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61 Glossary absorptance. The ratio of the flux absorbed by a medium to the incident flux. Note: The sum of the hemispherical reflectance, the hemispherical transmittance, and the absorptance is 1. adaptation. The process by which the retina becomes accustomed to more or less light than it was exposed to during the immediately preceding period, resulting in a change in the sensitivity of the eye to light. atmospheric transitivity. The ratio of the directly transmitted flux incident on a surface after passing through unit thickness of the atmosphere to the flux that would be incident on the same surface if the flux had passed through a vacuum. average initial illuminance. The average level of horizontal illuminance on the pavement area of a traveled way at the time the lighting system is installed, when lamps are new and luminaires are clean; expressed in average footcandles (lux) for the pavement area. average maintained illuminance. The average level of horizontal illuminance on the roadway pavement when the output of the lamp and luminaire is diminished by the maintenance factors; expressed in average footcandles (lux) for the pavement area. ballast. A device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current, and waveform) for starting and operating. bike lane. Any facility that explicitly provides for bicycle travel. bracket (mast arm). An attachment to a lamp post or pole from which a luminaire is suspended. candela (cd). The unit of luminous intensity (formerly âcandleâ). candela per square meter (cd/m2). The International System (SI) unit of luminance (photometric brightness) equal to the uniform luminance of a perfectly diffusing surface emitting or reflecting light at the rate of 1 lumen per square meter, or the average lumi- nance of any surface emitting or reflecting light at that rate; 1 candela per square meter equals 0.2919 footlambert. candlepower (cp). Luminous intensity expressed in candelas (not an indication of total light output). 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. central vision. The seeing of objects in the central, or foveal, part of the visual field, which is approximately 2Â° in diameter. Central vision permits seeing much finer detail than does peripheral vision.
62 Solid-State Roadway Lighting Design CIE. International Committee on Illumination, publishers of lighting references and design guides. coefficient of utilization (CU). The ratio of the luminous flux (lumens) from a luminaire received on the surface of the roadway to the lumens emitted by the luminaireâs lamps alone. complete interchange lighting. The lighting of a freewayâs through traffic lanes through the interchange, the traffic lanes of all ramps, the acceleration and deceleration lanes, all ramp terminals, and the crossroad between the outermost ramp terminals. constant-current transformer. A device (sometimes erroneously referred to as a âconstant- current regulatorâ) that automatically maintains a constant current in its secondary circuit under varying conditions of load impedance when supplied from a constant potential source. contrast sensitivity. The ability to detect the presence of luminance differences. Quantitatively, contrast sensitivity is equal to the reciprocal of the contrast threshold. contrast threshold. The minimal perceptible contrast for a given state of adaptation of the eye; also defined as the luminance contrast detectable during some specific fraction of the times it is presented to an observer. cutoff. Designation for luminaire light distribution limiting uplights. See IESNA classifications. diffuse reflectance. The ratio of the flux leaving a surface or medium by diffuse reflection to the incident flux. diffuser. A device to redirect or scatter the light from a source, primarily by the process of diffuse transmission. disability glare. Glare resulting in reduced visual performance and visibility, often accompanied by discomfort. See veiling luminance. discomfort glare. Glare producing discomfort. It does not necessarily interfere with visual performance or visibility. equipment factor. A factor used in calculations of illuminance or luminance to compensate for light losses resulting from normal production tolerances of commercially available luminaires as compared with laboratory photometric test models. fixture. See luminaire. footcandle (fc). The unit of illumination when the foot is taken as the unit of length. It is the illumination on a surface 1 square foot in area on which there is a uniformly distributed flux of 1 lumen, or the illumination produced on a surface, all points of which are at a distance of 1 foot from a directionally uniform point source of 1 candela. footlambert (fL). A unit of luminance (photometric brightness) equal to 1/p candela per square foot, or to the uniform luminance of a perfectly diffusing surface emitting or reflecting light at the rate of 1 lumen per square foot, or to the average luminance of any surface emitting or reflecting light at that rate. See luminance and candela per square meter. foveal vision. See central vision. 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. See disability glare and discomfort glare. Note: The magnitude of the sensation of glare depends on factors such as the size, position, and luminance of a source, the number of sources, and the luminance to which the eyes are adapted.
