Temperature to voltage conversion

The oldest conversion mechanism, the thermoelectric effect, was discovered by Seebeck (1826). If two dissimilar conductors form part of an electrical circuit, as shown in Eig. 5.11.3, and junction 2 is heated while the rest of the circuit is kept at a constant temperature, a voltage, U, can be measured at the terminals. The polarity of U changes if junction 2 is cooled instead. To heat the junction by 

Shortly after the discovery of the thermoelectric effect by Seebeck, Peltier (1834) found the inverse phenomenon. Suppose all junctions shown in Fig. 5.11.3 are at ambient temperature. Then an external current source is connected to the terminals. This current causes Joule heating, i R, as expected, but, in addition, one junction experiences cooling while the other shows excess heating. The Peltier effect has been exploited to construct small refrigerators. Thermoelectric coolers, specifically designed for the operation of infrared detectors, are commercially available. They are able to cool a small detector up to 50 or even more degrees below ambient (Wolfe Zissis, 1978). 

A third thermoelectric effect, discovered by W. Thomson (1843), later Lord Kelvin, is also related to the Seebeck and Peltier effects. Thomson found that even in a conductor made of one substance, but with a temperature gradient, heat can be removed or added depending on whether the electric current and the temperature gradient coincide or point in opposite directions. 

Thermocouples, also called thermopiles, in which several couples are combined electrically, played an important role as infrared detectors in the last decades of the nineteenth and the first half of the twentieth century. Thermocouples provide low impedance energy converters, well-suited to drive sensitive galvanometers. Before the use of electronic amplification this was of great value. Even today many laboratory spectrometers still use thermopiles, but, in nearly all cases, with electronic amplifiers and recorders. An excellent discussion of various constmction techniques and test results of thermoelectric detectors is given by Smith et al., (1957). 

With the advent of cryogenic detectors thermoelectric elements have lost most of their importance, except for applications where the use of cryogens is not necessary or not possible. The latter was the case for the Voyager spacecraft, where the use 

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All radiometric devices must convert infrared energy into electrical signals. The fundamental properties of infrared converters, commonly called detectors, are analyzed in Section 5.10. In Section 5.11 the operating principles, noise limitations, and several temperature to voltage conversion mechanisms of thermal detectors are treated. Properties and noise characteristics of quantum detectors are the subject of Section 5.12. In many cases radiometric instruments must be calibrated in intensity and wavenumber. For best results calibration techniques are part of the instrument design. Several calibration methods are treated and their merits discussed in Section 5.13. Finally, Section 5.14 deals with considerations encountered in the... 

Another temperature to voltage conversion mechanism, used in pneumatic detectors, is based on the gas thermometer. The best known representative of this type of detector is the Golay cell. The infrared absorber is imbedded in a small volume of gas contained in a sealed cell. Radiation heats the absorber and the surrounding gas. The associated increase in pressure causes a small displacement of a thin membrane. In the Golay cell this displacement is sensed by the deflection of an external light beam and registered by an appropriate detector (Golay 1947 ... 

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