Responsivity thermal detector

A simplistic model, the DETACT-QS model used to predict the thermal response of detectors and sprinklers, has been subjected to the ASTM El 355 guidelines as a test case Oanssens, 2002). Over five years of effort have been dedicated to this evaluation, showing the difficulty in performing a detailed, recognized validation process. 

In order for a detector to follow the intensity modulation of the sample in the frequency range of 50 kHz, it must have a response time on the order of a few microseconds or less. Standard infrared detectors are thermal detectors which... 

The most commonly used mid-infrared thermal detectors arc TGS (Iriglycinc sulphate) and DTGS (deuterated triglycinc sulphate). The variation in temperature on the surface of the detector causes the appearance of an electrical signal. The response is constant over the spectral domain. In the far-infrared, a bolometer (Si or InSb detector) with high detective capacity is preferred. 

Thermal and photoconductive detectors are used to measure radiation intensities, but all have relatively slow responses and are subject to drift. The lead sulfide or telluride photoconductive cell has a response time of about 0.5 ms, but sensitivity decreases sharply above 2900 cm" for the sulfide and above 1700 cm- for the telluride. Thermal detectors are employed at longer wavelengths. The simplest of these is the thermocouple, which has a relatively slow response (about 60 ms), and several are usually linked to form a thermopile. Bolometers... 

A thermopile is a thermal detector capable of measuring the total incident radiant flux (in W/m2) in the UV, visible, and IR regions of the spectrum, with a response that is essentially independent of wavelength. It is the reference against which the other devices can be calibrated, but is not recommended for direct use because it is expensive and not a straightforward instrument to use correctly (Jagger, 1985). 

Figure 22.4 Topical peaks registered from repetitive assays using an FI system with thermal detectors. The responses were obtained after injection of urea, penicillin V, and a mixture of the two into a system with an integrated thermal biosensor for simultaneous determinations of multiple analytes. Both signals were exothermic. The upper trace was registered with a thermistor pair called To-T. The lower trace was registered using another thermistor pair, T2-T3. In this case, the direction of the penicillin peaks was reversed by alternating the recorder polarity. A, response to 30 mmol T urea B, influence of the urease reaction on on the thermistor pair T2-T3 C, response to 30 mmol 1 penicillin V D, simultaneous responses for urea (5-60 mmol 1" ) in the mixed samples E, simultaneous responses for penicillin V (5-60 mmol I" ) in the mixed samples (from [47], with permission).

The detection of molecules in a molecular beam by a bolometer is based on the bolometer s response to the total beam energy, including the center of mass translational energy (Zen, 1988). The bolometer consists of a liquid-helium-cooled thermocouple whose electrical response varies rapidly with the energy of the bolometer. The low temperature is necessary in order to reduce the heat capacity of the thermocouple, thereby increasing its sensitivity, as well as to minimize the thermal detector noise. 

Because heating and cooling of a macroscopic sample is a relatively slow process, thermal detectors as a class are slower in their rate of response than photon ones, although some thermal detectors are faster than selected photon ones. It is convenient to think of thermal detectors as having millisecond response times and photon ones as having microsecond ones, but this is clearly only a rough rule to which there are many exceptions. 

Washwell et al. [2.116] have developed the pertinent equations for the Nernst photosignal in terms of the material parameters. They have exploited the effect in Bi and Bi Sbj as an uncooled infrared detector. Like other uncooled thermal detectors, the speed of response is somewhat slow. The authors believe it can be improved to the microsecond range. 

Fig. 2.13. Ideal spectral responses of photon and thermal detectors for unit radiant power per unit wavelength interval...

Because the performance of infrared detectors is limited by noise, it is important to be able to specify a signal-to-noise ratio in response to incident radiant power. An area-independent figure of merit is D ( dee-star ) defined as the rms signal-to-noise ratio in a 1 Hz bandwidth per unit rms incident radiant power per square root of detector area. D can be defined in response to a monochromatic radiation source or in response to a black body source. In the former case it is known as the spectral D, symbolized by Df X, f, 1) where A is the source wavelength,/is the modulation frequency, and 1 represents the 1 Hz bandwidth. Similarly, the black body D is symbolized by Z> (T,/1), where T is the temperature of the reference black body, usually 500 K. Unless otherwise stated, it is assumed that the detector Held of view is hemispherical 2n ster). The units of D are cm Hz Vwatt. The relationship between )J measured at the wavelength of peak response and D" (500 K) for an ideal photon detector is illustrated in Fig. 2.14. For an ideal thermal detector, Df = D (T) at all wavelengths and temperatures. 

For typical detector design falls within the range of milliseconds to seconds. This is much longer than the typical response time of a photon detector. For some applications this puts thermal detectors at a disadvantage with respect to photon detectors, but when all the systems tradeoffs are taken into account this disadvantage may not be as great as it would at first sight seem. 

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