Detector, thermal

Because of their wavelength-independent sensitivity, thermal detectors are useful for calibration purposes, e.g., for an absolute measurement of the radiation power of cw lasers, or of the output energy of pulsed lasers. In the rugged form of medium-sensitivity calibrated calorimeters, they are convenient devices for any laser laboratory. With more sophisticated and delicate design, they have been developed as sensitive detectors for the whole spectral range, particularly for the infrared region, where other sensitive detectors are less abundant than in the visible range. 

For a simple estimate of the sensitivity and its dependence on the detector parameters, such as the heat capacitance and thermal losses, we shall consider the following model [4.99]. Assume that the fraction p of the incident radiation power P is absorbed by a thermal detector with heat capacity H, which is connected to a heat sink at constant temperature (Fig. 4.73a). When G is the thermal conductivity of the link between the detector and the heat sink, the temperature T of the detector under illumination can be obtained from 

If the time-independent radiation power Pq is switched on at r = 0, the time-dependent solution of (4.112) is 

The temperature T rises from the initial value at r = 0 to the temperature T = Ts + at for = oo. The temperature rise 

In general, P will be time dependent. When we assume the periodic function 

At the frequency w = G/H, the amplitude AT decreases by a factor Vl compared to its DC value. 

Because of their wavelength-independent sensitivity, thermal detectors are useful for calibration purposes, e.g., for absolute measurements of the radiation power of cw lasers or of the output energy of pulsed lasers. In 

In case of constant radiation power P, the temperature T rises from its initial value to its stationary value (dT/dt = 0), 

In this type of detectors material properties are modified by heating, typically independently on the detected radiation wavelength, and this is used to generate electric output or some other type of readout signal. Table 1.3 lists some basic types of thermal IR photodetectors arranged according their detection mechanisms. 

For a majority of the existing thermal detectors there is a trade-off between flie specific detectivity and the maximum response speed— a detector may be smaller and faster, or larger and more sensitive. There is a frequency-dependent thermodynamical limit of the detectivity of thermal detectors. Instead of that theoretical limit, a more realistic Havens limit is used in practical situations [5]. Generally speaking, thermal detectors tend to be at the slower side of the response. 

The second limitation is connected with spectral selectivity. Thermal detectors receive radiation in the whole electromagnetic spectmm. They are limited only by the optical properties of the material at the device surface. Their spectral characteristics may be modified e.g., by using a bandpass filter or by properly choosing the material at the device surface. 

Some of the quoted performance limitations seriously compromise the applicability of thermal detectors for some key applications, especially those requiring high D f product. 

On the other hand, the majority (although not all—see the last three bolometer-type devices in Table 1.3) thermal detectors do not require cooling not even in far and extreme infrared range. This makes them very convenient for applications where spectral selectivity is not critical. Their integration with processing circuitry is excellent, since thermal detectors are usually fabricated using Si-compatible technologies. 

If the interferometer mirror speed is such that the optical velocity is 0.316 cm s (HeNe laser frequency of 5 kHz), 4000-cm radiation is modulated at 1.25 kHz (see Eq. 2.11). Thus, the response time of a detector for FT-IR spectrometry must be less than 1 ms. Although several cryogenically cooled detectors have response times this low, the only mid-infrared detectors that have an appropriate combination of high speed, reasonably good sensitivity, low cost, good linearity, and operation at or near room temperature are the pyroelectric bolometers. 

These detectors incorporate as their heat-sensing element ferroelectric materials that exhibit a large spontaneous electrical polarization at temperatures below their Curie point. If the temperature of ferroelectric materials is changed, the degree of polarization also changes. If electrodes are placed on opposite faces of a thin slice 

To a good approximation, Ry is proportional to lf For a rapid-scanning interferometer, therefore, the optical velocity V should be as low as possible. Two factors usually determine the value of V when a DTGS detector is being used. First, V must be high enough that the SNR of the interferogram at the centerburst is 

In addition to the responsivity and the NEP, other parameters are of concern in selecting a detector for a particular application. The time constant or the response to signal modulation frequency is important, and so is the linearity or at least the reproducibility. In a more practical sense mechanical integrity, insensitivity to high-energy particle radiation, convenience in matching preamplifier characteristics and that of a bias power supply, temperature range, and other subtleties need to be considered in a detector selection process. 

