IR detectors performance parameters
Signal-to-noise ratio (S/N)
It is defined as the ratio between the signal power and the noise power
S/N= PNEP
P = Incident radiant power received by the detector [W]
NEP = Noise Equivalent Power. It is defined as the signal power that gives a signal-to-noise ratio of one in a one hertz output bandwidth [W]
The higher this ratio, the best signal you get. To improve the S/N, infrared detectors must be cooled. Several cooling methods are available including thermoelectric cooling, cryogenic cooling (e.g. using liquid nitrogen) and mechanical cooling such as stirling coolers.
Responsivity R
Responsivity is the ability of the detector to convert the incoming radiation into an electrical signal. Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, responsivity measures the electrical output per optical input.
R= SP ∗ A
S = Signal output [V]
P= Incident radiant power received by the detector [W/cm2]
A = Detector Active Area [cm2]
Noise equivalent power (NEP)
A photodetector produces some noise output with a certain average power even when it does not get any input radiation. This noise output is proportional to the square of the r.m.s. voltage or current amplitude. The noise-equivalent power (NEP) of a detector is the optical input power (P) which produces an additional output power identical to the noise power for a given bandwidth (Δf). In other words, the NEP is the light power required to obtain a signal to noise ratio S/N of 1, that is, the light level required to produce a signal current equivalent to the noise current. The units of NEP are watts per square root hertz. NEP indicates the lower limit of light detection: a smaller NEP corresponds to a more sensitive detector.
NEP= P∗AS/NΔf−−−√
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[W/Hz−−−√]
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P = Incident radiant power received by the detector [W]
A = Detector active area [cm2]
Δf = Noise bandwidth [Hz]
S/N = Signal to Noise ratio
Specific detectivity D* (D-star)
D* is the photo sensitivity per unit active area of a detector. D* is conveniently used to compare the performances of various detector types since it is area-independent. D* is the signal-to-noise ratio at a particular electrical frequency, and in a 1 Hz bandwidth when 1 Watt of radiant power is incident on a 1 cm² active area detector. In other words it is equal to the reciprocal of the noise-equivalent power (NEP), normalized per unit area.
D∗=S/NΔf−−−√PA√=A√NEP
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[cmHz−−−√W]
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P = Incident radiant power received by the detector [W]
A = Detector active area [cm2]
Δf = Noise bandwidth [Hz]
S/N = Signal to Noise ratio
In general the measurement conditions of D* are expressed in the format of D* (X, Y, Z), where X is the temperature [K] or wavelength [μm] of a radiant source, Y is the chopping frequency [Hz], and Z is the noise bandwidth [Hz]. The units of D* are centimeter-square root-hertz per watt, sometimes referred to as “Jones” units. The higher D*, the better the detector. D* values are very high
Noise equivalente temperature difference NETD
NETD is a widely used performance parameter that characterizes the sensitivity of thermal imaging sensors. NETD 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). Essentially, it specifies the minimum detectable temperature difference. Typically NETD is expressed in units of Kelvin (K). Cooled infrared camera systems typically have low noise levels, in the range of 10 – 30mK. Uncooled infrared cameras systems are typically noisier, in the range of 30 – 120mK.
One important parameter that needs to be taken into account when specifying the NETD value of a thermal imaging camera is the lens aperture (or f-number). In fact, the lens f-number will directly affect the sensitivity of the camera. NETD values of different detectors can be compared only by using a lens with the same f-number.