 Hz. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. dimming ballast - A device that provides the ability to adjust light levels by reducing the lamp current. Most dimming ballasts are electronic. power - The power used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power for a lamp and ballast or driver combination. Power is typically reported in the SI units of watts. photosensor - A device used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. nadir - In the lighting discipline, nadir is the angle pointing directly downward from the luminaire, or 0°. Nadir is opposite the zenith. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels. hysteresis - The dependence of the output of a system not only on its current input, but also on its history of past inputs. The electric light level set by a photosensor with hysteresis, for a certain photocell input signal, depends on whether that photocell signal is increasing or decreasing. Hysteresis provides stable operation in switching photosensors but is undesirable in dimming photosensors. Glossary | Plasma Lighting Systems | Lighting Answers | NLPIP
    Volume 13 Issue 1
July 2015    

Glossary

Sources of term definitions: National Lighting Product Information Program (NLPIP), Lighting Research Center’s Lighting Education Online, the IEEE Standard Dictionary of Electrical and Electronics Terms (IEEE Std 100-1996).


A-lampThe incandescent lamp most commonly used in North American households. The "A" designation refers to the lamp's bulbous shape.

active powerthe system input power (in watts) for a lamp-ballast combination.

amalgamAn alloy of mercury with other metals. Some CFLs use a mercury amalgam rather than standard mercury. An amalgam keeps mercury pressure in the discharge near its optimal value as lamp temperature changes. Amalgam lamps can produce more than 90 percent of maximum light output over a wide temperature range, but they can take longer to reach their full light output when started.

ambient temperatureThe temperature of the surrounding air that comes into contact with the lamp and ballast. Ambient temperature affects the light output and active power of fluorescent lamp/ballast systems. Each fluorescent lamp-ballast system has an optimum ambient temperature at which it produces maximum light output. Higher or lower temperatures reduce light output. For purposes of lamp/ballast tests, ambient temperature is measured at a point no more than 1 meter (3.3 feet) from the lamp and at the same height as the lamp.

amplitudeThe maximum absolute value attained by a periodic wave.

ANSI codeAmerican National Standards Institute (ANSI) code that indicates the electrical operating designation of the lamp, which must match that of the ballast.

apertureThe diameter in the opening of a downlight, in inches (in.). Sometimes manufacturers will round up to the next whole-inch increment.

aperture diameterThe diameter of a reflector cone opening, expressed in inches.

apparent powerThe product of root-mean-square (rms) voltage and rms current.

applicationThe use to which a lighting system will be put; for example, a lamp may be intended for indoor residential applications.

arc tubeAn envelope, usually quartz or ceramic that contains the arc of a discharge light source.

average rated lifeThe number of hours at which half of a large group of product samples fail under standard test conditions. Rated life is a median value; any lamp or group of lamps may vary from the published rated life.

ballastA device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation.

ballast accessThe opening through which the ballast in a luminaire can be installed or replaced, either through the aperture or from above the luminaire.

ballast efficacy factor (BEF)Sometimes called ballast efficiency factor, ballast efficacy factor is the ratio of the ballast factor to the active power (in watts), usually expressed as a percent. It is used as a relative measurement of the system efficacy of the fluorescent lamp/ballast combination.

ballast factor (BF)The ratio of the light output of a fluorescent lamp or lamps operated on a ballast to the light output of the lamp(s) operated on a standard (reference) ballast. Ballast factor depends on both the ballast and the lamp type; a single ballast can have several ballast factors depending on lamp type.

ballast rated lifeThe number of hours at which half of a group of ballasts fail under standard test conditions. Rated life is a median value of life expectancy; any ballast, or group of ballasts, may vary from the published rated life.

barn doorsTypically, four adjustable shields that are attached to the face of the luminaire to reduce glare.

beam angleThe angle at which luminous intensity is 50 percent of the maximum intensity.

beam appearanceThe description of a beam’s image on a wall as determined by subjective visual evaluations.

beam appearanceThe description of the beam's image on a wall as determined by subjective visual evaluations of each lamp. The descriptive categories used are smooth, cloud, two-contour, ripple, and variegated.

beam spreadThe 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. Common beam spreads are known as spot, narrow, narrow flood, and flood.

bi-level switchingControl of light source intensity at two discrete levels in addition to off.

binTo sort or classify light sources (such as light emitting diodes) into groups according to their luminous intensity or color appearance.

blackbody radiatorA temperature radiator of uniform temperature whose radiant output in all parts of the spectrum is the maximum obtainable from any temperature radiator at the same temperature. Such a radiator is called a blackbody because it absorbs all the radiant energy that falls upon it. All other temperature radiators can be classed as non-blackbodies. Non-blackbodies radiate less in some or all wavelength intervals than a blackbody of the same size and the same temperature.

Brewster's angleThe incident angle of light on the surface of a medium for which the reflected and transmitted light are perpendicular to each other. This angle depends on the refractive index of the medium. It defines the angle of maximum polarization for a medium.

brownout circuitryFor exit signs, brownout circuitry is designed to switch the sign over to battery supply if the voltage of the utility-supplied power drops below a specified value. Brownout circuitry is an option for some signs.

bulb designationAn abbreviation of the shape and size of a lamp's outer envelope. The letter or letters indicate the shape and the numbers indicate the bulb's maximum diameter in eighths of an inch.

bulb finishThe coating, if any, that is applied to the inside surface of the bulb. Finishes are either clear, phosphor coated, or diffuse.

candelaThe Systeme International d'Unities (SI) of luminous intensity. One candela is one lumen per steradian. Formerly, candle.

capacitorA device used in electric circuitry to temporarily store electrical charge in the form of an electrostatic field. In lighting, a capacitor is used to smooth out alternating current from the power supply.

cathode-disconnect ballastAn electromagnetic ballast that disconnects the electrode-heating circuit after the lamps are started. Cathode-disconnect ballasts operate lamps at 60 hertz; they are sometimes called "hybrid" or "low-frequency electronic" ballasts. They operate lamps at lower power than other magnetic ballasts that produce similar light output.

center beam candlepower (CBCP)Center beam candlepower is the luminous intensity at the center of a beam, expressed in candelas (cd).

chromaticity The dominant or complementary wavelength and purity aspects of the color taken together, or of the aspects specified by the chromaticity coordinates of the color taken together. It describes the properties of light related to hue and saturation, but not luminance (brightness).