In Section 5.11, we discuss thermal detectors, their responsivity, their noise characteristics, and we provide examples of energy conversion mechanisms as well as a few data on actual detector performance. Then, in Section 5.12 we discuss photodetectors and we provide similar information on these types of transducers. 

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For most points requiring temperature monitoring, either thermocouples or resistive thermal detectors (RTD s) can be used. Each type of temperature transducer has its own advantages and disadvantages, and both should be considered when temperature is to be measured. Since there is considerable confusion in this area, a short discussion of the two types of transducers is necessaiy. 

Nonspectroscopic detection schemes are generally based on ionisation (e.g. FID, PID, ECD, MS) or thermal, chemical and (electro)chemical effects (e.g. CL, FPD, ECD, coulometry, colorimetry). Thermal detectors generally exhibit a poor selectivity. Electrochemical detectors are based on the principles of capacitance (dielectric constant detector), resistance (conductivity detector), voltage (potentiometric detector) and current (coulometric, polarographic and amperometric detectors) [35]. 

IR detectors convert (thermal) radiation energy into electrical signals. Two classes of such detectors exist thermal detectors and quantum detectors. 

The measurement of the cosmic microwave background. Far infrared astronomers were the first to develop thermal detectors. Some of the resulting technologies, such as neutron transmutation doping (NTD) [3], have been transferred to particle detection sensors and have allowed many groups (e.g., ref. [4-11] to make rapid progress). 

An extremely simplified scheme of a calorimeter (composite thermal detector) is shown in Fig. 15.6. The temperature of an absorber A (TA) is measured by a thermometer T. A thermal conductance G forms a thermal link with the heat sink B at the temperature Ts. In the ideal adiabatic situation (G = 0), an absorption of an energy AE produces an absorber temperature increase ... 

Horikawa [126] has adapted a thermal detector for the determination of formic, acetic, and propionic acids by liquid chromatography. 

Thermal or heat detectors respond to the energy emission from a fire in the form or heat. The normal means by which the detector is activated is by convention currents of heated air or combustion products or by radiation effects. Because this means of activation takes some time to achieve thermal detectors are slower to respond to a fire when compared to some other detection devices. 

IR spectrometers have the same components as UY/visible, except the materials need to be specially selected for their transmission properties in the IR (e.g., NaCl prisms for the monochromators). The radiation source is simply an inert substance heated to about 1500 °C (e.g., the Nernst glower, which uses a cylinder composed of rare earth oxides). Detection is usually by a thermal detector, such as a simple thermocouple, or some similar device. Two-beam system instruments often work on the null principle, in which the power of the reference beam is mechanically attenuated by the gradual insertion of a wedge-shaped absorber inserted into the beam, until it matches the power in the sample beam. In a simple ( flatbed ) system with a chart recorder, the movement of the mechanical attenuator is directly linked to the chart recorder. The output spectrum is essentially a record of the degree of... 

Detection of the middle and far range of infrared radiation requires thermal detectors, the simplest of which is a thermocouple, in which the change in temperature at one junction of the thermocouple results in a small voltage being produced. Although simple in design, thermocouples lack sensitivity. Bolometers are more sensitive and are based on the fact that as the temperature of a conductor... 

Thermal detector Senses when temperatures exceed a set threshold (fixed temperature detector) or when the rate of change of temperature increases over a fixed time period (rate-of-rise detector). 

Multi-sensor detector Is a combination of photoelectric and thermal detectors. The photoelectric sensor serves to detect smoldering fires, while the thermal detector senses the heat given off from fast-burning/flaming fires. 

It is a commonplace that FTIR-based analyzers are the predominant technology for mid-infrared applications. This arises from a unique tie-in between the inherent advantages of the FTIR method and serious limitations in the mid-infrared range. The most serious problem for mid-infrared spectroscopy is the very low emissivity of mid-infrared sources combined with the low detectivity of mid-infrared thermal detectors. 

Resistive Thermal Detectors (RTDs) RTDs determine temperature by measuring the change in resistance of an element due to temperature. Platinum is generally utilized in RTDs because it remains mechanically and electrically stable, resists contaminations, and can be highly refined. The useful range of platinum RTDs is... 

Depending on their function, radiometers can be either thermal or photon detectors. In thermal detectors, the incident photon energy is converted into heat, which is then measured. The measured data are independent of wavelength. Photon detectors are based on photoelectric effect and measure spectrum intensity. The results are dependent on wavelength. 

In thermal detectors, the incident photon energy is converted into heat, which is then measured. The measured data are independent of wavelength. 

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