| Lighting Glossary |
 |
| Term |
Definition |
| Accent Lighting | Directional lighting to emphasize a particular object or to draw attention to a part of the field of view. |
| Absorption | The dissipation of light within a surface or medium. |
| Accommodation | The process by which the eye changes focus from one distance to another. |
| Adaptation | The process by which the visual system becomes accustomed to more or less light than it was exposed to during an immediately preceding period. It results in a change in the sensitivity of the eye to light. |
| Alternating Current (AC) | Flow of electricity which cycles or alternates direction many times per second. The number of cycles per second is referred to as frequency. Most common frequency used in this country is 60 Hertz (cycles per second). |
| Ambient Lighting | Background or fill light in a space. |
| Amperes (amps or A) | The unit of measurement of electric current. |
| Back Lighting | The illumination provided for scenery in off-stage areas visible to the audience |
| Baffle | An opaque or translucent element that serves to shield a light source from direct view at certain angles, or serves to absorb unwanted light. |
| Ballast | An auxiliary device consisting of induction windings wound around a metal core and sometimes includes a capacitor for power correction. It is used with fluorescent and HID lamps to provide the necessary starting voltage and to limit the current during operation. |
| "Batwing" Distribution | Candlepower and distribution which serves to reduce glare and veiling reflections by having its maximum output in the 30° to 60° zone. |
| Bulb | A source of electrically powered light. This term is used to distinguish between an assembled unit consisting of a light source in a housing called a lamp and the internal source. |
| Candela | The unit of measurement of luminous intensity of a light source in a given direction. |
| Candlepower | Luminous intensity expressed in candelas |
| Candlepower Distribution Curve | A curve, generally polar, representing the variation of luminous intensity of a lamp or luminaire in a plane through the light center. |
| Cavity Ratio | A number indicating cavity proportions calculated from length, width and height |
| Coefficient of Utilization (CU) | The percent of rated bare-lamp lumens that exit the fixture and reach the work plane. The CU acounts for light directly from the luminaire as well as light reflected off the room surfaces. The CU value is used in lighting calculations to estimate light levels or the quantity of luminaires needed. The CU is determined from a photometric test and is typically published on product catalog sheets in a tabular form. |
| Colorimetry | The measurement of color. |
| Color Rendering Index (CRI) | Measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source of comparable color temperature. |
| Color Temperature | The absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source. |
| Contrast | The difference in brightness (luminance) of an object and its background |
| Cutoff Luminaires | Outdoor luminaires that restrict all light output to below 85° from vertical. |
| Digital Addressable Lighting Interface (DALI) | An open communications protocol used by multiple control and ballast manufacturers for digital control. |
| Dimming Ballast | Special lamp ballast, which when used with a dimmer control, permits varying light output. |
| Direct Current (DC) | Flow of electricity continuously in one direction from positive to negative. |
| Direct Lighting | Lighting involving luminaires that distribute 90 to 100% of emitted light in the general direction of the surface to be illuminated. The term usually refers to light emitted in a downward direction. |
| Direct Glare | Glare resulting from high luminance or insufficiently shielded light sources in the field of view. It usually is associated with bright areas, such as luminaires, ceilings and windows which are outside the visual tasks or region being viewed. |
| Discharge Lamp | A lamp in which light (or radiant energy near the visible spectrum) is produced by the passage of an electric current through a vapor or a gas. |
| Discomfort Glare | Glare producing discomfort. It does not necessarily interfere with visual performance or visibility. |
| Emergency Lighting | Lighting system designed to provide minimum illumination required for safety, during power failures. |
| Efficacy | See Lamp Efficacy. |
| Efficiency | See Luminaire Efficiency. |
| Equivalent Sphere Illumination (ESI) | The level of sphere illumination which would produce task visibility equivalent to that produced by a specific lighting environment. |
| Fill Light | Illumination added to reduce shadows or contrast range. |
| Fixture | Another name for a luminaire. |
| Floodlighting | A system designed for lighting a scene or object to a luminance greater than its surroundings. It may be for utility, advertising or decorative purposes. |
| Fluorescent Lamp | A low-pressure mercury electric-discharge lamp in which a phosphor coating transforms some of the ultraviolet energy generated by the discharge into light. |
| Footcandle (fc) | The unit of illuminance when the foot is taken as the unit of length. It is the illuminance on a surface one square foot in area on which there is a uniformly distributed flux of one lumen. |
| Foot Lambert (fl) | A unit of luminance of perfectly diffusing surface emitting or reflecting light at the rate of one lumen per square foot. |
| General Lighting | See Ambient Lighting. |
| Glare | The sensation produced by luminance within the visual field that is sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort, or loss in visual performance and visibility. |
| Group Relamping | Relamping of a group of luminaires at one time to reduce relamping labor costs. |
| Hazardous Area | Hazardous areas are locations where atmospheres may be exposed to the release of flammable dusts, vapors, or gases in exposive concentrations. The National Electrical Code requires that these areas be classified and sets rules for the types of luminaires that may be installed in them. Luminaires are typed in Article 500 of the NEC as Class I, Class II, and Class III locations. All electrical equipment must be tested and listed (or approved) by class, division and group for use in each respective area. The hazardous materials defined in each of these classifications are; Class I; Flammable gases or vapors, Class II; Combustible dust, and Class III; Combustible fibers or flyings. Each class is subdivided into two divisions depending on the likelihood that the hazard will be present. Divison 1 applies to an area where the hazardous condition would normally exit, while Division 2 applies to an area where there is a potential for the hazardous condition to exist.
