What is emissivity?

Related services Emissivity / emittance

It is well known that there are three heat transfer modes:

  • Conduction
  • Convection
  • Radiation

Emissivity is the key material surface property related to radiative heat transfer. In radiative heat transfer, a surface exchanges heat with the surroundings via radiation (electromagnetic wave):

  • The surface emits radiation to the surroundings (characterized by its emissivity)
  • The surface absorbs radiation emitted by the surroundings (characterized by its absorptivity)
  • The surface reflects radiation emitted by the surroundings (characterized by its reflectivity)

There are two relationships:

  • Emissivity = Absorptivity (Kirchhoff’s law of thermal radiation)
  • Absorptivity + Reflectivity = 1 (conservation of energy)

It is easy to calculate the absorptivity and reflectivity, when the emissivity is known.

Most natural surfaces are with high emissivity, around 0.9. Reflective metal surfaces are with low emissivity, around 0.05 or lower. Listed in the table below are the performances of high emissivity and low emissivity surfaces:

High emissivity surfaceLow emissivity surface
Radiation emission to surroundingsEmits more radiationEmits less radiation
Absorption/Reflection of radiation from surroundingsAbsorbs more radiation
Reflects less radiation
Absorbs less radiation
Reflects more radiation
Overall radiative heat transfer with surroundingsStonger radiative heat transferWeaker radiative heat transfer

In summary, emissivity is a material surface property characterizing its radiative heat transfer ability. A surface with high emissivity has stronger radiative heat transfer with the surroundings; a surface with low emissivity has weaker radiative heat transfer with the surroundings.

For insulation applications, surfaces with low emissivity are preferred, due to the weaker radiative heat transfer (and therefore better insulation).

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