Emissivity modeling of metals has been developed to elucidate behavior during the growth of oxide film, and the modeling results have been compared with experimental results. To express emissivities, pseudo-optical constants of a bare metal and of an oxide film obtained by an elipsometer are substituted into the model equations. Emissivity behavior during the growth of an oxide film upon the surface of a specimen is shown in terms of spectral, directional, and polarized characteristics, and it coincides with the experimental results, both quantitatively and qualitatively. The modeling is simple and provides useful guidance for the development of emissivity-compensated radiation thermometry.
An apparatus of the radiometric emissivity measurement of a metal has been developed. The apparatus possesses some features that it can keep a metallic sample under specific conditions such as vacuum, oxidizing, and deoxidizing atmosphere. Using the apparatus, spectral (from visible to 10 μm infrared wavelengths), directional (normal and 80°), and polarized emissivities (p and s polarized) of metals can be measured and can be utilized to investigate the behavior of the emissivity of the specimen. In this article, the concrete design of the apparatus is described and some examples of measurements of emissivities under some specific conditions are introduced. The peculiar behaviors of the emissivity of a metal in the early stage when the oxide film is grown on its surface are discussed.
This article describes some considerations for designing a practical radiation thermometry system for a glossy metal moving through a high temperature furnace, such as a continuous annealing furnace. In order to accomplish this task, two problems must be solved. The emissivity compensation of an object must be calculated and the furnace's background radiation noise must be eliminated. The authors have proposed a method that uses the radiance's polarized directional properties to simultaneously measure the emissivity and temperature to solve the first problem and a technique using a pseudo-blackbody installed in the furnace to solve the second problem. During heating, there is a one-to-one correspondence between the emissivity and the ratio of p-and s-polarized radiances for metals. This characteristic has successfully led to the development of a method for simultaneously measuring the emissivity and temperature of metals regardless of a potential large change in emissivity. Introducing a pseudo-blackbody radiator into a furnace removes the background radiation noise. Moreover, the blackbody radiator supplies a constant reference radiance. This reference plays an important role in maintaining the principle of emissivity-compensated radiation thermometry inside the furnace. Experimental results have simultaneously measured the emissivity and temperature of stainless steel at 1300 K with errors of 12% and 0.96%, respectively. These values were attained even though the s-polarized emissivities change from 0.25 to 0.75 at a wavelength of 0.9 m. These errors can be achieved by designing the apparatus to have a solid angle, the aperture of the pseudo-blackbody subtended by a measuring point of the specimen, of more than 0.02 steradians. The accuracy of this method is heavily dependent upon the specimen's surface roughness. The maximum surface roughness that allows for the successful utilization of this method is Ra = 0.12 m.
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