CIEAbbreviated as CIE from its French title Commission Internationale de l'Eclairage, the International Commission on Illumination is a technical, scientific, and cultural organization devoted to international cooperation and exchange of information among its member countries on matters relating to the science and art of lighting.

coefficient of utilization (CU)Coefficient of utilization is the ratio of the luminous flux (lumens) received on a plane to the light output (lumens) of the lamps. Coefficient of utilization depends on luminaire efficiency, distribution of light from the luminaire, size and shape of the room, and reflectances of surfaces in the room. Specifiers use the coefficient of utilization to evaluate how effectively a luminaire delivers light to a workplane.

color appearanceThe resultant color perception that includes the effects of spectrum, background contrast, chromatic adaptation, color constancy, brightness, size and saturation.

color consistencyThe measure of how close in color appearance random samples of a lamp or source tend to be.

color matchingThe action of making a color appear the same as a given color. Often used as a method of evaluating the ability of a light source to render colors faithfully.

color renderingA general expression for the effect of a light source on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference light source.

color rendering index (CRI)A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural.

color rendering index (CRI)A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural.

color shiftThe change in a lamp’s correlated color temperature (CCT) at 40% of the lamp’s rated life, in kelvin (K).

color stabilityThe ability of a lamp or light source to maintain its color rendering and color appearance properties over its life. The color properties of some discharge light sources may tend to shift over the life of the lamp.

color variationLamps of the same type made by the same manufacturer may exhibit a certain degree of variation in color, even when operated under the same conditions and seasoned for the same about of time.

combined uncertaintyCombined uncertainty is calculated by finding the sum of the squares of sample random variability (standard deviation) and laboratory measurement uncertainty and taking the square root of that sum.

compact fluorescent lamp (CFL)A family of single-ended fluorescent-discharge light sources with small-diameter [16-millimeter (5/8-inch) or less] tubes.

compatible ballastsAn abbreviated list of common ballasts that will provide the necessary circuitry for a photosensor to operate correctly. Other ballasts may also be compatible; contact the photosensor manufacturer for details.

conductionThe process of removing heat from an object via physical contact with other objects or materials, usually metals.

constant-wattage autotransformer (CWA)The most common type of ballast used for HID lamps, it maintains a constant power (wattage) supply to the lamp when system input voltage fluctuates.

continuous dimmingControl of a light source's intensity to practically any value within a given operating range.

continuously variable signalA signal that communicates data that can have a theoretically unlimited number of possible values between two end points. Examples include voltage, temperature, and illuminance.

contrastAlso known as luminance contrast, it is the relationship between the luminances of an object and its immediate background.

control signal rangeThe range of the electrical signal (in volts) that a control device uses to signal the dimming level to a ballast.

convectionThe process of removing heat from an object through the surrounding air.

correlated color temperature (CCT)A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT.

cosine distributionA property of a light source such that its luminous intensity in a particular direction is proportional to the cosine of the angle from the normal to the source.

CSACanadian Standards Association.

current crest factor (CCF)Defined as the peak current divided by the root-mean-square (rms) current of a lamp. Current crest factor ranges from 1 to infinity. ANSI requires current crest factor to be less than 1.7. Lamp manufacturers usually will not warranty their lamps when operated on a ballast having a current crest factor greater than 1.7.

current THDA measure of the degree to which the current waveform deviates from sinusoidal, expressed as a percentage. See total harmonic distortion (THD).

cutoff angleThe angle of light distribution from a luminaire, measured upward from nadir, between the vertical axis and the first line at which the bare source (lamp) is not visible.

cutoff classificationThe classification system of the Illuminating Engineering Society of North America (IESNA) that describes the light distribution of anoutdoor luminiare. Cutoff classifications define the luminous intensity limits in two illumination zones that occur within the range of 80° to 180° above nadir. North America (IESNA) that describes the light distribution of an outdoor luminaire. Cutoff classifications define the luminous intensity limits in two illumination zones that occur within the range of 80° to 180° above nadir.

cutoff luminaireIESNA classification that describes a luminaire having a light distribution in which the candela per 1000 lamp lumens does not numerically exceed 25 (2.5%) at or above an angle of 90° above nadir, and 100 (10%) at or above a vertical angle of 80° above nadir. This applies to all lateral angles around the luminaire.

degree of polarizationA measure of the amount of light polarization ranging from 0 to 100 percent.

dichroic coating (dichroic filter)A multi-layer coating that transmits certain wavelengths and reflects those not transmitted.

diffuser materialDiffusers scatter the light from a luminaire in all directions. Most diffusers in commodity residential-grade luminaires are made of plastic, usually acrylic or polycarbonate. Other materials include glass and alabaster.

dimming ballastA device that provides the ability to adjust light levels by reducing the lamp current. Most dimming ballasts are electronic.

direct digital control (DDC)The technology used by the components of a distributed control system. Direct digital control modules exchange digitally encoded signals with each other, indicating the status of devices connected to the network and executing commands when appropriate. Each module contains a programmable microprocessor, hardware for at least one type of network connection, and some means of detecting or changing a device's status.

direct lightLight emitted by a luminaire in the general direction of the task to be illuminated. The term usually refers to light emitted in a downward direction.

