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Lighting Glossary

- A -

AC (Alternating Current) An electric current that reverses direction in a circuit at regular intervals.

Accent Lighting Directional lighting to emphasize a particular object or surface feature or draw attention to a part of the field view.

Adaptation The process by which the retina becomes accustomed to more or less light that it was exposed to during an immediately preceding period. It results in a change in the sensitivity to light.

Ambient Lighting The lighting throughout an area that produces general illumination.

Amperes ("Amps.") A measure of electrical current. In incandescent lamps, the current is related to voltage and power as follows: Watts (power) = Volts x Amps (current).

Arc An electric discharge characterized by high cathode current densities and a low voltage drop at the cathode.

Arc Lamp An electric lamp that produces light by an arc made when a current passes between two incandescent electrodes surrounded by ga

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


- B -

Ballast a device used to provide the starting voltage or to stabilize the current in a circuit (as of a fluorescent lamp). 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) The fractional flux of a fluorescent lamp operated on a ballast compared to the flux when operated on the standard ballast specified for rating lamp lumens.

Ballast Hum Sound generated by the vibration of laminations in the iron core of the transformer or inductor present in the ballast.

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, while bipin bases are common for linear fluorescent lamps. Sample Base Types

Beam Angle The angle between the two directions for which the intensity is 50% of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the beam angle is generally given for two planes at 90 degrees, typically the maximum and minimum angles.

Beam Lumens The total flux in that region of space where the intensity exceeds 50% of the maximum intensity.

Beam Spread (In any plane) The angle between the two directions in the plane in which the intensity is equal to a stated percentage of the maximum beam intensity.

Blackbody A temperature radiator of uniform temperature whose radiant exitance 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.

Black Light A popular term for UV energy near the visible spectrum (320 to 400 nm)

Bollard A short, thick post with a light at its top, used for grounds and outdoor walkway lighting.

Bulb a generic term for a source created to produce optical radiation.

Bulb Size Bulb shape followed by its size (the maximum diameter of the bulb expressed in eighths of an inch). For Compact Fluorescent products, "S", "D",and "T", are used to represent Single, Double, and Triple sizes. The code also includes a reference such as T4 to represent the size of the tube. Rectangular headlamps are designated as "Rect" and the number of millimeters horizontally.

Brightness The attribute by which an area of color of finite size is perceived to emit, transmit, or reflect a greater or lesser amount of light. No judgment is made as to whether the light comes from a reflecting, transmitting, or self-luminous object.

- C -

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 An obsolete term for luminous intensity; current practice is to refer to this simply as candelas.

Centigrade (C) Celsius temperature scale where 0&#deg;C=32&#deg;F.

Chromaticity Measure to identify the color of a light source, typically expressed as (x,y) coordinates on a chromaticity chart (See COLOR TEMPERATURE).

Chromaticity Coordinates A system for measuring the color of the light emitted from a light source--either a primary source like a lamp or a secondary source like an illuminated object. Usually two numbers, x and y coordinates ranging from 0 to 1 specify the chromaticity.

Coefficient of Utilization (CU) In general lighting calculations, the ratio of luminous flux (lumens) calculated as received on the work plane to the total luminous flux (lumens) emitted by the lamps alone.  It is equal to the product of room utilization factor and luminaire efficiency.

Color Rendering Index (CRI) An international system used to measure the degree of color shift objects undergo when illuminated by the light source as compared with those same objects when illuminated by a reference source of comparable color temperature.

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

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

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

- D -

Daylight Lamp A lamp producing a spectral distribution approximating that of a specified daylight.

Dichroic Reflector (or Filter) A reflector (or filter) that transmits certain wavelengths and reflects those not transmitted; the absorption is small.

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

- E -

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 (See LUMINOUS EFFICACY).

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

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". (See ELECTRONIC BALLASTS).

Electromagnetic Spectrum A continuum of electric and magnetic radiation encompassing all wavelengths.

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

- F -


Field Angle The angle between the two directions for which the intensity is 10% of the maximum intensity as measured in a plane through the nominal beam centerline.  For beams that do not possess rotational symmetry, the beam angle is generally given for two planes at 90 degrees, typically that maximum and minimum angles. Note that in certain fields of applications the angle of the 10% of maximum directions was formerly called beam angle.

Flicker The periodic variation in light level caused by AC operation that can lead to strobe effects.

Flood Used to refer to the beam pattern of a reflector lamp, which disperses the light over a wide beam angle, typically 20 degrees or more. ("Flood" as opposed to "spot")

Floodlight A projector designed for lighting a scene or object to a luminance considerably greater than its surroundings.  It usually is capable of being pointed in any direction and is of weatherproof construction.

