Lighting Research Center Lighting Research Center
    Volume 7 Issue 2
March 2003 (revised February 2007)    
candela - The Systeme International d'Unities (SI) of luminous intensity. One candela is one lumen per steradian. Formerly, candle. 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. 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. 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. 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. luminous flux - Luminous radiant power, measured in lumens. The overall light output of a lamp or luminaire. 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. steradian (sr) - A unit of measure equal to the solid angle subtended at the center of a sphere by an area on the surface of the sphere equal to the square of the sphere radius. 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. 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. direct uplight - Light emitted upward by a luminaire. cutoff classification - The classification system of the Illuminating Engineering Society of North America (IESNA) that describes the light distribution of anoutdoor luminiare. Cutoff classifications define the luminous intensity limits in two illumination zones that occur within the range of 80° to 180° above nadir. 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. 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. light trespass - A undesirable condition in which exterior light is cast where it is not wanted. 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. nadir - In the lighting discipline, nadir is the angle pointing directly downward from the luminaire, or 0. Nadir is opposite the zenith. 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.) 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. zenith - In the lighting discipline, zenith is the angle pointing directly upward from the luminaire, or 180. Zenith is opposite nadir. In astronomical usage, zenith is the highest point in the sky, directly above the observation point. 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.)
How is the issue of light pollution currently being addressed?

A number of educational, research, and developmental efforts addressing light pollution are underway, including work to design lighting equipment that produces less light pollution; but the main efforts currently involve education and legislation.

In response to the demand to reduce light pollution, research and development efforts have focused on advancements in technology to design luminaires to efficiently direct light where it is needed. Luminaire manufacturers have concentrated on providing highly efficient luminaires with given beam distributions while meeting the cutoff classifications set forth by the Illuminating Engineering Society of North America (IESNA) to reduce glare as well as wasted light. Advancements in lamp technologies have resulted in producing high efficiency light sources that reduce light pollution and have the added benefit of saving energy.

In an effort to increase public awareness on the topic of quality outdoor lighting, the number of state, local, and regional lighting ordinances is growing. Many states have adopted legislation controlling outdoor lighting, more states have pending outdoor lighting bills in front of their legislators. Numerous adopted and pending local ordinances pertain to the lighting of cities, towns, and counties. Figure 8 shows which states have adopted or pending statewide legislation.

Legislation typically includes requirements for full cutoff luminaires, minimum light levels, lumen or wattage limitations, light source type limitations, controlled operating periods, curfews, and the elimination of certain kinds of lighting. (Curfews for outdoor lighting are generally defined by local planning authorities based on anticipated use of the area and, thus, a need for lighting. During pre-curfew times, the need for lighting is warranted, so lighting levels are generally higher than during post-curfew times.) In addition to preserving dark skies, legislation is being justified on the basis of minimizing wasted energy and money, reducing unwanted light on adjoining properties (such as light in bedroom windows), reducing glare, and preserving animal breeding and migration habitats.

Figure 8. Adopted and pending legislation governing light pollution in the United States, as of January 2003

Luminaires are designed to have lighting distributions that are appropriate for specific applications. Virtually any luminaire can generate sky glow, light trespass, and glare if installed improperly or in the wrong application. These problems can be avoided by selecting luminaires that have the appropriate distribution for the application and installing them correctly to limit spill light and uplight.


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