3.1. Spectral Normal Emissivity Measurements

Various experimental units have been developed to measure the emissive properties of material surfaces. The main two categories include calorimetric measurement devices where the total hemispherical emissivity is measured and indirect radiometric methods. The indirect radiometric methods rely on Kirchhoff’s law of thermal radiation to calculate the emissive properties based on reflected radiation. However, calorimetric measurement devices and indirect radiometric methods are being replaced by more advanced direct radiometric methods, where wavelength dependent emissive properties can be measured directly at high temperatures. In this case, the advantage of calorimetric measurement devices, namely operation at high temperatures, is combined with measuring the wavelength dependency, which is the main benefit of using radiometric methods. The direct radiometric units designed to measure the spectral normal emissivity mainly consist of a reference source of radiation, an FTIR spectrometer (for example Nicolet series from Thermo Fisher Scientific Inc., Waltham, MA, USA), a sample heating method, and a surface temperature detector [43].

The spectral normal emissivity measurement unit of the New Technologies Research Center at the University of West Bohemia, shown in Figure 2, is described in detail by Honnerova et al. [38]. A sample is heated in dry air using a 400 W fiber laser [44] while the surface temperature is controlled using a calibrated infrared camera as temperature detector [45]. The computer-controlled calibrated laboratory blackbody allows the unit to perform measurements up to 1255 K. The FTIR spectrometer allows measurements in the wavelength range from 1.38–26 µm, with the lower wavelength limit being determined by the radiation source temperature. Disk-shaped samples of the studied (un)coated material with a 25 mm diameter and a 5 mm thickness are used to perform the measurements. Refractory samples tend to be porous and are designed to have a low thermal conductivity. This insulating nature of refractory complicates laser heating, limiting the maximum operating temperature to 873 K.

Schematic overview of the spectral normal emissivity measurement device at the New Technologies Research Center at the University of West Bohemia.