8 Emissivity
8.1 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 (ε – 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.
8.2 Determination of unknown emissivity
► First determine the actual temperature of the measuring object with a thermocouple or contact sensor.
Second, measure the temperature with the infrared thermometer and modify the emissivity until the
displayed result corresponds to the actual temperature.
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