Fluorescence The emission of light as the result of, and only during, the absorption of radiation of shorter wavelengths.

Fluorescent Lamp A high efficiency lamp utilizing an electric discharge through inert gas and low pressure mercury vapor to produce ultraviolet (UV) energy. The UV excites phosphor materials applied as a thin layer on the inside of a glass tube which makes up the structure of the lamp. The phosphors transform the UV to visible light.

Foot Candles 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. See also Lux.

Footlambert An obsolete term referring to a luminance of 1/pi candelas per square foot.

Fovea, Foveal Vision A small region of the retina, subtending about 2 degrees, that contains cones but no rods and that forms the site of most distinct vision

- G -

General Lighting  Lighting designed to provide a substantially uniform level of illuminance throughtout an area, exclusive of any provision for special local requirements.

Glare Visual discomfort caused by excessive brightness is called discomfort glare. If task performance is affected it is called disability glare. Glare can be direct glare or indirect (reflected) glare (See VEILING REFLECTIONS and VISUAL COMFORT PROBABILITY). back to top

- H -


Halogen Lamp A halogen lamp is an incandescent lamp with a filament that is surrounded by halogen gases, such as iodine or bromine. Halogen gases allow the filaments to be operated at higher temperatures and higher efficacies. The halogen participates in a tungsten transport cycle, returning tungsten to the filament and prolonging lamp life.

Hertz (Hz) Unit used to measure frequency of alteration of current or voltage

High-Bay Lighting Interior lighting where the roof trusses or ceiling height is greater than approximately 7.6 m (25 ft) above the floor.

High Intensity Discharge (HID) Lamp An electric- discharge lamp in which the light-producing arc is stabilized by bulb wall temperature, and the arc tube has a bulb wall loading in excess of 3 W/cm2.  HID lamps include groups of lamps known as mercury, metal halide, and high-pressure sodium.

High-Pressure Sodium (HPS) A high-intensity discharge (HID) lamp in which light is produced by radiation from sodium vapor operating at a partial pressure of about 1.33 X 10^4 Pa (100 Torr). Includes clear and diffuse-coated lamps.

- I -

Ignitor An electronic device, either by itself or in association with other components, that generates voltage pulses to start discharge lamps without preheating of electrodes.

Illuminance The "density" of light (lumens/area) incident on a surface; i.e. the light level on a surface. Illuminance is measured in footcandles or lux.

Incandescent Lamp A light source that generates light utilizing a thin filament wire (usually of tungsten) heated to white heat by an electric current passing through it.

Indirect Lighting Lighting involving luminaries that distribute 90 to 100% of the emitted light upward.

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

Instant Start Lamp A fluorescent lamp designed for starting by a high voltage without preheating of the electrodes.

Isocandela Plot A line plotted on any appropriate set of coordinates to show all the points on a surface where the illuminance is the same.  A series of such lines for various illuminance values is called an isolux diagram.

- K -

Kelvin A unit of temperature used to designate the color temperature of a light source. A temperature scale where each degree is the same size as a centigrade degree, but the Kelvin scale has its zero at 273 degrees C.

Kilowatt (kW) The measure of electrical power equal to 1000 watts.

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

- L -

Lamp A generic term for a source created to produce optical radiation. By extension, the term is also used to denote sources that radiate in regions of the spectrum adjacent to the visible..

Lamp Description The lamp's identification code. For Projection lamps, this is a 3-letter-number code uniquely identifying the lamp for ordering purposes. In some instances, lamps with 3-letter (ANSI) codes are offered in more than one design voltage, in which case the voltage required should also be specified when ordering.

Lamp Types Filament lamps: Incandescent, Halogen, Halogen-IR. Discharge Lamps: Fluorescent, HID (High Intensity Discharge) HID Lamps: Mercury, HPS (High Pressure Sodium), MH (Metal Halide) and CMH (Ceramic Metal Halide)

Lens A glass or plastic element used in luminaires to change the direction and control the distribution of light rays; also, the part of the eye that allows objects at different distances to be focused onto the retina.

Light Center Length (L.C.L.) The distance between the light center to a specified reference point on the lamp.

Light Emitting Diode (LED) a p-n junction solid-state diode whose radiated output is a function of its physical construction, material used, and exciting current.  The output can be in the IR or in the visible region.

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 Meter (See ILLUMINANCE METER) A common name for an illuminance meter.

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 Efficiency The ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.

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.

