Glossary of terms related to lighting and light pollution

Footcandle - a unit of measurment of illuminance, or the amount of light falling on a given amount of surface area. For familiar nighttime situations this ranges from about one fc as a reasonable limit to light trespass to more than a hundred fc at a grossly overlit gas station canopy. A reasonably illuminated gas station, found mostly at older facilities, is about 35 fc.

Full cutoff - a description of light fixtures that cut off the light above the horizon--that is, they do not shine light upward above the fixture (into the sky). These include the so-called "shoebox" design fixture used in many parking lots as well as better street lights and wall packs (like the Criterion line from GE Lighting Systems).

Glare - light seen directly from the lighting fixture's bulb. Glare is much brighter than what is being illuminated, so it actually reduces the visibility of the area lighted.

Spill light - light that goes beyond the area that was intended to be illuminated. This may also be the same as light trespass, where the light spills across a property line.