optris PI 160 Operator's Manual page 64

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Figure 33: Spectral emissivity of several materials: 1 Enamel, 2 Plaster, 3 Concrete, 4 Chamotte
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.