Temperature is the most commonly measured physical quantity. It tells us how hot or cold a body or substance is, and is determined by the average kinetic energy contained in that body or substance. Temperature is measured with thermometers. We know several types of them. One of the types are radiation thermometers or pyrometers. With them, the international temperature scale above the freezing point of silver (961,78 °C) is realized. Radiation thermometers or pyrometers are taking the advantage of the fact, that all objects with a temperature above absolute zero (T = 0 K) are radiating a certain amount of thermal energy. Radiation thermometers are detecting this radiation with a detector and converting it into an electric signal. On the basis of that signal they perform complicated calculation and display the temperature. One of the advantages of radiation thermometers is their non-contact use, which on the other hand also entails many negative effects on their measurements. Measurements can also be negatively affected by relative humidity, which causes atmospheric absorption at certain wavelenghts, which leads to higher or lower measured body or substance temperature. That happens because the atmosphere, due to its lack of transparency, does not allow sufficient heat radiation to be transmitted from body or substance to the radiation thermometer. To avoid this problem, we must choose radiation thermometers that measure in the range of appropriate wavelenghts, so that the impact of atmospheric absorption is minimized. After all we can say that knowing the negative effects on radiation thermometers is thus very important for accurate measurements with them.
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