Noise-equivalent power
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Noise-equivalent power (NEP) is the radiant power that produces a signal-to-noise ratio of unity at the output of a given optical detector at a given data-signaling rate or modulation frequency, operating wavelength, and effective noise bandwidth.
Note 1: Some manufacturers and authors define NEP as the minimum detectable power per square root bandwidth. When defined this way, NEP has the units of watts per (hertz)1/2. Therefore, the term is a misnomer, because the units of power are watts.
Note 2: Some manufacturers define NEP as the radiant power that produces a signal-to-dark-current noise ratio of unity. The NEP measurement is valid only if the dark-current noise dominates the noise level.
Source: from Federal Standard 1037C and from MIL-STD-188