direct luminaireA luminaire that emits light in the general direction of the task to be illuminated. The term usually refers to luminaires that emit light in a downward direction.

direct uplightLight emitted upward by a luminaire.

disability glareA type of glare that causes a loss of visibility from stray light being scattered within the eye.

discomfort glareThe sensation of annoyance or even pain induced by overly bright sources.

distributed control systemA control system in which the computing hardware and software are contained in a network of control modules or multi-circuit control panels physically distributed throughout the facility.

driverFor light emitting diodes, a device that regulates the voltage and current powering the source.

dynamic outdoor lightingOutdoor lighting that varies light level or other characteristics automatically and precisely in response to factors such as vacancy or the type of use of an outdoor location.

efficacyThe ratio of light output (in lumens) to input power (in watts), expressed as lumens per watt (LPW).

efficacyThe ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt.

electrode preheat currentThe current flowing through a lamp's electrodes to heat them during starting.

electrodesThe structure that serves as the electric terminals at each end of electric discharge lamps.

electromagnetic interference (EMI)The interference of unwanted electromagnetic signals with desirable signals. Electromagnetic interference may be transmitted in two ways: radiated through space or conducted by wiring. The Federal Communications Commission (FCC) sets electromagnetic interference limits on radio frequency (RF) lighting devices in FCC Part 18.

electromagnetic waveA wave composed of perpendicular electric and magnetic fields. The wave propagates in a direction perpendicular to both fields.

electronic ballastA ballast that uses electronic components instead of a magnetic core and coil to operate fluorescent lamps. Electronic ballasts operate lamps at 20 to 60 kHz, which results in reduced flicker and noise and increased efficacy compared with ballasts that operate lamps at 60 Hz.

emergency optionsRefers to options available when exit signs are operated on a non-utility power supply such as a generator, a central battery unit that operates several exit signs, or an individual rechargeable battery. Options include whether or not the exit sign increases the brightness of the light source if the utility-supplied power fails.

field of viewThe area covered by an occupancy sensor, often reported (for wall-mounted sensors) as a horizontal field of view or (for ceiling-mounted sensors) as the solid angle of the cone-shaped coverage area.

filterA device that allows currents at certain frequencies to pass while those at other frequencies are blocked. Filters reduce conducted electromagnetic waves by grounding the current or by increasing the impedance to a specific frequency.

fixtureA complete lighting unit consisting of lamp or lamps and the parts designed to distribute the light, position and protect the lamp(s), and connect the lamp(s) to the power supply. (Also referred to as luminaire.)

flickerA rapid and continuous change in light levels caused by the modulation of the light output from fluorescent lamps.

fluorescent lampA low-pressure mercury electric-discharge lamp in which a phosphor coating on the inside of the glass tubing transforms most of the ultraviolet energy created inside the lamp into visible light.

footcandle (fc)A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent.

frequencyThe number of cycles completed by a periodic wave in a given unit of time. Frequency is commonly reported in cycles per second, or hertz (Hz).

full cutoff luminaireIESNA classification that describes a luminaire having a light distribution in which zero candela intensity occurs at or above an angle of 90° above nadir. Additionally, the candela per 1000 lamp lumens does not numerically exceed 100 (10%) at or above a vertical angle of 80° above nadir. This applies to all lateral angles around the luminaire.

full-spectrum color index (FSCI)A mathematical transformation of full-spectrum index into a zero to 100 scale, where the resulting values are directly comparable to color rendering index. An equal energy spectrum is defined as having an FSCI value of 100, a “standard warm white” fluorescent lamp has an FSCI value of 50, and a monochromatic light source (e.g., low pressure sodium) has an FSCI value of 0.

full-spectrum index (FSI)A mathematical measure of how much a light source's spectrum deviates from an equal energy spectrum, based on the slope of its cumulative spectrum.

fully shielded luminaireA luminaire that emits no direct uplight, but which has no limitation on the intensity in the region between 80° and 90°.

fundamentalThe component of a periodic wave that has the lowest frequency. It is also called the first-order harmonic.

gamut areaA measure of color rendering based upon volume in color space. It is the range of colors achievable on a given color reproduction medium (or present in an image on that medium) under a given set of viewing conditions.

gas-discharge lampsAn electric lamp that produces light from gas atoms excited by an electric current.

glareThe sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility.

glow currentThe flow of electrons away from a rapid-start lamp's electrodes during preheating. The higher the glow current, the faster the electrodes' emissive coating degrades, increasing lamp-end darkening and reducing lamp life.

gridThe combination of electric power plants and transmission lines operated by an electric utility.

groundedA circuit or metal object that is connected to the earth at one or more points. Done mostly for safety, grounding also reduces electromagnetic waves.

halo-phosphorsAlso referred to as halophosphates. Phosphors are the white powder inside fluorescent lamps that fluoresces (emits visible light) when excited by the ultraviolet radiation produced by the mercury vapor that is energized by the electric arc sustained inside the lamp. Phosphors are used to achieve high efficacy, good color rendering, and low lamp lumen depreciation. Halo-phosphors, however, are limited in their ability to provide a high color rendering index without sacrificing light output and are often mixed with other phosphors.

halogen cycleHalogen incandescent lamps are in the same family as standard incandescent lamps. The basic operating principle is the same, except that chemicals called halogens are introduced in the gas fill. When electricity passes through the lamp's filament, it is heated until it glows and emits light. In this process, tungsten from the filament evaporates and, over the life of the lamp, causes the glass bulb wall to slowly blacken and the filament to disintegrate until the lamp fails. Halogens remove evaporated tungsten from the glass wall and redeposit it back onto the filament. As a result, tungsten does not build up on the bulb, so the light output does not degrade as rapidly.

halogen lampAn incandescent lamp that uses a halogen fill gas. Halogen lamps have higher rated efficacies and longer lives than standard incandescent A-lamps.

