Glossary

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

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Ballast 
An auxiliary piece of equipment required to start and to properly control the flow of current to gas discharge light sources such as fluorescent and high intensity discharge (HID) lamps. Typically, magnetic ballasts (also called electromagnetic ballasts) contain copper windings on an iron core while electronic ballasts are smaller and more efficient and contain electronic components.
 
Ballast Factor (BF) 
This is the percentage of a lamp's rated lumen output that can be expected when operated on a specific, commercially available ballast. Note that the "rated output" is sometimes measured on a reference ballast unlike ones that actually operate the lamp in the field.
 
Ballast Losses 
Power or energy dissipated in the ballast as heat and not converted to lamp energy.
 
Base or Socket 
The socket is the receptacle connected to the electrical supply; the base is the end of the lamp that fits into the socket. There are many types of bases used in lamps, screw bases being the most common for incandescent and HID lamps.

Bollard 
A short, thick post used along pathways, walkways, curbs, crosswalks, and similar public spaces for the purpose of directing, controlling or limiting pedestrian and/or vehicular traffic. A bollard may come either with or without a light at its top.

 

BUG Rating
The BUG rating system was developed by the Illuminating Engineering Society (IES) to improve the means by which outdoor luminaires are evaluated and compared.  The BUG rating consists of three components that describe the way light is distributed from a luminaire - “Backlight”, “Uplight” and “Glare”.  Numerical values are given to each of these three components, and the result is a BUG rating for a given luminaire.  IES Technical Memorandum TM-15 is the document that defines the BUG rating system.

Bulb 
A loose way of referring to a lamp. "Bulb" refers to the outer glass bulb containing the light source.

 

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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.
 
Candlepower Distribution Curve
The photometric distribution curve is one of the lighting designer’s most valuable tools. It is a cross-sectional "map" of intensity (candelas) measured at many different vertical angles. It is a two-dimensional representation and therefore shows data for one plane only. If the distribution of the unit is symmetric, the curve in one plane is sufficient for all calculations. If asymmetric, such as with street lighting and fluorescent units, three or more planes are required. In general, incandescent and HID reflector units are described by a single vertical plane of photometry. Fluorescent luminaires require a minimum of one plane along the lamp axis, one across the lamp axis and one at a 45° angle. The greater the departure from symmetry, the more planes that are needed for accurate calculations.
 
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.
 
Ceramic Metal Halide 
A type of metal halide lamp that uses a ceramic material for the arc tube instead of glass quartz, resulting in better color rendering (>80 CRI) and improved lumen maintenance.
 
Color Rendering Index (CRI) 
A measure of the degree of color shift that objects undergo when illuminated by a lamp, compared with those same objects when illuminated by a reference source of comparable correlated color temperature (CCT). A CRI of 100 represents the maximum value. A lower CRI value indicates that some colors may appear unnatural when illuminated by the lamp. Incandescent lamps have a CRI above 95. The cool white fluorescent lamp has a CRI of 62; fluorescent lamps containing rare-earth phosphors are available with CRI values of 80 and above.
 
Color Temperature (Correlated Color Temperature - CCT) 
A number indicating the degree of "yellowness" or "blueness" of a white light source. Measured in kelvins, CCT represents the temperature an incandescent object (like a filament) must reach to mimic the color of the lamp. 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.
 
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.
 
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.
 
Core & Coil Ballast 
A ballast that uses a "Core & Coil" assembly to operate fluorescent or HID lamps. Refers to copper windings on a steel core.
 
Cutoff Angle 
The 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 Classification 
The classification system of the Illuminating Engineering Society of 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 Luminaire 
IESNA 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.

 

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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.
 

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Efficacy 
A measurement of how effective the light source is in converting electrical energy to lumens of visible light. Expressed in lumens-per-watt (LPW) this measure gives more weight to the yellow region of the spectrum and less weight to the blue and red region where the eye is not as sensitive.
 
Efficiency 
The efficiency of a light source is simply the fraction of electrical energy converted to light, i.e. watts of visible light produced for each watt of electrical power with no concern about the wavelength where the energy is being radiated. For example, a 100 watt incandescent lamp converts 7% of the electrical energy into light; discharge lamps convert 25% to 40% into light. The efficiency of a luminaire or fixture is the percentage of the lamp lumens that actually comes out of the fixture 
 
Electromagnetic Ballast (Magnetic Ballast) 
A ballast used with discharge lamps that consists primarily of transformer-like copper windings on a steel or iron core. Also called "Core and Coil".
 
Electronic Ballast 
A short name for a fluorescent high frequency electronic ballast. Electronic ballasts use solid state electronic components and typically operate fluorescent lamps at frequencies in the range of 25-35 kHz. The benefits are: increased lamp efficacy, reduced ballast losses and lighter, smaller ballasts compared to electromagnetic ballasts. Electronic ballasts may also be used with HID (high intensity discharge).
 

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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.)
 