Each classification is also subdivided by groups representing the types of gas, or dust that will or might be present. Gases fall into Groups A, B, C, or D. Dusts fall into Groups E, F or G. There is no group subdivision for fibers or flyings.
Environmental Constraints
Lighting equipment must be chosen from the listing for the class, group, and divison of the hazardous material present in the areas where they are to be used. Improper application of a luminaire can result in fire or explosion, which could cause serious injury or death to the occupants. Classification of these areas within a plant must be made prior to selection of the light source and luminaire type. |
| High Intensity Discharge (HID) Lamp | A discharge lamp in which the light producing arc is stabilized by wall temperature, and the arc tube has a bulb wall loading in excess of three watts per square centimeter. HID lamps include groups of lamps known as mercury, metal halide, and high pressure sodium. |
| High Output Fluorescent Lamp | Operates at 800 or more milliamperes for higher light output than standard fluorescent lamp (430MA). |
| High Pressure Sodium (HPS) Lamp | High intensity discharge (HID) lamp in which light is produced by radiation from sodium vapor. Includes clear and diffuse-coated lamps. |
| Incandescence | The self-emission of radiant energy in the visible spectrum due to the thermal excitation of atoms or molecules. |
| Incandescent Lamp | A lamp in which light is produced by a filament heated to incandescence by an electric current. |
| Illuminance | The flow of light onto a surface. Expressed in footcandles (English units) or lux (metric units) |
| Instant Start Fluorescent Lamp | A fluorescent lamp designed for starting by a high voltage without preheating of the electrodes. |
| Inverse Square Law | The law stating that the illuminance at a point on a surface varies directly with the intensity of a point source, and inversely as the square of the distance between the source and the point. If the surface at the point is normal to the direction of the incident light, the law is expressed by fc=cp/d2. |
| Kelvin | Unit of measurement for color temperature. The Kelvin scale starts from absolute zero, which is -273° Celsius. |
| Kilowatt-Hour (KWH) | Unit of electrical power consumed over a period of time. KWH = watts/1000 x hours used.
Lamp An artificial source of light (also portable luminaire equipped with a cord and plug). |
| Lamp Efficacy | The ratio of lumens produced by a lamp to the watts consumed. Expressed as lumens per watt (LPW). |
| Lamp Lumen Depreciation (LLD) | Multiplier factor in illumination calculations for reduction in the light output of a lamp over a period of time. |
| Light | Radiant energy that is capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 770 nm. |
| Lens | Used in luminaires to redirect light into useful zones. |
| Louver | A series of baffles used to shield a source from view at certain angles or to absorb unwanted light. The baffles usually are arranged in a geometric pattern. |
| Low Pressure Sodium Lamp | A discharge lamp in which light is produced by radiation of sodium vapor at low pressure producing a single wavelength of visible energy, i.e. yellow. |
| Low Voltage Lamps | Incandescent lamps that operate at 6 to 12 volts. |
| Lumen | The unit of luminous flux. It is the luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela. |
| Luminaire | A compiete lighting unit consisting of lamp or lamps, the parts designed to distribute the light, and to position and protect the lamps (fixture housing) and any necessary starting components (ballasts), and to connect the unit to the power supply. |
| Luminaire Dirt Depreciation (LDD) | The multiplier to be used in illuminance provided by clean, new luminaires to the reduced illuminance that they will provide due to dirt collection on the luminaires at the time at which it is anticipated that cleaning procedures will be instituted. |
| Luminaire Efficacy Rating (LER) | NEMA document LE5 describes the calculation of LER as:
- LER = EFF x TLL x BF / Input Watts
- where:
- EFF = luminare efficiency
- TLL = # lamps per luminaire X rated lumens per lamp
- BF = ballast factor
- Input Watts = total system watts of the Luminaire
This results in a lumens-per-watt rating that can be used to compare the energy efficiency of various products.
