Experimental investigation of emissivity of aluminum alloys and temperature determination using multispectral radiation thermometry (MRT) algorithms

Experiments were performed to measure the emissivity spectra for aluminum (Al) surfaces that are subject to variations in alloy, temperature, heating time, and surface finish. The linear emissivity model (LEM) and log-linear emissivity (LLE) model were tested against thermocouple measurements to explore the accuracy of these models at inferring surface temperature. The data show emissivity decreases with increasing wavelength for lambda < 3.5 μm, but the trend is reversed at higher wavelengths. Except for AL 1100 (commercially pure Al), the emissivity of all alloys tested decreased 600-700 K and increased 700-800 K. The increased emissivity at 800 K was closely associated with a discoloration of surfaces from light gray into black. Surface roughness produced a 2- to 3-fold increase in emissivity compared with polished surfaces. Overall, the third-order LEM model showed the best overall accuracy for different alloys, temperatures, and surface roughness. Nonetheless, this study points to a need for more accurate models that could handle the diverse operating environment of Al processing plants.