Noise-equivalent flux density

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 $\nu$ 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

1. Malacara, Daniel (1988). Physical Optics and Light Measurements. Academic Press. p. 247. ISBN 9780124759718.
2. Benford, D. J.; Hunter, T. R.; Phillips, T. G. (1998). "Noise Equivalent Power of Background Limited Thermal Detectors at Submillimeter Wavelengths". International Journal of Infrared and Millimeter Waves 19 (7): 931. doi:10.1023/A:1022671223858.