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.

LIGHT LEVELS:

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.

HOURS OF OPERATION:

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).

GLARE AND LIGHT TRESPASS:

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.

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.

"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").