Discontinuity in normal spectral emissivity of solid and liquid copper at the melting point
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NONCONTACT type temperature measurement devices, such as one-color and two-color pyrometers, have been used considerably for process control in metallurgical processing, plastics processing, and semiconductor manufacturing.[1,2] The temperature measurement using such devices requires values of spectral emissivities for measured objects as input data, and the accuracy in measured temperature values is determined principally by that in spectral emissivity data.[3] Accordingly, it is very important to have accurate data for spectral emissivities of relevant materials. There are many data reported on the spectral emissivity of solid metals.[4] The spectral emissivity depends fundamentally upon the composition and temperature of the sample and the wavelength of light radiated from the sample. Furthermore, the spectral emissivity of solids is also very sensitive to the surface condition of the sample, i.e., the roughness of the sample surface.[5,6] This effect is considered to be a source of scatter in published data for solid metals. With respect to the spectral emissivity of liquids, the scatter arising from the difference in the roughness of the sample surface can be eliminated. In fact, it has been reported that measurements of spectral emissivities for liquid metals at their melting points have good reproducibility and that spectral emissivities of many liquid metals are almost constant, irrespective of temperature.[7,8,9] One-color pyrometers, therefore, are applied to the temperature measurements for liquid metals at high temperatures. Many investigations have been attempted to determine spectral emissivities for liquid metals such as gold,[9–13] silcopper,[7,9,11–18] nickel,[9,11,12,15,16,19–21] ver,[9,11,12,14,15] iron,[11,12,15,16,19,22–24] and so on[12,15,16,21,24] at their melting points, since these data can be used in practice as standard values of emissivities for their liquids. These investigations can be divided into two groups from the viewpoint of the measurement method. In the first H. WATANABE, Graduate Student, M. SUSA, Associate Professor, and K. NAGATA, Professor, are with the Department of Metallurgical Engineering, Tokyo Institute of Technology, Tokyo 152, Japan. Manuscript submitted January 3, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
group,[9,13,20] the optical constants have been measured using ellipsometry, from which emissivities have been calculated using the theoretical relation. This method has a problem that it is difficult to detect only the polarized light reflected at the surface of liquid metals at high temperatures, since the metal surface itself is emitting radiation and the surface is not flat due to the surface tension. In the second group,[10–12,14–19,21–24] radiant temperatures of samples heated at their melting points have been measured using the one-color pyrometer. The emissivity has been calculated by substituting the measured radiant temperature into Wien’s equation. This method has a problem that the optical path for the measurement is different from that
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