Every body emits electromagnetic radiation above absolute zero (-273.15°C). There is a fixed relationship between the emitted radiation and the temperature of the body. The radiation capacity of a body – the emission – is of particular importance. It is made up of reflection, absorption and transmission; this interplay determines the emissivity.

The emissivity is specified as a value between 0.1 and 1.0. For almost all non-metals, the emissivity is between 0.98 and 0.70, with a few exceptions. Infrared temperature measurement utilises this emissivity for temperature control.

The sensor or pyrometer can be adjusted to the respective material of the measured object by setting the emissivity. Emissivity tables provide an overview of the emissivities of various materials.

However, the values in the tables are not absolute values, as the emissivity of a material is influenced by various factors. These include:

• Temperature of the measurement object
• Wavelength in µm at which the measurement is taken
• Geometry of the surface (flat, concave, convex)
• Surface finish (polished, rough, oxidised, sandblasted)
• Transmittance (e.g. with thin plastic films)
• Spectral range of the measurement
• Angle of the measurement