Noise equivalent temperature difference
Encyclopedia
Noise-equivalent temperature (NET) is a measure of the sensitivity of a detector of thermal radiation in the infrared
, terahertz or microwave
portions of the electromagnetic spectrum
. It is the amount of incident signal temperature
that would be needed to match the internal noise of the detector such that the signal-to-noise ratio
is equal to one. Often the spectrum of the NET is reported as a temperature per root bandwidth. A detector that measures power is often interested in the analogous noise-equivalent power
(NEP). If a relation between intensity and temperature is well defined over the pass band, as in the case of a blackbody, then the NET simply scales with the NEP.
If a detector is limited by either shot noise
or Johnson noise then the NET can be decreased by using an increased integration time. The NET of flicker noise
limited detectors can not be reduced by increased integration time.
Typically uncooled bolometric detectors
have NET figures of 80-200 mK
. Cooled photon detecting infrared detectors using materials such as HgCdTe (LWIR or MWIR) or InSb (MWIR) can approach a NET figure of 10 mK. In the microwave radiation region NET values are typically several hundred millikelvins to several kelvins.
For a particular mean signal temperature there is a fundamental limit to NET given by the natural thermodynamic fluctuations of the photon flux from the source under investigation.
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, terahertz or microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
portions of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. It is the amount of incident signal temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
that would be needed to match the internal noise of the detector such that the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
is equal to one. Often the spectrum of the NET is reported as a temperature per root bandwidth. A detector that measures power is often interested in the analogous noise-equivalent power
Noise-equivalent power
Noise-equivalent power 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. An output bandwidth of one hertz is equivalent to half a second of integration time. The units...
(NEP). If a relation between intensity and temperature is well defined over the pass band, as in the case of a blackbody, then the NET simply scales with the NEP.
If a detector is limited by either shot noise
Shot noise
Shot noise is a type of electronic noise that may be dominant when the finite number of particles that carry energy is sufficiently small so that uncertainties due to the Poisson distribution, which describes the occurrence of independent random events, are of significance...
or Johnson noise then the NET can be decreased by using an increased integration time. The NET of flicker noise
Flicker noise
Flicker noise is a type of electronic noise with a 1/ƒ, or pink power density spectrum. It is therefore often referred to as 1/ƒ noise or pink noise, though these terms have wider definitions...
limited detectors can not be reduced by increased integration time.
Typically uncooled bolometric detectors
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...
have NET figures of 80-200 mK
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. Cooled photon detecting infrared detectors using materials such as HgCdTe (LWIR or MWIR) or InSb (MWIR) can approach a NET figure of 10 mK. In the microwave radiation region NET values are typically several hundred millikelvins to several kelvins.
For a particular mean signal temperature there is a fundamental limit to NET given by the natural thermodynamic fluctuations of the photon flux from the source under investigation.
See also
- Noise-equivalent powerNoise-equivalent powerNoise-equivalent power 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. An output bandwidth of one hertz is equivalent to half a second of integration time. The units...
- Specific detectivitySpecific detectivitySpecific detectivity, or D*, for a photodetector is a figure of merit used to characterize performance, equal to the reciprocal of noise-equivalent power , normalized per unit area....
- Minimum resolvable temperature differenceMinimum resolvable temperature differenceMinimum resolvable temperature difference is a measure for assessing the performance of infrared cameras, and is inversely proportional to the modulation transfer function....
External links
- Expanding the Vision of Sensor Materials (1995) National Materials Advisory Board (NMAB)