halophosphatesThe class of phosphors commonly used in fluorescent lamps. Halophosphates are limited in their ability to provide a high color rendering index without sacrificing light output. Standard T12 lamps containing halophosphates are the most common and least expensive fluorescent lamps, but United States federal regulations require that all fluorescent lamps must meet minimum efficacy and CRI standards, and 40-watt T12 halophosphate lamps do not meet these standards. T8 lamps usually contain both halophosphates and rare-earth phosphors.

harmonicFor a distorted waveform, a component of the wave with a frequency that is an integer multiple of the fundamental

harmonic distortionDistorted waveshapes contain components with frequencies that are multiples of the fundamental frequency. These higher frequency components are known as harmonics.

harmonicsDistortions of a periodic sinusoidal waveform represented as a harmonic series of sinusoidal waveforms of different amplitude and phase. A harmonic series is a group of different frequency waveforms that are multiples of the lowest or fundamental frequency.

heat sinkingAdding a material, usually metal, adjacent to an object in order to cool it through conduction.

high-intensity discharge (HID)An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps.

high-pressure sodium (HPS)A high-intensity discharge lamp type that uses sodium under high pressure as the primary light-producing element. HPS lamps produce light with a correlated color temperature (CCT) of approximately 2000 kelvins, although CCTs for lamps having higher CRI values range from 2200 to 2700 kelvins. Standard lamps have a CRI value of 22; others have CRI values from 60 to 80. HPS lamps are among the most efficacious light sources, with efficacies as high as 150 lumens per watt, although those with higher CRI values have efficacies as low as 25 lumens per watt.

high-wattage compact fluorescent lampAbbreviated as HW-CFL, sometimes called "high lumen CFLs", these lamps are a larger cousin to regular CFLs, usually much larger in size and with higher wattages and light output.

horizontal illuminanceThe average density of luminous flux incident on a horizontal surface, measured in footcandles (fc) or lux (lx). One fc equals 10.76 lx.

horizontal rotation rangeThe total angular horizontal rotation of the lamp-reflector assembly.

hueThe attribute of a light source or illuminated object that determines whether it is red, yellow, green, blue, or the like.

hysteresisThe dependence of the output of a system not only on its current input, but also on its history of past inputs. The electric light level set by a photosensor with hysteresis, for a certain photocell input signal, depends on whether that photocell signal is increasing or decreasing. Hysteresis provides stable operation in switching photosensors but is undesirable in dimming photosensors.

ignitorA device, either by itself or in association with other components, that generates voltage pulses to start discharge lamps.

illuminanceThe amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent.

illuminanceThe density of luminous flux incident upon a surface. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux.

illuminationThe process of using light to see objects at a particular location.

impedanceA measure of the total opposition to current flow in an alternating current circuit. The unit of impedance is the ohm Ω .

incident angleThe angle between a ray of light reaching a surface and a line normal (perpendicular) to that surface.

indicationThe process of using a light source as something to be seen as in signaling.

indirect lightingLight arriving at a surface after reflecting from one or more surfaces (usually walls and/or ceilings) that are not part of the luminaire.

infrared radiationAny radiant energy within the wavelength range of 770 to 106 nanometers is considered infrared energy. (1 nanometer = 1 billionth of a meter, or 1 X 10-9 m).

initial light outputA lamp's light output, in lumens, after 100 hours of seasoning.

instant startA method of starting fluorescent lamps in which the voltage that is applied across the electrodes to strike the electric arc is up to twice as high as it is with other starting methods. The higher voltage is necessary because the electrodes are not heated prior to starting. This method starts the lamps without flashing. It is more energy efficient than rapid or preheat starting, but results in greater wear on the electrodes during starting. The life of instant-start lamps that are switched on and off frequently may be reduced by as much as 25 percent relative to rapid-start operation. However, for longer burning cycles (such as 12 hours per start), there may be no difference in lamp life for different starting methods.

intensity (luminous intensity)Total luminous flux within a given solid angle, in units of candelas, or lumens per steradian.

interoperabilityThe ability to communicate such information as temperature, illuminance levels, status of security devices, and occupancy among building systems and their controls.

inverterAlso known as “power inverter.” A device used to convert direct current (dc) electricity into alternating (ac) current.

irradianceThe density of radiant flux incident on a surface.

isotemperatureA set of coordinates within which all points have the same temperature. In a color space diagram, isotemperature lines represent lights with identical correlated color temperatures.

junction temperatureFor light emitting diodes, the temperature of the light-emitting portion of the device (see PN junction), which is inversely correlated with its light output.

KelvinColor temperature is measured in degrees Kelvin, which indicathe hue of a specific type of light source. Higher temperatures indicate whiter, "cooler" colors, while lower temperatures indicate yellower, "warmer" colors.

lampA radiant light source.

lamp base positionThe location of the lamp socket, either in the center of the top of the ballast or on the side of the ballast. Modular ballasts for circular compact fluorescent lamps (CFLs) have a lamp socket located at the end of a wiring harness.

lamp currentThe current flowing between a lamp's electrodes during operation.

lamp efficacyThe ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt (LPW).

lamp electrode voltageVoltage to the electrodes to operate a lamp.

lamp envelopeThe shape of either the bare lamp or the capsule surrounding the lamp. Common shapes include quad, triple tube, four-tube, coiled tube, A-line, circular, square, globe, capsule (bullet), reflector, and decorative.

lamp lifeThe median life span of a very large number of lamps (also known as the average rated life). Half of the lamps in a sample are likely to fail before the rated lamp life, and half are likely to survive beyond the rated lamp life. For discharge light sources, such as fluorescent and HID lamps, lamp life depends on the number of starts and the duration of the operating cycle each time the lamp is started.

