Measurement of spectral emissivity and absorptivity of laser beam at high temperatures

Optical properties of materials have significant effect on laser processes. Laser beam interacts with a material as light and absorptivity of the material to this light determines the amount of heat that is coupled to the material. The other light is lost. Emissivity determines cooling effects during and after the process by radiation heat transfer and is also important in noncontact temperature measurement. Absorptivity, emissivity and other optical properties depend not only on the material, but also on the surface state, roughness, oxidation, contamination, incidence angle and also temperature. In our laboratory, we have developed a measurement system for direct spectral emissivity measurement of coatings and bulk materials at high temperatures up to 1223 K. As for a given wavelength, temperature and angle the absorptivity is equal to emissivity, we can determine absorptivity of the material at high temperature for a given laser. The measurement method uses a 400 W fiber laser with scanning head as a heat source for sample heating, FTIR spectrometer for emission measurement, high temperature blackbody as a reference source of radiation and infrared camera for surface temperature measurement. Examples of measurement are shown – for refractory steel and a high temperature coating on steel at temperatures from 673 K to 1173 K. The laboratory is also equipped with room temperature reflectivity and transmissivity measurement accessories and so absorptivity and emissivity can be indirectly determined also at room temperature. The measured values at high and room temperatures can be used for numerical modelling of laser processes or for example for choosing of right laser for a given material.