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.
[1] It is a measure used by astronomers in determining the accuracy of observations.
[2] The NEFD can be related to a light detector's noise-equivalent power for a collection area A and a photon bandwidth
(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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