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Chapter 3:
Radiation Thermometry Principles
There are literally hundreds of different radiation thermometers on the market, comprising a range of different types with a multitude of different models and specifications. Radiation thermometer types include spectral-band thermometers, total radiation thermometers, ratio and multi-wavelength thermometers, thermal imagers, laser pyrometers, gold-cup pyrometers, disappearing-filament thermometers, and fiber-optic thermometers. The common feature of these radiation thermometer types is that they are all radiometers, calibrated in terms of the spectral radiance of a blackbody at known temperatures. A radiometer is simply an electro-optical system designed to measure radiant power. Radiation thermometers are designed in such a way that the measured radiant power is proportional to the radiance of the target within a specified wavelength range. Radiance is of fundamental importance to radiation thermometry for two reasons. Firstly, the spectral radiance of a blackbody is precisely described by Planck's law (Equation (2.1)) in terms of temperature only. This makes it relatively simple to convert a radiometer into a radiation thermometer. Secondly, radiance has the useful property that it is invariant along an elemental beam of radiant flux [5]. This means that, in properly designed radiometers, the measured signal is independent of the distance from the target and of the optical system employed to relay the flux from the target to the detector. The optical system may be comprised of any appropriate combination of lenses, mirrors, filters and apertures, and may be described as imaging or non-imaging. The only significant difference between an imaging radiometer (one that uses lenses or mirrors to focus the radiation onto the detector) and a non-imaging radiometer is that the former provides a smaller well-defined field of view on the target. The size of the field of view is an important consideration in almost all applications of radiation thermometry, particularly when tube measurements are made over long distances inside a furnace. In this chapter we focus mainly on spectral-band thermometers, as these find the most widespread use in furnace applications, and indeed in the majority of other applications. The gold-cup pyrometer is another device that is suited to furnace measurements, although due to its cumbersome nature and finite measuring distance its use is mainly limited to that of a reference instrument. Thermal imaging equipment, with its ability to gather large amounts of data in a short period of time, is becoming more and more important in quantitative analysis of furnace tube temperatures. These and other instruments are also discussed in this chapter, along with a warning on the use of ratio and multi-wavelength thermometers in industrial furnaces.
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