lamp lifeThe number of hours at which half of a large group of lamps have failed when operated under standard testing conditions.

lamp lumen depreciation (LLD)The reduction in lamp light output that progressively occurs during lamp life.

lamp operating currentCurrent flowing through a lamp during normal operation.

lamp quantity and typeThe number of lamps (in parentheses) used by the luminaire, followed by a generic designation indicating the type.

lamp rated lifeThe number of operating hours at which half of a large group of product samples are expected to fail. The rated life is a median value of life expectancy; individual lamp life may vary considerably from the published rated life and operating conditions (e.g., temperature, hours per start) may affect actual life because rated life is based on standard test conditions. In addition, the way a product fails can vary by technology. For example, incandescent lamps abruptly stop producing any light while LEDs are considered to have failed when their light output drops below a certain fraction of the initial level.

lamp shield typeThe material used in a luminaire to shield the lamp from the environment. Lamp shields are required by Underwriters Laboratories for some lamp types.

lamp starting currentCurrent flowing through a lamp during starting operation.

light lossThe reduced light output caused by a circuit-level power reducer expressed as a percentage of the light output without the circuit-level power reducer. (Full system output minus reduced output with a lighting-circuit power reducer divided by the full system output times 100.)

light pollutionAn unwanted consequence of outdoor lighting that includes such effects as sky glow, light trespass, and glare.

light power density (LPD)Sometimes referred to as power density. A measurement of the ratio of light output in an area and the electric power used to produce that light. LPD is determined by dividing the total light output by the total wattage consumed and is measured in lumens per watt.

light trespassA undesirable condition in which exterior light is cast where it is not wanted.

light-emitting diode (LED)A solid-state electronic device formed by a junction of P- and N-type semiconductor material that emits light when electric current passes through it. LED commonly refers to either the semiconductor by itself, i.e. the chip, or the entire lamp package including the chip, electrical leads, optics and encasement.

line voltageThe voltage supplied by the electric power infrastructure, typically 110-120 Vac at 60 Hz for homes in North America.

load capacityThe maximum total power that can be connected to an occupancy sensor.

load sheddingThe practice of turning off electrical devices during peak energy demand hours to reduce building energy use.

louverA fixed shield, usually divided into small cells, that is attached to the face of a luminaire to reduce direct glare.

low battery voltage disconnectIndicates whether or not an exit sign has a circuit that is designed to disconnect the battery after it is discharged. This circuit prevents damage to the battery. Lead acid and lead calcium batteries need this circuit, but nickel cadmium batteries do not.

low-voltage circuit protectionProtection for a ballast’s low-voltage control circuit from high voltage spikes. Does not apply to high-voltage controls.

lumen (lm)A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation.

lumen (lm)A unit measurement of the rate at which a lamp produces light. A lamp's lumen output rating expresses the total amount of light the lamp emits in all directions per unit time.

lumen depreciationThe decrease in lumen output that occurs as a lamp is operated, until failure. Also referred to as lamp lumen depreciation (LLD).

lumen maintenanceThe lumens produced by a light source at any given time during its operating life as a percentage of its lumens at the beginning of life.

lumen maintenanceThe ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1.

luminaireA complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.)

luminaire angleThe vertical (altitude) angle used in luminaire photometry to express the direction of the light output being measured. Light coming straight down is at 0° (the nadir).

luminaire efficacyThe ratio of the measured light output of a luminaire to its active power, expressed in lumens per watt (LPW).

luminaire efficiencyThe ratio, expressed as a percentage, of the light output of a luminaire to the light output of the luminaire's lamp(s). Luminaire efficiency accounts for the optical and thermal effects that occur within the luminaire under standard test conditions.

luminanceThe photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (square feet or square meter).

luminance contrastLuminance contrast quantifies the relative brightness of an object against its background. It can range from zero and one. The closer the luminance contrast is to one, the greater the relative brightness of the object against its background.

luminous fluxLuminous radiant power, measured in lumens. The overall light output of a lamp or luminaire.

luminous intensityThe luminous flux on a small surface centered on and normal to the direction divided by the solid angle (in steradians) that the surface subtends at the source. Luminous intensity can be expressed in candelas or in lumens per steradian.

lux (lx)A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle.

MacAdam ellipseResearcher David L. MacAdam showed that a just noticeable difference (JND) in the colors of two lights placed side-by-side was about three times the standard deviation associated with making color matches between a reference light and a test light (MacAdam 1942, Wyszecki and Stiles 1982). These JNDs form an elliptical pattern of "constant discriminability" in a chromaticity space, centered on the chromaticity of a reference light, known as MacAdam ellipse.

maximum ambient temperatureThe maximum ambient temperature for which a compact fluroescent lamp (CFL) product is warranted to achieve rated life.

maximum ballast case temperatureThe maximum temperature of the ballast case for which the manufacturer’s life rating is valid.

maximum relative light outputIlluminance measured at a fixed distance from the lamps.

mean light outputLight output typically evaluated at 40% of rated lamp life. In combination with initial light output, mean light output may be used to estimate lamp lumen depreciation.

medium bi-pinA type of connector commonly used on T-8 and T-12 fluorescent lamps. Two small pins protrude from the lamp ends, which are inserted into a socket in the fixture.

mercury vapor (MV) lampA high-intensity discharge lamp type that uses mercury as the primary light-producing element. Mercury vapor lamps produce light with a CCT from 3000 to 7000 K. Mercury vapor lamps with clear outer bulbs have CRI values from 15 to 25, whereas phosphor-coated lamps have CRI values from 40 to 55. Mercury vapor lamps are less efficacious than other HID lamp types, typically producing only 30 to 65 LPW, but they have longer lamp lives and lower initial costs than other HID lamp types.

