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).

EN 12898:2019 glass emissivity measurement

Related services Emissivity / emittance, Glass optical & thermal properties

We are able to measure glass emissivity according to EN 12898:2019, with the PerkinElmer Spectrum Two FTIR spectrometer and PIKE 10Spec 10-degree specular reflection accessory.

The total normal emissivity (εn) result is reported. The corrected emissivity (ε) is calculated and presented in the appendix. For glass U-value calculation, the corrected emissivity should be used.

Solar absorptance and thermal emittance of transparent/translucent materials

Related services Solar reflectance index (SRI), Emissivity / emittance

We had a post on the sample requirements of translucent membrane products for SRI testing before: SRI testing: Can membrane products be tested without substrate? Essentially, if a transparent/translucent material is to be laid on top of an opaque substrate, it is required to test the transparent/translucent material together with the substrate.

There are also scenarios that transparent/translucent materials are with standalone installations and without substrates, .e.g. canopies made of fabric materials. In such cases, it is sometimes necessary to determine the solar absorptance and thermal emittance of transparent/translucent materials.

Solar absorptance of transparent/translucent materials

According to ASTM E903, the solar absorptance of a transparent/translucent material can be calculated as:

Solar absorptance = 1 – solar transmittance – solar reflectance

The solar transmittance and solar reflectance can be measured with a UV/VIS/NIR spectrophotometer.

Thermal emittance of transparent/translucent materials

It is out of the scope of ASTM C1371 to measure the thermal emittance of a transparent/translucent material. A manufacturer’s technical note is available to measure the IR transmittance and thermal emittance of transparent/translucent materials, with a portable emissometer.

SRI of transparent/translucent materials

It is out of the scope of ASTM E1980 to determine the solar reflectance index (SRI) of transparent/translucent materials. As suggested by its title, ASTM E1980 is for “horizontal and low-slope opaque surfaces”.