- M -

Mercury Lamp A high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation from mercury operating at a partial pressure in excess of 10^5 Pa. Includes clear, phosphor-coated (mercury-fluorsecent), and self-ballasted lamps.

Metal Halide Lamp A high intensity discharge (HID) lamp in which the major portion of the light is produced by radiation of metal halides and their products of dissocation- possibly in combination with meallic vapors such as mercury.  Includes clear and phosphor-coated lamps.

Mounting Height The vertical distance between the roadway surface and the center of the apparent light source of a luminaire.

- N -

Nanometer A unit of wavelength equal to one billionth of a meter.

- O -

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

- P -

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

Photopic Vison mediated essentially or exclusively by the cones.  It is generally associated with adaptation to a luminance of at least 3.4 cd/m^2.

Preheat Lamp A fluorescent lamp designed for operation in a circuit requiring a manual or automatic starting switch to preheat the electrodes in order to start the arc.

- R -

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

Rapid Start Lamp A fluorescent lamp designed for operation with a ballast that provides a low-voltage winding for preheating the electrodes and initiating the arc without a starting switch or the application of high voltage.

Rated Lamp Life The life value assigned to a particular type lamp.  This is commonly a statistically determined estimate of average or of median operational life.  For certain lamp types other criteria than failure to light can be used; for example, the life can be based on the average time until tha\e a\lamp type produces a given fraction of initial luminous flux.

Reflectance The ratio of light reflected from a surface to that incident upon it.

Reflector Lamp (R) An incandescent filament or electric-discharge lamp  in which the outer blown glass bulb is coated with a reflecting material so as to direct the light. The light-transmitting region can be clear, frosted, patterned, or phosphor coated.

Room Cavity Ratio (RCR) A number indicating room cavity proportions, calculated from the length, width, and height.

- S -

Scotopic Vision mediated essentially or exclusively by the rods. It is generally associated with adaptation to a luminance below about 0.034 cd/m^2

Spacing to Mounting Height Ratio The ratio of the actual distance between luminaire centers to the mounting height above the workplane. Also, an obsolete term that described a characteristic of interior luminaires.

Specular Reflection Reflection from a smooth, shiny surface, as opposed to diffuse reflection.

Starter A device used in conjunction with a ballast for the purpose of starting an electric-discharge lamp.

Sunburn Inflammation with reddening (erythema) of the skin, of variable degree, caused by exposure to direct or diffuse solar radiation or artifical optical radiation.

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

- T -

T-12, T-8, T-5 A designation for the diameter of a tubular bulb in eighths of an inch; T-12 is 12 eighths of an inch, or 11/2 inches; T-8 is 1 inch, and so on.

Task Lighting Lighting directed to a specific surface or area that provides illumination for visual tasks.

Transients High voltage surges through an electrical system caused by lightning strikes to nearby transformers, overhead lines or the ground. May also be caused by switching of motors or compressors, as well as by short circuits or utility system switching. Can lead to premature ballast failure.

Troffer A long recessed lighting unit usually installed with the opening flush with the ceiling. The term is derived from "trough" and "coffer"

Tungsten-Halogen Lamp A gas-filled tungsten filament incandescent lamp containing a certain proportion of halogens in an inert gas whose pressure exceeds 3 atm.

- U -

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

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


Valance Lighting Lighting comprising light sources shielded by a panel parallel to the wall at the top of a window.

Veiling Reflection Regular reflections that are superimposed upon diffuse reflections from an object that partially or totally obscure the details to be seen by reducing the contrast. This sometimes is called reflected glare.  Another kind of veiling reflection occurs when one looks through a plate of glass.  A reflected image of a bright element or surface can be seen superimposed on what is viewed through the glass plate.

Visual Comfort Probability (VCP) The rating of a lighting system expressed as a percent of people who, when viewing from a specified location an in a specified direction, will be expected to find it accepatable in terms of discomfort glare.  Viusal comfort probability is related to the discomfort glare rating (DGR).

Visual Task Conventionally designates those details and objects that must be seen for the performance of a given activity, and includes the immediate background of the details or objects.

Volt The difference in electrical potential between two points in a circuit.  It is also called the electromotive force.  The symbol often used in equations is "E", although "V" is acceptable.

- W -

Watts A unit of electrical power. Lamps are rated in watts to indicate the rate at which they consume energy. (See KILOWATT HOUR).

Wavelength The distance between two successive points of a periodic wave, in the direction of propagation, at which the oscillation has the same phase.

Work Plane The plane on which a visual task is usually done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 m (30 in.) above the floor.