Glossary 63 high-intensity discharge lamp. A general group of lamps that includes mercury, metal halide, and high-pressure sodium lamps. high-mast lighting. Illumination of a large area by means of a group of luminaires designed to be mounted in fixed orientation at the top of a high mastâgenerally 80 feet (25 meters) or higher. high-pressure sodium lamp. A sodium vapor lamp in which the partial pressure of the vapor during operation is on the order of 104 N per m2 (0.1 atmosphere). illuminance. The density of the luminous flux incident on a surface; it is the quotient derived by dividing the luminous flux by the area of the surface when the latter is uniformly illuminated. Illuminating Engineering Society of North America (IESNA). Publishers of lighting references and design guides. isocandela line. A line plotted on any appropriate coordinates to show directions in space about a source of light in which the candlepower is the same. For a complete exploration, the line always is a closed curve. A series of such curves, usually for equal increments of candlepower, is called an isocandela diagram. isolux (isofootcandle) line. A line plotted on any appropriate coordinates to show all the points on a surface where the illumination is the same. For a complete exploration, the line is a closed curve. A series of such lines for various illumination values is called an isolux (isofootcandle) diagram. lambert (L). A unit of luminance (photometric brightness) equal to 1/p candela per square centimeter. lambert surface. A surface that emits or reflects light in accordance with Lambertâs cosine law. A lambert surface has the same luminance regardless of viewing angle. lamp lumen depreciation (LLD) factor. The multiplier to be used in illumination calculations to relate the initial rated output of light sources to the anticipated minimum rated output based on the relamping program to be used. (See the discussion of light loss factor in Chapter 10). light center (of a lamp). The center of the smallest sphere that would completely contain the light-emitting element of the lamp. light center length (of a lamp). The distance from the light center to a specified reference point on a lamp. light loss factor. A depreciation factor applied to the calculated initial average luminance or illuminance. lighting unit. The assembly of pole or standard with bracket and luminaire. low-pressure sodium lamp. A sodium vapor lamp in which the partial pressure of the vapor during operation is on the order of 30 mm of mercury (0.04 atmosphere). lumen (lm). The unit of luminous flux. It is equal to the flux through a unit solid angle (steradian), from a uniform point source of one candela (candle), or to the flux on a unit surface, all points of which are at unit distance from a uniform point source of one candela. luminaire. A complete fixture consisting of a lamp or lamps together with the parts designed to distribute the light, position and protect the lamps, and connect the lamps to the power supply. luminance (photometric brightness). (1) Luminance in a direction, at a point on the surface of a source of a receiver, or of any other real or virtual surface, is the quotient of the luminous
64 Solid-State Roadway Lighting Design flux leaving, passing through, or arriving at an element of the surface surrounding the point and propagated in directions defined by an elementary cone containing the given direction, by the product of the solid angle of the cone and the area of the orthogonal projection of the element on the surface on a plane perpendicular to the given direction. See candela per square meter. (2) The luminous intensity of any surface in a given direction per unit of projected area of the surface as viewed for that direction. Note: In the defining equation, q is the angle between the direction of observation and the direction normal to the surface. In common usage, the term âbrightnessâ usually refers to the intensity of sensation resulting from viewing surfaces or spaces from which light comes to the eye. This sensa- tion is determined in part by the definitely measurable luminance defined above and in part by conditions of observation, such as the state of adaptation of the eye. In much of the literature the term âbrightness,â used alone, refers to both luminance and sensation. The context usually indicates which meaning is intended. luminance ratio. The ratio between the luminance (photometric brightness) of any two areas in the visual field. luminous efficacy (of a source of light). The quotient of the total luminous flux emitted by the total lamp power input. It is expressed in lumens per watt. luminous flux. Lumens. lux (lx). The International System (SI) unit of illumination. It is the illumination on a surface 1 square meter in area on which there is a uniformly distributed flux of 1 lumen or the illumi- nation produced at a surface, all points of which are at a distance of 1 meter from a uniform point source of 1 candela. maintenance factor (MF). A factor formerly used to denote the ratio of the illumination of a given area after a period of time to the