Night visual range

From Glossary of Meteorology



night visual range

(Also called nighttime visual range, transmission range.) The maximum distance at which a normal observer can see a particular point light source under given atmospheric conditions.

For a light source of luminous power Pv (in lumens), uniform atmospheric extinction coefficient σ along the viewing direction, and threshold illuminance Ethresh for the observer, the night visual range x is
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Unlike the daytime visual range, the night visual range is determined by both the inverse-square law and atmospheric extinction. Note that for a light to be seen at a distance x > 0, its illuminance at the observer (Pv/x2) must exceed Ethresh. Even if σ is known, there is no unique night visual range because x also depends on Pv and the observer's dark adaptation.
See Allard's law


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