Noise-equivalent flux density
noise figure in optics
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 noise present in the system. It is a measure used by astronomers in determining the accuracy of observations.
The NEFD can be related to a light detector's noise-equivalent power for a collection area A and a photon bandwidth
ν
{\displaystyle \nu }
by:
N
E
F
D
=
η
N
E
P
A
ν
{\displaystyle \mathrm {NEFD} =\eta {\frac {\mathrm {NEP} }{A\nu }}}
,
where a factor
η
{\displaystyle \eta }
(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.
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