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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 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.