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Emissivity
thermoIMAGER TIM
11. Emissivity
11.1 Definition
The intensity of infrared radiation, which is emitted by each body, depends on the temperature as well as on
the radiation features of the surface material of the measuring object. The emissivity (e – Epsilon) is used as
a material constant factor to describe the ability of the body to emit infrared energy. It can range between 0
and 100 %. A “blackbody” is the ideal radiation source with an emissivity of 1.0 whereas a mirror shows an
emissivity of 0.1.
If the emissivity chosen is too high, the infrared thermometer may display a temperature value which is much
lower than the real temperature – assuming the measuring object is warmer than its surroundings. A low
emissivity (reflective surfaces) carries the risk of inaccurate measuring results by interfering infrared radiation
emitted by background objects (flames, heating systems, chamottes). To minimize measuring errors in such
cases, the handling should be performed very carefully and the unit should be protected against reflecting
radiation sources.
Fig. 27 Capability of an object to emit radiation Fig. 28 Spectral emissivity of some materials
1
1) 1 = Enamel, 2 = Plaster, 3 = Concrete, 4 = Chamotte
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