|
| Luminance | The amount of light reflected or transmitted by an object. |
| Luminous Efficacy | Quantifies lumens produced per unit of power (watts) consumed. Can be used to evaluate the energy efficiency of a lamp or a luminaire. Measured in lumens per watt. |
| Luminous Efficiency | Total lumen out put of a luminaire expressed as a percent of rated bare-lamp lumens (as determined by photometric tests). Luminous efficiency quantifies only the amount of light emitted from a luminaire. It does ot describe the quality of the light from the luminaire. |
| Luminous Flux | The flow of light from a lamp or luminaire. Measured in lumens. |
| Luminous Intensity | Measured in a photometric test to describe the intensity of light in a particular direction. Measured in candelas. |
| Lux | The metric unit of illuminance. One lux is one lumen per square meter (lm/m2). |
| Maintenance Factor (MF) | A factor used in calculating illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, dirt accumulation on luminaire and room surfaces, lamp depreciation, maintenance procedures and atmosphere conditions. |
| Mercury Lamp | A high intensity discharge (HID) lamp in which the major portion of the light is produced by radiation from mercury. Includes, clear, phosphor-coated 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 dissociation-possible in combination with metallic vapors such as mercury. Includes clear and phosphor coated lamps. |
| Nadir | Vertically downward directly below the luminaire or lamp; designated as 0°. |
| Parabolic Louvers | A grid of baffles which redirect light downward and provide very low luminaire brightness. |
| Photometrics | A photometric test measures the quantity and direction of light emitted from a uminaire. Photometrics refers to the measured values. |
| Photometry | The measurement of light quantities. |
| Point Method Lighting Calculation | A lighting design procedure for predetermining the illuminance at various locations in lighting installations, by use of luminaire photometric data. |
| Polarization | The process by which the transverse vibrations of light waves are oriented in a specific plane. Polarization may be obtained by using either transmitting or reflecting media. |
| Power Factor | Ratio of: Watts (volts x amperes) Power factor in lighting is primarily applicable to ballasts. Since volts and watts are usually fixed, amperes (or current) will go up as power factor goes down. This necessitates the use of larger wire sizes to carry the increased amount of current needed with Low Power Factor (L.P.F) ballasts. The addition of a capacitor to an L.P.F. ballast converts it to a H.P.F. ballast. |
| Raw Footcandles | See Footcandles. |
| Reflection | Light bouncing off a surface. In specular reflection the light strikes and leaves a surface at the same angle. Diffuse reflected light leaves a surface in all directions. |
| Reflectance | Sometimes called reflectance factor. The ratio of reflected light to incident light (light falling on a surface). Reflectance is generally expressed in percent. |
| Reflected Glare | Glare resulting from specular reflections of high luminances in polished or glossy surfaces in the field of view. It usually is associated with reflections from within a visual task or areas in close proximity to the region being viewed. |
| Reflector | A device used to redirect the light flow from a source by bouncing it off the surface. |
| Refraction | The process by which the direction of a ray of light changes as it passes obliquely from one medium to another in which its speed is different. |
| Room Cavity Ration (RCR) | A number indicating room cavity proportions calculated from length, width and height. |
| Spacing Criterion (SC) | A value, calculated from photometric data, that is used to estimate how far apart luminaires can be spaced to maintain uniform illuminance on the work plane. The spacing criterion value is multiplied by the luminaire mounting height above the work plane to estimate an acceptable center-to center luminaire spacing. |
| Spacing Ratio | Ratio of the distance between luminaire centers to the mounting height above the work-plane for uniform illumination. |
| Spacing to Mount Height Ratio | M(S/MH) - A value, calculated from photometric data, that is used to estimate how far apart luminaires mounted in a row can be spaced to maintain uniform illuminance on the work plane. The spacing criterion value is multiplied by the luminaire mounting height above the work plane to estimate an acceptable spacing. For luminaires mounted in a rectangular array, the spacing criterion provies a better indication of the spacing required to achieve uniform illuminance. |
| Spectral Energy Distribution (SED) | Curves A plot of the level of energy at each wavelength of a light source. |
| Sphere Illumination | The illumination on a task from a source providing equal luminance in all directions about that task, such as an illuminated sphere with the task located at the center. |
| Surface Mounted Luminaire | A luminaire that is mounted directly on a ceiling. |
| Suspended (Pendant) Luminaire | A luminaire that is hung from a ceiling by supports. |
| Task Lighting | Lighting directed to a specific surface or area that provides illumination for visual tasks. |
| Transformer | A device to raise or lower electric voltage. |
| Transmission | The passage of light through a material. |
| Visual Comfort Probability (VCP) | The rating of a lighting system expressed as a percent of people who, when viewing from the specified location and in a specified direction, will be expected to find it acceptable in terms of discomfort glare. |
| Visual Field | The field of view that can be perceived when the head and eyes are kept fixed. |
| Wall Wash Lighting | A smooth even distribution of light over a wall. |
| Watt (W) | The unit for measuring electric power. It defines the power or energy consumed by an electrical device. The cost of operating an electrical device is determined by the watts it consumes times the hours or use. It is related to volts and amps by the following formula: Watts = Volts x Amps. |
| Work Plane | A horizontal surface where visual tasks are performed. The work plane is typically ground level for outdoor applications and 2.5 feet for office appplications (corresponding with desk height). In some cases, a work plane may be vertical, such as a library stack or warehouse rack. |
| Zonal Cavity Method Lighting Calculation | A lighting design procedure used for predetermining the relation between the number and types of lamps or luminaires, the room characteristics, and the average illuminance on the work-plane. It takes into account both direct and reflected flux. |
|
Glossary1
|
Glossary2
|
