Night visual range: Difference between revisions
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{{Term | |||
|Display title=night visual range | |||
{{ | |Definitions={{Definition | ||
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|Meaning=(''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. | |||
|Explanation=For a [[light]] source of [[luminous]] power ''P''<sub>''v''</sub> (in lumens), uniform atmospheric [[extinction coefficient]] σ along the viewing direction, and [[threshold illuminance]] ''E''<sub>''thresh''</sub> for the [[observer]], the night visual range ''x'' is <blockquote>[[File:ams2001glos-Ne6.gif|link=|center|ams2001glos-Ne6]]</blockquote> Unlike the [[daytime visual range]], the night visual range is determined by both the [[inverse-square law|inverse-square law]] and atmospheric [[extinction]]. Note that for a light to be seen at a distance ''x'' > 0, its [[illuminance]] at the observer (''P''<sub>''v''</sub>/''x''<sup>2</sup>) must exceed ''E''<sub>''thresh''</sub>. Even if σ is known, there is no unique night visual range because ''x'' also depends on ''P''<sub>''v''</sub> and the observer's [[dark adaptation]]. <br/>''See'' [[Allard's law]] | |||
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Latest revision as of 09:16, 27 March 2024
(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
See Allard's law
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
