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What is Emissivity?

  • June 28 2023
  • Riho Kato
放射温度計 AD-5635


An infrared thermometer is a measuring instrument that detects the amount of infrared energy radiated from

the surface of a material using an infrared sensor and measures the surface temperature of the material based on that energy.


The incident infrared energy on a material, as shown in the diagram, can be categorized into three cases:

absorption by the material, reflection at the surface, and transmission through the material.The behavior of incident energy

Furthermore, all materials emit infrared energy.

The absorbed infrared energy and the infrared energy radiated from the material's surface are equal in quantity.

Therefore, the emissivity (ε) is equal to the absorptivity (α).



The ideal substance that absorbs all incident energy without reflecting or transmitting it,

with emissivity (ε) and absorptivity (α) both equal to 1, is referred to as a "blackbody."

However, in reality, most materials exhibit some degree of reflection and transmission.


The relationship between emissivity, transmittance, and reflectance is defined by the equation

emissivity + transmittance + reflectance = 1.




As mentioned above, objects do not emit infrared energy at 100%,

so the emitted radiant energy is less than that of a blackbody at the same temperature (emissivity = 1).


Therefore, if temperature is measured directly, the displayed temperature will be lower than the actual temperature.


For this reason, when using an infrared thermometer for temperature measurement,

it is necessary to set the emissivity to account for how much infrared energy is being radiated by the object being measured,

relative to a blackbody at the same temperature (emissivity = 1.0).


Furthermore, the emissivity of a material is influenced by its composition and surface condition.

The table below provides a list of representative emissivity values for different materials.


Various Emissivity Values 2


Please note that the listing for the emissivity of human skin is provided for reference purposes.

When using an infrared thermometer, it can only measure the surface temperature of the skin, not the internal body temperature.

Therefore, it is not possible to measure body temperature using an infrared thermometer.