optris PI LightWeight kit Operator's Manual page 75

Emissivity:
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.
Figure 19: Capability of an object to emit radiation
I
Radiation
ε
Emission
ρ
Reflection
τ
Transmission
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ε + ρ+ τ = 1