optris LS – E2006-01-A
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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.
Determination of unknown Emissivities
► First, determine the actual temperature of the measuring object with a thermocouple or contact sensor.
Secondly, measure the temperature with the infrared thermometer and modify the emissivity until the
displayed result corresponds to the actual temperature.
► If you monitor temperatures of up to 260 °C you may place a special plastic sticker onto the measuring
object, which covers it completely. Now set the emissivity to 0,95 and take the temperature of the sticker.
Afterwards, determine the temperature of the adjacent area on the measuring object and adjust the
emissivity according to the value of the temperature of the sticker.
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