Setting for Accuracy

Blackbody and Emissivity

Blackbody

An ideal object which absorbs all incident light (electromagnetic wave) and reflects none.

Emissivity

The ratio of the radiation emitted by an object surface to the radiation emitted by a blackbody at the same temperature. According to Kirchhoff’s law of thermal radiation, an emissivity of an object is equal to a radiation absorption rate of it.

        “emissivity ε” (= absorption rate) = 1 - “reflectance” - “transmittance”

If a reflectance and a transmittance are zero, such an object must be a blackbody and an emissivity is 1.

*Object examples of an emissivity close to 1

       Pottery  --- Emissivity 0.90

       Concrete --- 0.94 

*Typically a transmittance of metal is zero and its reflectance is large.
  Therefore, an emissivity of metal is relatively small.

     Example: Aluminum (Oxide surface) --- Emissivity 0.2 to 0.4

                    Copper (Oxide surface)     ---         0.4 to 0.8

                    Iron (Oxide surface)         ---         0.5 to 0.9

     An emissivity of metal has a range of values.
     Because the metal surface oxidizes in the air and therefore the reflectance changes according to its surface condition.

Infrared thermometer and Emissivity

The infrared thermometer does not detect all of the radiant light, but actually IT-480 detects infrared light with wavelengths from 8 to 14 microns. HOR team defines that the ideal object which is zero reflectance and zero transmittance for the infrared light with 8 to 14 microns is the blackbody i.e. emissivity ε=1 for an infrared thermometer; IT-480.

Temperature measurement of a low emissivity object

  • Firstly, emissivity setting of IT-480 must be adjusted to the suitable value.
  • If a reflectance of the object to be measured is high, non-negligible errors may occur depending on mixing radiant infrared lights from the surrounding objects to a radiant infrared light from the object to be measured. Similarly, if a transmittance of the object is high, radiant infrared lights from objects behind the object to be measured propagate through the object and are mixed in a radiant infrared light from the object.
  • In the case of an emissivity is 0.1, ninety percent of the detected infrared light by the IR detector of IT-480 is background infrared light. And in this case, for example, if a surface temperature of the object to be measured is 100ºC, actual measured temperature value becomes close to room temperature.
  • Temperature measurement with a demo unit is strongly recommend to confirm the applicability at the measurement site.

 Emissivity Setting Error and Temperature Measurement Error

      Following plot is an example of theoretical calculations about HOR infrared thermometers.


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