Working toward legislation to curb light pollution in Illinois.

Illinois Coalition for Responsible Outdoor Lighting

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Outdoor Illumination Principles

Since earliest times, humans have sought to bring light to their surroundings at night. Campfires could help keep dangerous nocturnal animals at bay; torches could allow activity to continue when there was not enough natural light to see by. We continue the practice of lighting up the night today, and our light sources have become tremendously more powerful. Unfortunately, many are used in forms no more advanced than a caveman's flaming torch, and both their tremendous energy consumption and their unintended stray illumination are taking high tolls on our environment.

All lighting installations should be engineered with three main factors in mind:
--What is the precise area which needs illumination?
--What level of illumination is required?
--For what time periods is the illumination needed?
An installation which doesn't meet our needs is insufficient; one which illuminates beyond the intended area, illuminates to too high a level, or operates at times when illumination is not needed is wasting energy.

Fixtures are often specified for new or replacement installations by architects or committees who will look at a catalog and pick a design which will look good in the daytime, without considering the energy cost of running them night after night, year after year, or the impact (good or bad) which nighttime lighting can have on an installation or its neighborhood.

Illuminating An Area

Any outdoor illumination should serve a purpose. The most common one is to illuminate an area where people must see things after dark to perform some type of activity; this may be to operate vehicles, walk, play sports, or shop. Responsibly engineering a lighting installation involves determining what light level is needed for the activity, and designing a set of fixtures to deliver only that amount, only to the area which needs illumination.

The most efficient way to illuminate an area lighting is usually to install fixtures overhead, shining their light downward. Most standard types of light fixture will best illuminate the area under the fixture, with the light level tapering off the further one moves along the ground away from the spot under the light; in other words, the fixture creates a cone of good illumination under it.

Fixture spacing is determined by the quality of lighting needed for the activity. Pedestrian and vehicular movement can often be served by having the light cones of adjacent fixtures just intersect, as in the top row of the illustration below; high light level activities such as automobile sales lots (during business hours) may need closer light spacing for more uniform, multi-directional illumination.

Diagram of exterior light fixture spacing.

Two rows of lights; the top one with the fixtures spaced so the fully illuminated zones meet, and the lower one with the zones overlapping, so each spot is illuminated from multiple directions.


The amount of lighting an area receives can be measured in several ways. The most common system is to measure the amount of light falling on a given surface area; the primary units used for this are foot-candles and lux.

Different nocturnal activities require different levels of lighting to be accomplished safely and successfully. As noted above, proper engineering of outdoor lighting installations includes determining the appropriate levels, and sizing and spacing fixtures to achieve levels within the appropriate range over the area to be illuminated (and to not spill illumination outside of that area). Activities such as navigating through a parking lot may take as little as .25 foot-candles; walking up a stairway may need illumination in the 1-5 foot-candle range, and safely fueling a vehicle 10 foot-candles.


Exterior lights should only be operated when needed.Operating exterior (and interior) lighting outside of the hours when it is actually needed is a source of tremendous energy waste in our state. Timers or photoelectric switches may turn lights on at dusk, and off at dawn, when in reality they are only needed during evening hours, or can at least be turned down to a lower level for the majority of the night. Business exterior lights can be turned down to a security level (including extinguishing lighted advertising) after business hours, or at least after the time when most potential customers are at home for the night (lighting ordinances often give 11:00pm as an accepted time for this). Auto dealerships can turn off 3/4 of their display yard lights, and still have enough remaining for security, saving large amounts on their utility bills, and losing very little in impulse buying. Sports fields, which use greatly more electrical lighting than any other outdoor area, should have curfews, and be required to turn off their field lights immediately after games are over (and, sports managers should be encouraged to schedule daytime games as much as possible, when the sun is providing excellent lighting without any detrimental environmental impact!) Even cities can dim their downtown lighting during the late-night hours when all businesses are closed (and downtown parking is often not allowed).


The human eye is capable of adapting to see over a wide range of lighting conditions, from intensely sunlit day to faintly moonlit night. When the eye is adapted to lower lighting conditions, such as outside after dark, and a distinct light source is presented which "overloads" it with too high a light level, glare occurs. We end up neither being able to see the bright light nor the formerly visible nighttime scene around it well.

The lampshade was invented to remove glare.Soon after the invention of the electric lamp came the development of the lampshade; people found that having a glaring light bulb in view actually made it harder to see in a room after dark. The only place we are commonly exposed to bare light bulbs nowadays is in poorly designed outdoor lighting.
As noted above, exterior light fixtures are generally used to illuminate an area below and around them. Direct lighting which escapes to the sides will shine into the eyes of people approaching the area, causing glare. The light output of a fixture, therefore, falls into three main classes: The desired light, which directly illuminates the surfaces within the area which the fixture is intended to illuminate; sideways-spilling light, which can cause glare to anyone looking toward the area, and which consists of energy being wasted by the fixture; and any light which shines upwards from the fixture, which is entirely wasted (unless the light is intended to illuminate the undersides of airplanes, birds, or other objects passing overhead). As described on our Lighting Studies> Fixtures page, the fixture in the diagram below could be replaced with one which focuses its lamp's entire light output in the desired lighting area, illuminating that area to the same brightness while using a lamp which consumes just third of the electricity of the one in the acorn globe.

Diagram illustrating how much of a ligh fixture's output can be waste.

"Light trespass" is the term used to describe artificial light which shines from an area of intended illumination to adjacent areas. If your neighbors' lights shine brightly into your yard and windows, that is light trespass. If the streetlight outside keeps you awake at night, that is light trespass. If you look up in the clear sky at night, and see no stars, only the glow of many lights making your sky bright, that is light trespass (although this last variety also goes by the commonly used name of "light pollution").

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