Penrose method

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 $\nu$ by:

$NEFD=\eta {\frac {NEP}{A\nu }}$, where a factor $\eta$ (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.

References

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