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In [[optics]] the '''noise-equivalent flux density (NEFD)''' or '''noise-equivalent irradiance (NEI)''' of a system is the level of [[flux]] density required to be equivalent to the [[Signal noise|noise]] present in the system.<ref>{{cite book |title=Physical Optics and Light Measurements |last=Malacara |first=Daniel |page=247 |year=1988 |publisher=[[Academic Press]] |isbn=9780124759718 }}</ref> It is a measure used by astronomers in determining the accuracy of observations.<ref>{{cite doi |10.1023/A:1022671223858}}</ref>

The NEFD can be related to a light detector's [[Noise-Equivalent Power]] for a collection area ''A'' and a photon bandwidth <math>\nu</math> by:

<math>NEFD = \eta \frac{NEP}{A \nu}</math>, where a factor <math>\eta</math> (often 2, in the case of switching between measuring a source and measuring off-source) accounts for the photon statistics for the mode of operation.

==References==
{{Reflist}}

{{DEFAULTSORT:Noise Equivalent Flux Density}}
[[Category:Physical quantities]]
[[Category:Vector calculus]]


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