initial illumination on the same area. mean lamp lumens. Mean lumen output of a lamp; it is calculated by determining the area beneath the lumen maintenance characteristic curve of that source over a given period of time and dividing that area by the time period in hours. mercury lamp. An electric discharge lamp in which the major portion of the radiation is produced by the excitation of mercury atoms. metal halide lamp. A discharge lamp in which the light is produced by the radiation from a mixture of a metallic vapor (for example, mercury) and the products of the disassociation of halides (for example, halides of scandium or sodium). mounting height (MH). The vertical distance between the roadway surface and the center of the apparent light source of the luminaire. non-cutoff. The luminaire light distribution category when there is no limitation in candle- power in the zone above maximum candlepower. See IESNA classifications. overhang. The distance between a vertical line passing through the luminaire and the curb or edge of the roadway. partial interchange lighting. Lighting consisting of a few luminaires located in the vicinity of some or all ramp terminals, intersections, or other decision-making areas. pedestrian ways. Public sidewalks for pedestrian traffic generally not within rights-of-way for vehicular traffic roadways. Included are skywalks (pedestrian overpasses), subwalks
Glossary 65 (pedestrian tunnels), walkways giving access to park or block interiors, and crossings near centers of long blocks. point of fixation. A point or object in the visual field at which the eyes look and upon which they are focused. rapid-start fluorescent 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. reaction time. The interval between the beginning of a stimulus and the beginning of the response of the observer. reflector. A device used to redirect the luminous flux from a source by the process of reflection. refractor. A device used to redirect the luminous flux from a source, primarily by the process of refraction. semicutoff. Designation for luminaire light distribution when the candlepower per 1,000 lamp lumens does not numerically exceed 50 (5%) at an angle of 90Â° above nadir (horizontal) and 200 (20%) at a vertical angle of 80Â° above nadir. This applies to any lateral angle around the luminaire. spacing. For roadway lighting, the distance between successive lighting units, measured along the center line of the street. spacing-to-mounting height ratio (S/MH). The ratio of the distance between luminaire centers to the mounting height above the roadway. street lighting luminaire. A complete lighting device consisting of a light source together with its direct appurtenances such as globe, reflector, refractor, housing, and such support as is integral with the housing. The pole, post, or bracket is not considered part of the luminaire. street lighting unit. The assembly of a pole or lamppost with a bracket and a luminaire. transverse roadway line (TRL). Any line across the roadway that is perpendicular to the curb line. uniformity of illuminance. The ratio of average footcandles (lux) of illuminance on the pavement area to footcandles (lux) at the point of minimum illuminance on the pavement, commonly called the uniformity ratio. uniformity of luminance. The ratio of the average level of luminance to the minimum point of luminance or of the maximum point of luminance to the minimum point. The average- to-minimum method uses the average luminance of the roadway design area between two adjacent luminaires divided by the lowest value at any point in the area. The maximum- to-minimum method uses the maximum and minimum values between the same adjacent luminaires. The uniformity of luminance (average/minimum and maximum/minimum) considers the traveled portion of the roadway, except for divided highways that have different designs on each side. utilization efficiency. A plot of the quantity of light falling on a horizontal plane both in front of and behind the luminaire. It shows only the percentage of bare lamp lumens that fall on the horizontal surface and is plotted as a ratio of the width of the area to the mounting height of the luminaire. veiling luminance. Luminance superimposed on the retinal image that reduces the image contrast. The veiling effect may be produced by bright sources in the visual field.
66 Solid-State Roadway Lighting Design visibility. The quality or state of being perceivable by the eye. Visibility may be defined in terms of the distance at which an object can just be perceived by the eye, or it may be defined in terms of the contrast or size of a standard test object observed under standardized viewing conditions and having the same threshold as the given object. visual acuity. A measure of the ability to distinguish fine details. Quantitatively, it is the reciprocal of the angular size (in minutes) of the critical detail that is just large enough to be seen. visual angle. The angle subtended by an object or detail at the point of observation. It is usually measured in minutes of arc. walkway. A sidewalk or pedestrian way.