Floodlight 
A luminaire used to light a scene or object to a level much brighter than its surroundings. Usually floodlights can be aimed at the object or area of interest.
 
Fluorescent 
The fluorescent lamp produces light by activating selected phosphors on the inner surface of the bulb with ultraviolet energy, which is generated by a mercury arc. Because of the characteristics of a gaseous arc, a ballast is needed to start and operate fluorescent lamps.
The advantages of the fluorescent light source include improved efficacy and longer life than incandescent lamps. Efficacies for fluorescent lamps range anywhere from 50 to 100 lumens per watt. Their low surface brightness and heat generation make them ideal for offices and schools where thermal and visual comfort are important.
The disadvantages of fluorescent lamps include their large size for the amount of light produced. This makes light control more difficult, which results in a diffuse, shadowless environment. Their use in outdoor areas becomes less economical because light output of a fluorescent source is reduced at low ambient temperatures
 
Foot Candles (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.
 
Full Cutoff Luminaire 
IESNA 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.

 

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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.
 

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High Intensity Discharge (HID) and LPS 
High intensity discharge sources include mercury vapor, metal halide, and high pressure sodium (HPS) lamps. Light is produced in HID and low pressure sodium (LPS) sources through a gaseous arc discharge using a variety of elements. Each HID lamp consists of an arc tube which contains certain elements or mixtures of elements which, when an arc is created between the electrodes at each end, gasify and generate visible radiation. The major advantages of HID sources are their high efficacy in lumens per watt, long lamp life and point-source characteristic for good light control. Disadvantages include the need for a ballast to regulate lamp current and voltage as well as a starting aid for HPS and some MH and the delay in restriking after a momentary power interruption.
 
High Power Factor
A ballast whose power factor is corrected to 90% or greater by the use of a capacitor.
 
High Pressure Sodium (HPS) 
In the 1970s, as increasing energy costs placed more emphasis on the efficiency of lighting, high pressure sodium lamps (developed in the 1960s) gained widespread usage. With efficacies ranging from 80 to 140 lumens per watt, these lamps provide about 7 times as much light per watt as incandescent and about twice as much as some mercury or fluorescent. The efficacy of this source is not its only advantage. An HPS lamp also offers the longest life (24,000+ hrs.) and the best lumen maintenance characteristics of all HID sources. The major objection to the use of HPS is its yellowish color and low color rendition. It is ideal mainly for some warehouse and outdoor applications.
 

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Ignitor 
An electronic device providing a high voltage pulse to initiate an electrical discharge. Typically, the ignitor is paired with or is a part of the ballast
 
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.
 
Illumination 
The process of using light to see objects at a particular location
 
Incandescent Lamp
An incandescent lamp is the light source most commonly used in residential lighting. Light is produced in this source by a wire or filament being heated to incandescence (emitting light) by a flow of current through it. The short life and low efficacy (lumens per watt) of this source limit its use mostly to residential and decorative commercial lighting. Efficacy varies with wattage and filament type, but generally ranges from 15 to 25 lumens per watt for general service lamps. The incandescent source does, however, produce light in a well-accepted warm tone. It is more convenient than other light sources because it can be run directly on line current and therefore does not require a ballast. It can also be dimmed using relatively simple equipment. It is available in different bulb sizes, shapes and distributions to add a decorative touch to an area.
 
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.
 
Induction Lamp 
Induction lamps are electrodeless fluorescent lamps driven by high-frequency current, typically between 250kHz and 2.65mHz, usually via an external generator. They are available in limited wattages and are known for exceptionally long service life: up to 100,000 hours. Lamp efficacies typically range from 64 to 88 lumens per watt. Color rendition with induction lamps is very good. Although not easily optically controllable in a luminaire because of the large lamp size, induction lighting is often employed in applications where luminaires may be very difficult to access or where maintenance costs are a strong factor in the lighting design and installation. Initial system purchasing costs are high compared to the best HID or fluorescent systems.
 
Isofootcandle Chart 
Isofootcandle charts are used to describe the light pattern produced by a luminaire. These charts are derived from the candlepower data and show exact plots or lines of equal footcandle levels on the work plane when the fixture is at a designated mounting height. Use of isofootcandle charts in determining illuminance at designated points will be discussed in the point calculations section.
 

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

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Lamp Efficacy
The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt (LPW).
 
Lamp Life 
The 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 Lumen Depreciation (LLD) 
The reduction in lamp light output that progressively occurs during lamp life.
 
Lamp 
The term used to refer to the complete light source package, including the inner parts as well as the outer bulb or tube. "Lamp", of course, is also commonly used to refer to a type of small light fixture such as a table lamp.
 
Light Emitting Diode (LED) 
A solid that directly converts electrical impulses into light. LEDs used for general illumination incorporate fluorescent materials (i.e. phosphors) to change the color characteristics of the emitted light.
 
Light Loss Factor 
The product of all factors that contribute to lowering the illumination level including reflector degradation, dirt, lamp depreciation over time, voltage fluctuations, etc.
 