metal halide (MH) lampA high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements. Metal halide lamps have better color properties than other HID lamp types because the different additives produce more visible wavelengths, resulting in a more complete spectrum. Metal halide lamps are available with CCTs from 2300 to 5400 K and with CRI values from 60 to 93. Efficacies of metal halide lamps typically range from 75 to 125 LPW.

metal halide lampA high-intensity discharge (HID) lamp that uses mercury and several halide additives as light-producing elements. Metal halide lamps have better color properties than other HID lamp types because the different additives produce light distributed over more visible wavelengths, resulting in a more complete spectrum. Metal halide lamps are available with CCTs from 2300 to 5400 K and with CRI values from 60 to 93. Efficacies of metal halide lamps typically range from 75 to 125 LPW.

metamersLights of the same color but of different spectral power distribution.

miniature bi-pinA type of connector commonly used on T-5 lamps. Similar in design to but smaller than medium bi-pin connectors, it uses two small pins that protrude from the lamp ends and are inserted into a fixture socket.

minimal erythema dose (MED)The quantity of ultraviolet radiation (expressed in Joules per square meter) required to produce the first perceptible, redness reaction on human skin with clearly defined borders. MED can vary significantly depending on factors such as skin pigmentation.

minimum ambient temperatureThe minimum temperature at which a compact fluorescent lamp (CFL) product is warranted to start.

minimum bulb wall temperature (MBWT)The temperature of the coldest spot on a lamp’s bulb wall. MBWT is determined by the ambient temperature, the heat generated within the luminaire, and the luminaire’s heat dissipation effectiveness. The coldest spot on a lamp wall is where the mercury vapor tends to condense because pressure is lowest there.

minimum dimmed levelThe lowest dimmed level achieved by a ballast, expressed as a percentage of that ballast’s maximum light output.

minimum load requirementThe minimum power required for an occupancy sensor to operate properly.

minimum required efficacyThe minimum lamp efficacy required by EPACT, expressed in lumens per watt (LPW).

minimum starting temperatureThe minimum ambient temperature at which a ballast will reliably start fluorescent lamps.

monochromaticFor light, consisting of a single wavelength and having a very saturated color.

multitapA passive distribution component composed of a directional coupler and a splitter with two or more output connections.

nadirIn the lighting discipline, nadir is the angle pointing directly downward from the luminaire, or 0°. Nadir is opposite the zenith.

noncutoff luminaireIESNA classification that describes a luminaire light distribution in which there is no candela limitation in the zone above maximum candela. (See also cutoff classification and cutoff angle.)

open-circuit voltageThe voltage applied across the output terminals of a ballast when no load is connected. Open-circuit voltage is the voltage applied across a lamp circuit to start the lamp. After starting, the voltage rapidly decreases and stabilizes at the operating voltage.

operating cycleThe frequency with which lamps are cycled on and off.

operating electrode voltageThe voltage that a ballast supplies to a lamp's electrodes.

operating positionThe manufacturer-recommended operating position for a lamp.

operating voltageThe voltage a ballast supplies to a lamp's electrodes.

PAR lampAn incandescent or tungsten-halogen incandescent lamp with a hard glass bulb and an interior reflecting surface, a precisely placed filament, and a lens to control beam spread. The lens is hermetically sealed to the reflector. Metal halide PAR-lamps are also now available.

pendant mountingA suspension device between a mount and a luminaire.

phase displacementThe extent to which voltage and current waveforms are out of synchronous phase with one another. Current lags or leads voltage, depending on whether the current waveform crosses a reference point after or before the voltage waveform, respectively. Phase displacement can be expressed as a unit of time, as a fraction of the period, or as an angle in degrees with one period corresponding to 360 degrees. When voltage and current are synchronized, phase displacement is zero.

phosphorsMaterials used in a light source to produce or modify its spectral emission distribution. In fluorescent and high intensity discharge lamps, the phosphors fluoresce (emit visible light) when excited by ultraviolet radiation produced by mercury vapor inside the lamp when energized by an electric arc. In a light emitting diode, phosphors convert short-wavelength light or ultraviolet radiation produced by a semiconductor die into longer-wavelength light, usually with the goal of producing white illumination.

photonA small bundle or quantum of electromagnetic energy, including light.

photopicVision mediated essentially or exclusively by the cones. It is generally associated with adaptation to a luminance of at least 3.4 cd/m2.

photosensorA device used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient.

photovoltaic (PV)Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels.

PN junctionFor light emitting diodes, the portion of the device where positive and negative charges combine to produce light.

polarized lightLight whose vibrations are oriented in (or around, for partially polarized light) a specific plane.

position factorThe light output of the lamp in a certain position divided by the light output of the lamp in the base-up position.

positive affectRelatively mild shifts in current mood in a positive direction.

powerThe power used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power for a lamp and ballast or driver combination. Power is typically reported in the SI units of watts.

power factor (PF)The ratio of active power (in watts) to apparent power (in rms volt-amperes), power factor is a measure of how effectively an electric load converts power into useful work. Power factor (PF) is calculated using the equation PF = (active power) / [(rms voltage) x (rms current)]. Phase displacement and current distortion both reduce power factor. A power factor of 0.9 or greater indicates a high power factor ballast.

power line carrier (PLC)A system that transmits high-frequency (50 to 500 kHz) analog or digital signals via the power lines of a building. These signals control devices such as luminaires or contain voice transmissions such as intercom messages. Some commercial and residential energy management systems also use power line carrier systems.

power qualityThe degree to which current and voltage wave forms conform to a sinusoidal shape and are in synchronous phase with each other. Poor power quality results when the wave forms are distorted and/or out of phase and can interfere with data communications, cause inefficient operation or failure of other electrical equipment on the same supply line, and result in excessive current in electrical distribution lines.

power reduction efficiency factorA measure of the efficiency of a power reducer, representing the reduced light output in percent from a lighting-circuit power reducer divided by the reduced active power in percent from a lighting circuit power reducer.

