Noise Equivalent Power

Definition: Noise Equivalent Power (NEP) quantifies the sensitivity of a detector (also often used for the instrument to define the sensitivity of the entire system). It is the minimum input power required to produce a usable output signal in the presence of noise, or in other words, a measure of the weakest optical signal that can be detected. Therefore, it is desirable to have a NEP as low as possible since a low NEP value corresponds to a lower noise floor and, consequently, a more sensitive detector. A low NEP is beneficial even at higher input intensities since it will lead to lower noise characteristics in the output signal.Technically, NEP is the input power that produces a signal-to-noise ratio (SNR) of one in a 1 Hertz (Hz) bandwidth. For photodetectors it is given in Watts per square root of Hertz (W/√Hz). However, it is also not uncommon to list a system’s sensitivity in terms of its total individual bandwidth in Watts only. Typical dimensions in the field of fNIRS are in the magnitude of pW To calculate Noise Equivalent Power, it is necessary to know the spectral sensitivity of the photodetector, dark noise performance, measurement bandwidth, and the current-to-voltage conversion factor.

Alternative definition: Noise equivalent power is a metric used to evaluate the sensitivity of a detector, which is the minimum input power required to produce a usable output signal in the presence of noise. The lower the NEP value, the more sensitive the detector is, since a low NEP corresponds to a lower noise floor. NEP is calculated as the input power that produces a signal-to-noise ratio (SNR) of one in a 1 Hertz (Hz) bandwidth, typically measured in Watts per square root of Hertz (W/√Hz). In the field of fNIRS, NEP values are typically in the magnitude of picoWatts (pW).

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References: 10.3390/s20102831, 10.3390/s21186106, 10.1016/j.neuroimage.2022.119216, Corpus ID: 7011179, https://www.thorlabs.com/images/TabImages/Noise_Equivalent_Power_White_Paper.pdf

Related terms: detector, performance, sensitivity