4. Different methods for measuring thermal emissivity

The measurement techniques presented in this document can only be applied correctly to opaque materials, i.e. materials that are opaque under a very small thickness, of the order of a micrometer. It is possible to measure the radiative parameters (reflectance and transmittance) of non-opaque materials, but defining emissivity is tricky because of the difficulty of defining a reference temperature when the material is not isothermal, which is quite often the case.