preheatA method of starting fluorescent lamps in which the electrodes are heated before a switch opens to allow a starting voltage to be applied across the lamp. With preheat starting, the lamp flashes on and off for a few seconds before staying lit because several starting attempts may be necessary to establish the electric arc across the lamp electrodes. Often, the luminaire's start button must be held down until the lamp lights. Preheat ballasts are less energy efficient than rapid-start or instant-start ballasts.

preheat timeFor rapid-start lamps, the time from the onset of lamp current to the lamp arc's striking, during which the lamp electrodes are heated to ease starting.

preheating timeAlso referred to as preheat time and lamp preheat time. The length of time that a ballast heats a lamp’s electrodes before initiating the lamp arc. Rapid start ballasts preheat a lamp before initiating the arc in order to ease starting. Too short or too l

primaryAny one of three lights in terms of which a color is specified by giving the amount of each required to match it by additive combination.

prismatic lensAn optical component of a luminaire that is used to distribute the emitted light. It is usually a sheet of plastic with a pattern of pyramid-shaped refracting prisms on one side. Most ceiling-mounted luminaires in commercial buildings use prismatic lenses.

programmed startRefers to a type of rapid start ballast that optimizes the starting process by waiting until the lamp’s electrodes have been heated to apply the starting voltage, thus easing the load to the electrode and extending lamp life. Standard rapid start ballasts heat the electrodes during the starting process to allow quicker starting without flicker.

pulse-width modulationOperating a light source by very rapidly (faster than can be detected visually) switching it on and off to achieve intermediate values of average light output; the frequency and the duty cycle (percentage of time the source is switched on) are important parameters in the modulation.

R lampA common reflector 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.

rapid startA method of starting fluorescent lamps in which the electrodes are heated prior to starting, using a starter that is an integral part of the ballast. Heating the electrodes before starting the lamps reduces the voltage required to strike the electric arc between the electrodes. A rapid-start system starts smoothly, without flashing.

rare-earth phosphorsA group of phosphors containing rare-earth elements. Rare-earth phosphors are used in fluorescent lamps to achieve high efficacy and better color rendering. They produce light in very narrow wavelength bands.

rated average lamp lifeAlso referred to as lamp rated life. Lamps are tested in controlled settings and the point at which 50% of a given sample burns out is listed as the lamps’ rated average lamp life.

rated lamp lifeThe number of hours at which half of a group of product samples fail. The rated life is a median value of life expectancy; any lamp or group of lamps may vary from the published rated life. Rated life is based on standard test conditions.

rated light outputThe sum of the initial rated lamp lumens of the lamp(s) that were supplied with the luminaire.

rated light output from lamp(s)The sum of the initial rated lamp lumens of the lamp(s) that were supplied with the luminaire.

rated lumenAlso referred to as rated light output from lamp in lumens. Lumen refers to a unit measurement of the rate at which a lamp produces light. A lamp’s light output rating expresses the total amount of light emitted in all directions per unit time. Manufacturers rate their lamps’ initial light output after 100 hours of operation.

RE70Designation referring to lamps that use rare-earth phosphors and have color-rendering index values of 70-79.

RE80Designation referring to lamps that use rare-earth phosphors and have color-rendering index values of 80-89.

RE80 HLO, LLAn RE80 lamp with additional enhancements of high light output (HLO) and/or long life (LL).

RE90Designation referring to lamps that use rare-earth phosphors and have color-rendering index values equal to or greater than 90.

reactive powerPower that creates no useful work. It results when current is not in phase with voltage. It is calculated using the equation reactive power = V x A x sin(q) where q is the phase displacement angle.

reflectanceA measure of the ability of an object to reflect or absorb light, expressed as a unitless value between 0 and 1. A perfectly dark object has a reflectance of 0, and a perfectly white object has a reflectance of 1.

relative beam diameter (manufacturer)The normalized beam diameter based on manufacturer-supplied beam angles.

relative beam diameter (NLPIP)The normalized beam diameter based on NLPIP-measured values.

relative CBCP (manufacturer)The normalized center beam candlepower based on manufacturer-supplied values.

relative system efficacyThe ratio of relative light output (RLO) to system active power. For each lamp type, relative system efficacy is normalized to the highest value at the maximum light output level, which is assigned a relative system efficacy value of 100%.

restrike timeThe time required for a lamp to restrike, or start, and to return to 90% of its stabilized light output after the lamp is extinguished. Normally, HID lamps need to cool before they can be restarted.

rms currentRoot-mean-square current, a value that quantifies the magnitude of a current that varies with time (as in ac circuits). Rms current is calculated as the square root of the squared values of current over one complete cycle. Rms current delivers the same power to a resistive load as an equivalent steady dc current.

root-mean-square (rms)The effective average value of a periodic quantity such as an alternating current or voltage wave, calculated by averaging the squared values of the amplitude over one period and taking the square root of that average.

semiconductorA material whose electrical conductivity is between that of a conductor and an insulator; the conductivity of most semiconductors is temperature dependent.

semicutoff luminaireIESNA classification that describes a luminaire light distribution in which the candela per 1000 lamp lumens does not numerically exceed 50 (5%) at or above an angle of 90° above nadir, and 200 (20%) at or above a vertical angle of 80° above nadir. This applies to all lateral angles around the luminaire.

sensitivity adjustmentA trim potentiometer (sometimes called a "trim pot") or a set of dip switches used to refine the response function of a photosensor. Some photosensors include a remote trim pot that allows for adjustment at a distance from the photosensor housing.

shieldingBlocking an electric or magnetic field with a metallic substance. The incident field induces currents in the metallic substance, and these currents induce a field that opposes the incident field. Shielding reduces radiated electromagnetic waves. Electronic components, wires, lamps, and devices can all be shielded.

