Noise-equivalent power

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Noise-equivalent power (NEP) is a measure of the sensitivity of an optical detector or detector system. It is defined as the signal power which gives a signal to noise ratio of 1 for an integration time of half a second, or more technically 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 [1]. Therefore, it is desirable to have as low an NEP as possible, since a small NEP means a high signal to noise ratio (SNR).

The advantage of expressing a detector's sensitivity in NEP as opposed to a SNR is that the NEP remains constant for a given detector. For example, if a detector was placed 1 meter from the source it was observing, it would have a particular SNR. If the detector was then moved to 2 meters from the source, the amount of power from the source reaching it would be reduced but the noise could be assumed to stay the same, thus the SNR would have been reduced. However the NEP would remain the same for both of these measurements.

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.

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

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