Dark Sky

If you were in a spacecraft looking back at the Earth at night, you would have no trouble seeing the evidence of habitation, for the earth glows with electric light. Although outdoor lighting serves security and economic purposes, its excessive use has created serious light pollution. The problem first came to public attention when astronomers working at observatories in the southwest United States noticed that their research efforts were thwarted by the light emanating from outdoor electric lights. But astronomers are not the only ones adversely affected by light pollution. Sky glow, glare, and spillover from outdoor lighting can create dangerous roadway conditions, and floodlight spilling onto personal property (“light trespass”) is not only a nuisance but can have adverse affects on people, plants, and animals.

  • Sky glow occurs in part from natural sources such as sunlight reflected off the moon, starlight, and atmospheric conditions, but most sky glow results from light that is emitted or reflected upward into the sky by unshielded or over-bright outdoor lights. Sky glow depends on weather conditions, the amount of dust and gas in the atmosphere, the amount of light, and the direction from which it is viewed. It is difficult to measure because it involves so many variables that change from moment-to-moment, but astronomers and lighting professionals are developing ways to record and evaluate sky brightness.
  • Light trespass occurs when light spills over into areas where it’s not wanted. For example, streetlights, floodlights, or advertising lights may shine through your window and right onto your pillow, making it hard for you to sleep. Like sky glow, light trespass is hard to measure because each instance is different and the whether it is “unwanted” is very subjective. Although light trespass has been largely ignored by modern lighting practices, it is coming under closer scrutiny and is the primary focus of many anti-light-pollution laws. In some cases, however, these laws are written by people with little or no professional lighting experience and often contain technical problems.
  • Glare is excessive and uncontrolled brightness that can be uncomfortable or disabling. Discomfort glare is annoying or painful; disability glare reduces visibility. In many commercial situations, premises are overlit with glaring lighting under the belief that an intense light level is required for security. In other instances, glare is created by advertising signage.

Unwanted light is a waste of money and energy. The International Dark-Sky Association, a non-profit organization dedicated to increasing awareness and resolving the problem of light pollution, performed an analysis of the costs of the excess light produced by streetlights in the United States. The study, set forth in an information sheet dated in 2000, considered the 175-watt dusk-to-dawn mercury vapor lamp widely used for yard lighting, security lighting, and street lighting. It assumed an annual operating cost for this lamp of approximately $70 per year, based on average energy costs at the time. In Tucson, Arizona, with a population of about 600,000, the local utility had over 20,000 of these lights, costing nearly 1.4 million dollars per year to operate (this figure is surely higher in 2006). The population of the United States is 500 times that of Tucson, so the Association estimated that the annual cost of operating this type of fixture throughout the United States was as much as $700 million. Because 30% of the light produced by mercury vapor streetlights is directed up into the sky where it is not used or needed, these lights waste about $200 million per year in operating costs alone. This figure increases when the environmental cost of generating the electricity to power these lights is taken into account.

To rectify light pollution, research and development efforts are underway to develop technology to direct light where it is needed, in the amount needed. For example, manufacturers are designing luminaires that are fully shielded on the top, directing the light beam only toward the ground and that reduce light levels to the lowest required for the application. Click here for the International Dark-Sky Association’s list of manufacturers of sky-friendly products and other resources.

Other associations such as the Illuminating Engineering Society of North America (IESNA), the Institute of Lighting Engineers (ILE), and the Commission Internationale de l’Eclairage (CIE) have developed or are developing classifications and guidelines for preventing light pollution. State and local governments are tightening their legislation controlling outdoor lighting, including requirements for shielded luminaries, lumen or wattage limitations, controlled operating periods, and the elimination of certain kinds of lighting.

Here’s an area where a little knowledge could go a long way. Including awareness of light pollution in all architecture, interior design, and engineering coursework and all lighting-related qualifying exams would help make more people aware of the issue. Let’s aim for a darker sky.

Here are a few more resources. A little research will find a lot more.

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