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Glossary of Lighting Terms

Source of terms definition: The National Lighting Product Information Program (NLPIP) at Rensselaer Polytechnic Institute's Lighting Research Center (LRC).

ANSI Code American National Standards Institute (ANSI) code that indicates the electrical operating designation of the lamp, which must match that of the ballast.
Aperture The diameter in the opening of a downlight, in inches (in.). Sometimes manufacturers will round up to the next whole-inch increment.
Average Rated Life The 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.
Beam Angle The angle at which luminous intensity is 50 percent of the maximum intensity.
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. Common beam spreads are known as spot, narrow, narrow flood, and flood.
Bi-Level Switching Control of light source intensity at two discrete levels in addition to off.
Bin To sort or classify light sources (such as light emitting diodes) into groups according to their luminous intensity or color appearance
Candela The Systeme International d’Unities (SI) of luminous intensity. One candela is one lumen per steradian. Formerly, candle.
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.
Center Beam Candlepower (CBCP) Center beam candlepower is the luminous intensity at the center of a beam, expressed in candelas (cd).
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 Appearance The resultant color perception that includes the effects of spectrum, background contrast, chromatic adaptation, color constancy, brightness, size and saturation.
Color Consistency The measure of how close in color appearance random samples of a lamp or source tend to be.
Color Matching The 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 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 Shift The change in a lamp’s correlated color temperature (CCT) at 40% of the lamp’s rated life, in kelvin (K).
Color Stability The 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.
Conduction The process of removing heat from an object via physical contact with other objects or materials, usually metals.
Continuous Dimming Control of a light source’s intensity to practically any value within a given operating range.
Contrast Also known as luminance contrast, it is the relationship between the luminances of an object and its immediate background.
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 3200K 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.
Diffuser Material Diffusers 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.
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 Luminaire A 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.
Disability Glare A type of glare that causes a loss of visibility from stray light being scattered within the eye.
Discomfort Glare The sensation of annoyance or even pain induced by overly bright sources.
Distributed Control System A control system in which the computing hardware and software are contained in a network of control modules or multicircuit control panels physically distributed throughout the facility.
Driver For light emitting diodes, a device that regulates the voltage and current powering the source.
Dynamic Outdoor Lighting Outdoor 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.
Efficacy The ratio of light output (in lumens) to input power (in watts), expressed as lumens per watt (LPW).
Emergency Options Refers 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 View The 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.
Filter A 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.
Fixture A 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.)
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.
Fully Shielded Luminaire A luminaire that emits no direct uplight, but which has no limitation on the intensity in the region between 80° and 90°.
Glare The 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
Grid The combination of electric power plants and transmission lines operated by an electric utility.
Grounded A circuit or metal object that is connected to the earth at one or more points. Done mostly for safety, grounding also reduces electromagnetic waves.
Heat Sinking Adding a material, usually metal, adjacent to an object in order to cool it through conduction.
Hue The attribute of a light source or illuminated object that determines whether it is red, yellow, green, blue, or the like.
Illuminance The 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.
Indirect Lighting Light arriving at a surface after reflecting from one or more surfaces (usually walls and/or ceilings) that are not part of the luminaire.
Infrared Radiation Any 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 Output A lamp’s light output, in lumens, after 100 hours of seasoning.
Interoperability The ability to communicate such information as temperature, illuminance levels, status of security devices, and occupancy among building systems and their controls.
Inverter Also known as “power inverter.” A device used to convert direct current (dc) electricity into alternating (ac) current.
Kelvin Color temperature is measured in degrees Kelvin, which indicate the hue of a specific type of light source. Higher temperatures indicate whiter, “cooler” colors, while lower temperatures indicate yellower, “warmer” colors.
Light Loss The reduced light output caused by a circuit-level power reducer expressed as a percentage of the light output without the circuitlevel power reducer. (Full system output minus reduced output with a lighting-circuit power reducer divided by the full system output times 100.)
Light Pollution An unwanted consequence of outdoor lighting that includes such effects as sky glow, light trespass, and glare.
Light Power Density 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 Trespass A 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 Voltage The voltage supplied by the electric power infrastructure, typically 110-120 Vac at 60 Hz for homes in North America.
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 Depreciation The decrease in lumen output that occurs as a lamp is operated, until failure. Also referred to as lamp lumen depreciation (LLD).
Light Maintenance The 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.
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.)
Luminaire Efficacy The ratio of the measured light output of a luminaire to its active power, expressed in lumens per watt (LPW).
Luminaire Efficiency The 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.
Luminance The 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 Contrast Luminance 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 Flux Luminous radiant power, measured in lumens. The overall light output of a lamp or luminaire.
Luminous Intensity The 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.
Photosensor A device used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient.
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.
Power Line Carrier (PLC) A system that transmits highfrequency (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 Quality The 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.
Prismatic Lens An 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.
Rated Lumen Also 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.
Semiconductor A material whose electrical conductivity is between that of a conductor and an insulator; the conductivity of most semiconductors is temperature dependent.
System Efficacy Also 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).
Tri-Level Switching Control of light source intensity at three discrete levels in addition to off.
Tri-Phosphor A 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.
Uniformity The 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.
Visual Performance The quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy.
Voltage Drop The difference between the voltages at the transmitting and receiving ends of a feeder, main, or service.
Voltage Regulation The 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.
Wavelength The 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)

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