sky glowBrightening of the sky caused by outdoor lighting and natural atmospheric and celestial factors.

skylightA device similar to a window that is placed in a roof, allowing sunlight to enter a structure, thus reducing the need for electric lighting. Skylights can be used to reduce peak load demand by taking advantage of sunlight during the peak demand time of the day.

sound ratingMagnetic ballasts sometimes produce a humming noise caused by vibration of the magnetic core. Electronic ballasts operate at high frequencies and are usually less noisy. Ballasts are rated from “A” to “F” based on their noise levels. Ratings define the range of ambient sound levels in which people will not notice the ballast noise. The higher the rating, the more noise that will be required to mask the ballast hum.

spectral power distribution (SPD)A representation of the radiant power emitted by a light source as a function of wavelength.

specular angleThe reflected angle of light striking a surface, which is equal to and in the same plane as the incident angle.

specular reflectionLight incident on a surface that is redirected at the specular angle. Glossy or shiny surfaces exhibit a high degree of specular reflection.

spill lightLight that falls outside of the area intended to be lighted.

standard deviationA measure of the average distance of a set of data points from their mean. A set of data points that are all close to their mean will have a smaller standard deviation than a set of points that are further from their mean.

starting methodThe method a ballast uses to start a lamp. For compact fluorescent lamps (CFLs), ballasts use one of three methods: preheat, instant start, or rapid start. Dimming electronic ballasts use one of these starting methods: rapid start, programmed start, or controlled rapid start.

starting timeThe time it takes the lamp to start from the point at which voltage is applied to the lamp until stable operation.

starting voltageThe voltage applied across the lamp during starting.

steradian (sr)A unit of measure equal to the solid angle subtended at the center of a sphere by an area on the surface of the sphere equal to the square of the sphere radius.

substrateFor light emitting diodes, the material on which the devices are constructed.

supply voltageThe voltage, usually direct, applied by an external source to the circuit of an electrode.

system efficacyAlso referred to as relative system efficacy, system efficacy is a measurement of a system'’s ability to convert electricity into light. Measured in lumens per watt (LPW), system efficacy is the ratio of the light output (in lumens) to the active power (in watts).

time delay rangeFor motion sensors, the range of time that may be set for the interval between the last detected motion and the turning off of the lamps.

total harmonic distortion (THD)A measure of the degree to which a sinusoidal wave shape is distorted by harmonics, with higher values of THD indicating greater distortion.

track head diameterThe size of the luminaire used in a track lighting system.

track luminaire optionsAccessories available for track luminaires.

track mountingFor track luminaires, the method by which the track is attached to the ceiling.

transformerTransformers are electrical devices with no moving parts, which change distribution voltages to higher or lower levels. When used with incandescent or halogen lamps, they typically step 120-V distribution downward to 12V, although 5.5V and 24-V models are also offered.

transientsFor an alternating current circuit, a momentary voltage surge, often at amplitudes 10 to 20 times the normal voltage.

tri-level switchingControl of light source intensity at three discrete levels in addition to off.

tri-phosphorA mixture of three phosphors to convert ultraviolet radiation to visible light in fluorescent lamps; each of the phosphors emits light that is blue, green or red in appearance with the combination producing white light.

tri-phosphorsTri-phosphors are a blend of three narrow-band phosphors (red, blue, and green) that provide improved color rendition and higher light output versus some other types of phosphors.

trim optionA decorative luminaire accessory.

ultrasonic frequencyThe frequency at which an ultrasonic sensor operates.

ultravioletAny radiant energy within the wavelength range 100 to 400 nanometers is considered ultraviolet radiation (1 nanometer = 1 billionth of a meter, or 1 X 10-9 m).

uniformityThe degree of variation of illuminance over a given plane. Greater uniformity means less variation of illuminance. The uniformity ratio of illuminance is a measure of that variation expressed as either the ratio of the minimum to the maximum illuminance or the ratio of the minimum to the average illuminance.

uplightLight directed upward at greater than 90° above nadir. The source of uplight can be from a combination of direct uplight and reflected light.

ventingHoles in the reflector assembly of a downlight.

vertical illuminanceThe average density of luminous flux incident on a vertical surface, measured in footcandles (fc) or lux (lx). One fc equals 10.76 lx.

visual performanceThe quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy.

voltage dropThe difference between the voltages at the transmitting and receiving ends of a feeder, main, or service.

voltage regulationThe change in output voltage that occurs when the load (at a specified power factor) is reduced from rated value to zero, with the primary impressed terminal voltage maintained constant.

wall-washingThe practice of illuminating vertical surfaces, such as walls. Wall-washer luminaries are designed to illuminate vertical surfaces.

warm-up timeThe time it takes for a lamp to produce 90% of its stabilized light output when it is started, unless otherwise indicated.

wavelengthThe distance between two corresponding points of a given wave. Wavelengths of light are measured in nanometers (1 nanometer = 1 billionth of a meter, or 1 X 10-9 m)

weightThe weight of a luminaire plus ballast (except for certain track luminaires with separately mounted ballasts, when the weight is that of the lamp and track head only). For modular compact fluorescent lamp (CFL) ballasts, the weight of the ballast without a lamp. For self-ballasted CFLs, "weight" indicates the total product weight.

x-barColor matching function x-bar, y-bar, z-bar are used to define the color-matching properties of the CIE 1931 standard observer. In 1931, CIE defined the color-matching functions x-bar, y-bar, z-bar in the wavelength range from 380nm to 780 nm at wavelength intervals of 5nm.

zenithIn the lighting discipline, zenith is the angle pointing directly upward from the luminaire, or 180°. Zenith is opposite nadir. In astronomical usage, zenith is the highest point in the sky, directly above the observation point.



Rensselaer Polytechnic Institute
LRC Intranet Web mail Lighting Research Center