Thermopiles spectrometer

The most common source in MIR spectrometers is a glowing ceramic bar, a so-called glowbar (or globar). More intense emission is provided by the Nernst glower due to its higher operation temperature (black body radiator). A thermocouple or a thermopile is commonly used as detector. The response behaviour of such detectors is slow, which prevents rapid scanning by dispersive MIR spectrometers. 

Newton used a liquid in glass-thermometer to study heat radiation. Rumford and Leslie used a difierential gas thermometer. Herschel reverted to the liquid thermometer, but this was soon replaced by the thermopile (Melloni [3.4]). Some time later (Langley [3.5]) the first bolometers were used. More recently the use of the gas thermometer, in the shape of the Golay [3.6] and Luft cells has been reintroduced and is now widely used in spectrometers. Another type of thermal detector now widely used is that utilizing the pyroelectric effect. In addition to these, several other detection processes have been suggested, including thermal expansion and changed dielectric properties with temperature. 

Figure 4 Plan view of a Hilger IR spectrometer made in 1918. A constant-deviation Wadsworth optical arrangement with a 60° rock salt prism was employed. The detector was a thermopile and the source a Nernst filament. From Hilger Journal, August 1955. Photograph courtesy Professor N. Sheppard, FRS, and reproduced with permission from Hilger Analytical.

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). 

Hickey Daniels, 1969). In a more recent version of a pneumatic detector, the displacement of the membrane is sensed electrostatically as in a condenser microphone (Chatanier Gauffre, 1971). Good pneumatic detectors have obtained NEP values of 2 to 3 X 10 W Hz 5. Golay cells are still in use in many laboratory spectrometers, but have not been applied to space-borne observations, probably because of their weight, complexity, and lower reliability compared to thermopiles and pyroelectric detectors, which are discussed next. 

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