Noise-equivalent power (NEP) is a measure of the sensitivity of a

photodetector Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical ...

or detector system. It is defined as the signal power that gives a

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...

of one in a one

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...

output bandwidth. An output bandwidth of one hertz is equivalent to half a second of integration time.The factor of one half is explained by the Nyquist–Shannon sampling theorem. The units of NEP are

watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...

s per

square root In mathematics, a square root of a number is a number such that y^2 = x; in other words, a number whose ''square'' (the result of multiplying the number by itself, or y \cdot y) is . For example, 4 and −4 are square roots of 16 because 4 ...

. The NEP is equal to the noise amplitude spectral density (expressed in units of

\mathrm/\sqrt

\mathrm/\sqrt

) divided by the

responsivity Responsivity is a measure of the input–output Gain (electronics), gain of a detector system. In the specific case of a photodetector, it measures the electrical output per optical input. A photodetector's responsivity is usually expressed in un ...

(expressed in units of

\mathrm/\mathrm

\mathrm/\mathrm

, respectively). The fundamental equation is

SNR = P/NEP

. A smaller NEP corresponds to a more sensitive detector. For example, a detector with an NEP of

10^ \mathrm/\sqrt

can detect a signal power of one picowatt with a signal-to-noise ratio (SNR) of one after one half second of averaging. The SNR improves as the square root of the averaging time, and hence the SNR in this example can be improved by a factor of 10 by averaging 100-times longer, i.e. for 50 seconds. If the NEP refers to the signal power absorbed in the detector, it is known as the electrical NEP. If instead it refers to the signal power incident on the detector system, it is called the optical NEP. The optical NEP is equal to the electrical NEP divided by the optical coupling efficiency of the detector system.

References and footnotes

See also