Thermal Emissivity measurement

The scientific and technical fields requiring knowledge of surface emissivity are extremely numerous. Emissivity is essential for calculating and optimizing heat exchange by radiation. Remote temperature measurement using optical pyrometry or thermography, and remote detection of objects, heat sources and targets are also common applications requiring knowledge and control of material emissivity. Today, the sharp drop in the price of thermal imaging cameras has made this measurement and diagnostic tool popular in many fields. The use of these cameras in a wide variety of conditions requires a better understanding of the emissivity of materials in order to make controlled measurements, or at least to avoid making gross diagnostic errors.

The emissivity of a material is highly dependent on the nature of the material, and can also depend on wavelength, surface morphology (roughness) and temperature. Numerous emissivity data are available in the technical and scientific literature for many materials, but the numerical values found can be extremely scattered for the same material. This is often due to the fact that the materials tested were slightly different (e.g. a more or less oxidized metal) or had different surface conditions. Data sources often fail to clearly specify the type of emissivity provided, the measurement conditions and the detailed characteristics of the materials tested. Uncertainties on the values available in the literature are generally not given. For the user, therefore, the only way to obtain reliable emissivity data is often to measure emissivity on the material used.

The first part of this article gives a brief overview of thermal radiation, the definitions of the most commonly used emissivities (directional or hemispherical emissivity, spectral or total emissivity) and the relationships between the radiative properties of materials and between normal emissivity and hemispherical emissivity. This document also presents commercially available emissivity measurement instruments and the main valid standards dealing with emissivity measurements. The difficulties and limitations of the measurement techniques presented are analyzed in detail, and examples of uncertainty source balances and numerical uncertainty budgets are given.

We recommend that you refer to [BE 8 210] for more comprehensive information on the thermal radiation of materials, theoretical relationships giving the emissivity of a material, emissivities...