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Appendix
each curve) increases to the power of 4 of the temperature. These relationships
were recognized by Stefan and Boltzmann in 1879 and illustrate that an
unambiguous temperature can be measured from the radiation signal. /1/, /3/, /4/
and /5/.
Fig. 3 Radiation characteristics of a blackbody in relation to its temperature. /3/.
Looking at Fig. 3, then, the goal should be to set up the IR thermometer for the
widest range possible in order to gain the most energy (corresponding to the
area below a curve) or signal from the target. There are, however, some
instances in which this is not always advantageous. For instance, in Fig. 3, the
intensity of radiation increases at 2 µm - much more when the temperature
increases than at 10 µm. The greater the radiance difference per temperature
difference, the more accurately the IR thermometer works. In accordance with the
displacement of the radiation maximum to smaller wavelengths with increasing
temperature (Wien's Displacement Law), the wavelength range behaves in
accordance with the measuring temperature range of the pyrometer. At low
temperatures, an IR thermometer working at 2 µm would stop at temperatures
below 600°C, seeing little to nothing since there is too little radiation energy. A
further reason for having devices for different wavelength ranges is the emissivity
pattern of some materials known as non-gray bodies (glass, metals, and plastic
films). Fig. 3 shows the ideal - the so-called "blackbody". Many bodies, however,
emit less radiation at the same temperature. The relation between the real
emissive power and that of a blackbody is known as emissivity å (epsilon) and
can be a maximum of 1 (body corresponds to the ideal blackbody) and a
minimum of 0. Bodies with emissivity less than 1 are called gray bodies. Bodies