Noise in thermal detectors

Instruments to measure the radiation field b. Noise in thermal detectors 

The precision of every radiometric instrument is ultimately limited by fundamental disturbances, which have a random character and which are predictable only in a statistical sense. These disturbances, called noise, originate from several physical mechanisms. In this section we consider noise in detectors only. Amplifier noise can be significant for modem cryogenic detectors and may influence the choice of the operating parameters or even the type of detector. We will address fundamental limits and not disturbances caused by poor electrical contacts, improper shielding, or faulty grounding, which we assume have all been eliminated. 

Except for ultrahigh frequencies the factor hf/kT[exp(hf/kT) — 1] is unity. For all practical purposes, electrical noise is in the Rayleigh-Jeans limit. Then the power Pa/ corresponds to a mean rms noise voltage of 

In order to express all noise sources in common terms, it is desirable to refer the Johnson noise produced by the electrical resistance of the infrared detector to radiative units and express it in terms of NEP [W Hz 5]. The total NEP of the system can then be found by quadratically adding the NEPs of all individual noise sources. Thus 

Johnson noise is not the only noise source present in detectors. In a thermal detector temperature changes are converted into electrical signals. Consequently, random fluctuations in the temperature of the detector translate directly into random noise at the output terminals. This noise, called temperature noise, is inherent to all thermal detectors. Statistical mechanics shows that the mean square fluctuation of 

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The topics included here are limited to the usual types of noise in the common types of infrared photon detectors. Noise in thermal detectors, such as temperature noise in bolometers, is not included. Noise associated with the avalanche process is omitted. The detailed noise theory of phototransistors, an extension of shot noise in photodiodes, is not included. Modulation noise, an example of which arises from conductivity modulation by means of carrier trapping in slow surface states, is not included. Pattern noise, due to the... 

In thermal detectors and especially in bolometers, the energy exchange between the sensing element and the heat sink through a thermal link of conductance G results in a thermal noise known as phonon noise. The NEP associated with this phonon noise, which is a white (frequency-independent) noise, is given by ... 

Optical receiver noise can arise partly from fundamental photon noise and partly from thermal noise in the receiver circuit. For CO2 detection, it was assumed that the optical receiver was an extended InGaAs detector, followed... 

The main limitation of photoelectric detectors is the noise cansed by thermal excitation of the carriers from the valence band or from the impnrity levels. If there is a large dark current (a cnrrent generated by the detector in the absence of incident hght), the sensitivity of the photoelectric detector becomes poor (only very intense beams will indnce an appreciable change in the detector condnctivity). In order to rednce the dark cnrrent, photoelectric detectors are nsnally cooled dnring operation. 

Noise and Drift. Electronic, pump, and photometric noise poor lamp intensity, a dirty flow cell, and thermal instability contribute to the overall noise and drift in the detector. Excessive noise can reduce the sensitivity of the detector and hence affect the quantitation of low-level analytes [13,14]. The precision of the... 

Cryogenics. At room temperature, thermal excitation of charge carriers within the Ge(Li) detector produces unacceptable levels of electrical noise in the external counting circuits. To overcome this effect it... 

It is also important that the oven not influence the detector temperature. This could happen, not only by thermal conduction, but also by heat transfer through the flowing carrier gas. The second effect is particularly bad because it is one way that variations in gas flow can cause noise or drift in the detector. 

The intrinsically low intensity of Raman scattering strongly influences both the sensitivity and penetration depth of SORS and its variants. Dominant noise components (photon shot noise or thermal/dark count [1]) can be minimised relative to signal by increasing absolute signal levels. In many Raman systems, collection optics, laser power and other relevant parameters are usually maximised for optimum performance of the system current detectors (CCD devices), for example, have detection efficiencies approaching 100%. Typically, acquisition time provides the only straightforward means available... 

All signal detectors are required to detect the signal against a background of noise . Therefore, the signal-to-noise ratio must be optimized or, put another way, for maximum sensitivity the noise has to be minimized. The sensitivity of any detector is determined by the noise level in the amplified output signal. In the case of a pyroelectric detector and its associated circuitry, the principal sources of noise are Johnson noise, amplifier noise and thermal fluctuations. 

In UV-vis-NIR spectrometers, the monochromator and detector are switched simultaneously. Step-like artifacts can be generated at this switch, and it is then questionable which part of the spectrum represents the correct absolute intensity. By nature, NIR detectors are susceptible to thermal radiation, and the step at the change-over to or from the NIR range and also the noise in the NIR range increase with temperature (Melsheimer et al., 2003). Sometimes authors present the UV-vis and NIR sections of the spectrum separately, disguising step-like artifacts at the transition. 

Next we must consider the different types of noise which can be present in the spectrum [2,9], Detector noise is generally due to random fluctuations in the detector, such as thermal noise, and is therefore independent of the signal level. This type of noise is proportional to the square root of the amount of time a given... 

There are two main sources of drift, both due to non equilibrium conditions in the column and the detector. If the detector, column and mobile phase are not in thermal equilibrium, then serious drift will occur. This can be eliminated by careful temperature control of column and detector. Another and more common source of drift arises when the stationary phase and mobile phase have not been given sufficient time to come into equilibrium. This type of drift often occurs when changing the mobile phase composition and mobile phase should be pumped through the chromatographic system until a stable baseline is achieved. Trace impurities in the mobile phase can cause prolonged drift and longterm noise and so very pure solvents must be used for the mobile phase. Distilled in glass solvents may not necessarily be sufficiently pure to ensure drift-free detector operation. 

The TCD measures changes In the thermal conductivity of the carrier gas, perturbed by the eluting analyte. Thermal conductivity detectors which rely on diffusion by the analyte to the hot filament generally have the lowest limit of detection (due to the very low noise of the detector), yet have very long time constants. Flow-through thermal conductivity detectors can retain peak symmetry without excessively large volumetric time constants. 

At a difference with thermal detectors, the background noise of photoconducting detectors is frequency-dependent. If it is assumed that the photoconductor is used to detect radiation at a frequency just above its cut-off frequency z/c, the detectors with a cut-off in the near IR display a much smaller background noise than those with a cut-off at lower energies. This is because in the near IR, the black body emissivity contribution at room temperature and below is very small. 

Low-frequency noise, referred to as 1// noise, has been observed in both thermal and photon detectors. Current noise that appears when an electrical current is passed through a resistor has this approximate spectral dependence. This noise has several origins, some of them technological, other more fundamental and its contribution can vary in different detectors. Besides the fact that the amplification of electric signals can be made more selective at high frequencies, the existence of this noise is an incentive to use, when possible, high modulation frequencies. 

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