True temperature measurement on metallic surfaces using a two-color pyroreflectometer method

In the most common case of optical pyrometry, the major obstacle in determining the true temperature is the knowledge of the thermo-optical properties for in situ conditions. We present experimental results obtained with a method able to determine the true temperature of metallic surfaces above 500 degrees C when there is not parasitic effect by surrounding radiation. The method is called bicolor pyroreflectometry and it is based on Planck's law, Kirchhoff's law, and the assumption of identical reflectivity indicatrixes for the target surface at two different close wavelengths (here, 1.3 and 1.55 microm). The diffusion factor eta(d), the key parameter of the method, is introduced to determine the convergence temperature T(*), which is expected to be equal to the true temperature T. Our goal is to asses this method for different metallic surfaces. The validation of this method is made by comparison with thermocouples. Measurements were made for tungsten, copper, and aluminum samples of different roughnesses, determined by a rugosimeter. After introducing a theoretical model for two-color pyroreflectometry, we give a description of the experimental setup and present experimental applications of the subject method. The quality of the results demonstrates the usefulness of two-color pyroreflectometry to determine the temperatures of hot metals when the emissivity is not known and for the commercially important case of specular surfaces.