optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

the noise-equivalent flux density (NEFD) or noise-equivalent irradiance (NEI) of a system is the level of

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...

density required to be equivalent to the

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...

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 Noise-equivalent power (NEP) is a measure of the sensitivity of a photodetector or detector system. It is defined as the signal power that gives a signal-to-noise ratio of one in a one hertz output bandwidth. An output bandwidth of one hertz is equi ...

for a collection area ''A'' and a photon bandwidth

\nu

by:

NEFD = \eta \frac

, 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

Physical quantities Vector calculus {{Optics-stub