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.
[1] An output bandwidth of one hertz is equivalent to half a second of integration time.
[2] The units of NEP are watts per square root hertz.
The NEP is equal to the noise amplitude spectral density (expressed in units of
can detect a signal power of one picowatt with a signal-to-noise ratio (SNR) of one after one half second of averaging.
If instead it refers to the signal power incident on the detector system, it is called the optical NEP.