Measurement methods

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Methods for measuring thermal radiative properties can be classified in two general categories: calorimetric and radiometric methods.

Total emissivity measurement of a metal strip: thermal radiation is detected from a central known area at uniform temperature.

In the radiometric method the radiant flux is measured directly and the desired property is computed as the ratio of radiant fluxes. This method is much more flexible since it allows emissivity to be measured at various wavelengths and angles. It can offer higher measurement accuracies but at the cost of more complicated equipment and a more rigorous assessment is needed of the measurement uncertainties.

In the calorimetric method emissivity is evaluated in terms of heat lost or gained by the material. Only total hemispherical emissivity is measured, i.e. the thermal radiation is measured over all wavelengths and angles. In general, the calorimetric method is relatively simple but of low precision.

NPL vacuum emissometer, 800 - 1800 C: a cylindrical sample is rotated within a metal heating tube having central holes for detecting sample and blackbody radiation, respectively.

The following table lists the most important techniques, using either calorimetric or radiometric methods. More details can be found via the links (pdf files).

Technique

Description

Calorimetric techniques Emissivity is evaluated in terms of heat lost or gained by the material. Both steady-state or transient operation may be used.
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Direct radiometric techniques Emissivity is given by the ratio of the measured specimen to blackbody radiances. Seven different apparatus are described here.
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Indirect radiometric techniques 

Blackbody reflectometer
Absolute spectral emissivity is derived through the measurement of spectral hemispherical-directional reflectance. Accurate technique used for characterisation of reference samples at room temperature.
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Integrating sphere reflectometer
One of the most used apparatus for deriving emissivity data in the visible and near-infrared, up to 2.5 m. Longer wavelength versions are also available.
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Integrating mirror reflectometer
An alternative to the integrating sphere reflectometer. Different types of integrating mirror may be used.
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Gonioreflectometer
A gonioreflectometer measures the bi-directional reflectance factor (BDRF) as a function of angle of reflectance (or incidence) for any given angle of incidence (or reflectance).
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Hybrid technique
A technique which combines the advantages of both direct and indirect radiometric techniques. Well suited for infrared characterisation up to 1000 C.
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