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Light and Dark Defined
BORTLE CLASS: A qualitative method of rating night skies based on visual observations. Developed by John Bortle, the scale ranges from Class 1 (pristine) to Class 9 (strongly light polluted).
FULLY SHIELDED: A lighting fixture that directs all light downward (below the horizontal) except for incidental reflections from supports or other structures.
GLARE: A common condition of natural and artificial lighting causes by excessive contrast between a bright source or brightly lit area and a dark surrounding area. Glare can cause viewers to look away, squint, be annoyed or compromise their ability to perform vital visual tasks.
GUIDANCE LIGHTING: Lighting that provides for navigation and safety via very low brightness lamps to mark a path, edge or roadway instead of the traditional approach of illuminating surfaces.
ILLUMINATION: The amount of light falling onto a surface measured in lumens per unit area. The footcandle is equal to one lumen per square foot. A lux is 1 lumen per square meter, approximately 1/10th of a footcandle.
LANDSCAPE LIGHTING: Luminaries mounted in or at grade (but not more than 3 feet above grade) and used solely for landscape rather than any area lighting.
LUMEN: The unit used to describe the amount of light radiated by a source.
LUMINAIRE (LIGHT FIXTURE): A complete lighting unit consisting of one or more electric lamps, the lamp holder, any reflector or lens, ballast (if any) and any other components and accessories
OBTRUSIVE LIGHT: Spill light that causes glare, annoyance, discomfort or loss of visual ability. Light pollution.
SPILL LIGHT: Light from a lighting installation that falls outside of the boundaries of the property on which it is located. Usually results in obtrusive light.
Source: International Dark-Sky Association
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