Emissivity characteristics of polished aluminum alloy surfaces and assessment of multispectral radiation thermometry (MRT) emissivity models

Emissivity characteristics were measured for several polished aluminum alloy samples over the spectral range of 2.05-4.72 μm and temperatures of 600-800 K. Overall, aluminum alloys buck the general trend of increasing emissivity with increasing temperature for metallic surfaces in the infrared range. Only AL 7150 follows the expected trend, while the emissivity of the other alloys decreases between 600 and 700 K and increases between 700 and 800 K, and the emissivity of commercially pure aluminum (AL 1100) decreases monotonically with increasing temperature. The experimental results are used to assess the accuracy of popular multispectral radiation thermometry (MRT) emissivity models for temperature measurement. It is shown that drastic changes in the shape of emissivity distribution preclude the use of a single function to accurately represent every band of the measured spectrum. Better predictions are achieved using the simplest form of MRT emissivity models and minimum number of wavelengths required by the model. Two relatively simple models are identified for best overall predictions for different alloys and temperatures. Despite the relative success of these two models, this study clearly demonstrates that improvements are required in both instrumentation and emissivity models to achieve acceptable accuracy in the implementation of radiation thermometry in the aluminum industry.