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All forms of thermometry require careful measurement practice to obtain accurate results. However, compared to other forms of thermometry, blind application of a radiation thermometer is much more likely to lead to measurement results that are widely different from the true temperatures we are trying to measure. This is because there are many external effects that influence the reading on a radiation thermometer. To be able to make confident temperature measurements we must have a basic understanding of the physical principles exploited by radiation thermometers. Radiation thermometers operate by detecting the thermal radiation emitted by objects and relating this to their temperature. Therefore, this chapter is primarily concerned with the nature of thermal radiation and, in particular, a concept known as blackbody radiation. Understanding blackbody radiation allows us to quantify, as we do in later chapters, the errors associated with radiation thermometry measurements and to identify measurement strategies that minimize the potential errors. This understanding also provides us with the means to design and choose different thermometers optimized for particular applications. We also introduce in this chapter the key radiative properties of materials, including emissivity and reflectivity, and explain how these influence the blackbody radiation that a radiation thermometer measures. Because radiation thermometers are able to measure the temperature of objects over large distances, we also look at atmospheric effects that can perturb the radiation as it travels towards the thermometer. However, before we delve into the principles of radiation thermometry, it is useful to review the ways in which heat is transferred from one body to another. This will help in understanding the origin of some of the difficulties faced in the measurement of temperature inside industrial furnaces.
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