Light Trespass (Spill Light) 
Light that is not aimed properly or shielded effectively can spill out at into areas that don't want it: it can be directed towards drivers, pedestrians or neighbors. It is distracting and annoying and can sometimes be disabling.   

Louver 
A fixed shield, usually divided into small cells, that are attached to the face of a luminaire to reduce direct glare.
 
Low Pressure Sodium (LPS)
Low pressure sodium offers the highest initial efficacy of all lamps on the market today, ranging from 100 to 180 lumens per watt. However, because all of the LPS output is in the yellow portion of the visible spectrum, it produces extremely poor and unattractive color rendition. Control of this source is more difficult than with HID sources because of the large size of the arc tube. The average life of low pressure sodium lamps is 18,000 hours. While lumen maintenance through life is good with LPS, there is an offsetting increase in lamp watts, reducing the efficacy of this lamp type with use.
 
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).
 
Lumen Maintenance
The ability of a lamp to retain its lumen 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.

Lumens
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.
 
Lumens/Watts 
A ratio expressing the luminous efficacy of a light source. Typical lamp efficacies:
• Thomas Edison's first lamp - 1.4 lpW
• Incandescent lamps - 10-40
• Halogen incandescent lamps - 20-45
• Fluorescent lamps - 35-105
• Mercury lamps - 50-60
• Metal halide lamps - 60-120
• High-pressure sodium lamps - 60-140
Note: The values above for discharge lamps do not include the effect of the ballasts, which must be used with those lamps. Taking ballast losses into account reduces "system" or lamp-ballast efficacies typically by 10-20% depending upon the type of ballast used.
 
Luminaire 
A complete lighting unit consisting of a lamp (or lamps), ballast (or ballasts) as required together with the parts designed to distribute the light, position and protect the lamps and connect them to the power supply. A luminaire is often referred to as a fixture.

Luminaire Efficiency 
The ratio of total lumens emitted by a luminaire to those emitted by the lamp or lamps used in that luminaire
 
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) and was formerly referred to as "photometric brightness."
 
Lux (lx) 
A unit of illuminance or light falling onto a surface. One lux is equal to one lumen per square meter. Ten lux approximately equals one footcandle.
 

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Medium Base
Usually refers to the screw base typically used in household incandescent lamps.
 
Mercury Vapor (MV) Lamp 
The mercury vapor source was the first HID lamp developed, filling the need for a more efficient, yet compact, high output lamp. When first developed, the major disadvantage of this lamp was its poor color rendition. The color of the deluxe white lamp is somewhat improved through use of a phosphor coated bulb wall. The life of mercury vapor lamps is good, averaging 24,000 hours for most larger wattage lamps. However, because the output diminishes so greatly over time, economical operational life is often much shorter. Efficacy ranges from 30 to 60 lumens per watt, with the higher wattages being more efficacious than the lower wattages.
As with other HID lamps, the starting of a mercury vapor lamp is not immediate. Starting time is short, though, taking 4-7 minutes to achieve maximum output depending upon the ambient temperature.

Metal Halide (MH) Lamp 
Metal halide lamps are similar in construction to mercury lamps with the addition of various other metallic elements in the arc tube. The major benefits of this change are an increase in efficacy to 60 to 100 lumens per watt and an improvement in color rendition to the degree that this source is suitable for commercial areas. Light control of a metal halide lamp is also more precise than that of a deluxe mercury lamp since light emanates from the small arc tube, not the total outer bulb of the coated lamp.
Pulse-start metal halide lamps have several advantages over standard (probe-start) metal halide: higher efficacy (110 lumens per watt), longer life, and better lumen maintenance.
A disadvantage of the metal halide lamp is its shorter life (7,500 to 20,000 hrs.) as compared to mercury and high pressure sodium lamps. Starting time of the metal halide lamp is approximately the same as for mercury lamps. Restriking after a voltage dip has extinguished the lamp, however, can take substantially longer, ranging from 4 to 12 minutes depending on the time required for the lamp to cool.

Mogul Base 
A screw base used on larger lamps, e.g. many HID lamps.

Mounting Height
Distance from the bottom of the fixture to either the floor or work plane, depending on usage. 
 

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Noncutoff Luminaire 
IESNA 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.)
 

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Photometry 
The measurement of light and related quantities.
 
Photopic 
Vision for which the cones in the eye are responsible; typically at high brightness and in the fovea or central region
 
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.
 
Pulse Start 
An HID ballast with a high voltage ignitor to start the lamp. 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.
 

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Semicutoff Luminaire
IESNA 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.
 
Sky Glow 
Brightening of the sky caused by outdoor lighting and natural atmospheric and celestial factors.
 
Spill Light 
Light that falls outside of the area intended to be lighted.
 

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Underwriters Laboratories (UL) 
A private organization which tests and lists electrical (and other) equipment for electrical and fire safety according to recognized UL and other standards. A UL listing is not an indication of overall performance.
 
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.

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

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