How is sky glow measured or calculated?

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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 - The rate of flow of light, measured in lumens. The overall light output of a lamp 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 spill 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.)

With the growing awareness of light pollution, lighting professionals only recently began measuring sky glow in an attempt to quantify its relationship to electric lighting. This is a very challenging task, because many factors play a role in sky glow. One must not only consider the lighting, but also the angular distribution of the light emitted from the luminaire, the light reflected from the ground and its angular distribution, as well as atmospheric effects of humidity and the interaction of light with aerosols (particles in the atmosphere that may be caused by manufactured pollutants, fire, volcanic eruptions, etc.), all of which can change from moment to moment.

When observing the night sky, professional astronomers often measure the dark portion of the sky with their astronomical equipment to have a background value they can use to compare their star signal against. Professional astronomers use an instrument called a photoelectric photometer to make these measurements. Typically such measurements are made at the zenith. Other methods to measure sky brightness are discussed in the CIE technical report, Guidelines for Minimizing Sky Glow (CIE 1997).

Many amateur and professional astronomers have recorded sky brightness measurements to monitor the increase in sky brightness (Lockwood et al. 1990; Upgren 1991). From these data, a number of metrics or calculation methods have been developed to estimate sky glow (Garstang 1986, 1989, 1991; Walker 1977). One very simple approach used by casual observers, for example, is to estimate sky glow by looking for the Big Dipper constellation and then counting how many stars are visible to the naked eye. However, none of the metrics directly relate electric lighting emissions to sky glow. For example, both Garstang (1986) and Walker (1977) developed relationships using population multiplied by a given number of lumens per person to include lighting in their calculations. However, the distribution of the light sources, the number of light sources, wattage, and the amount of reflected light are all excluded from the calculation. Without the ability to model this lighting information, it is difficult to determine how to minimize sky glow while minimizing the impact on the use of light at night.