Radiation thermometry, which enables non-contact temperature measurements of a moving target, plays a crucial role in temperature control during steel production. However, radiation thermometers implemented in manufacturing lines frequently encounter practical problems such as emissivity fluctuation, obstruction in the optical path, and background radiation. Herein, a new approach for solving the obstruction problem arising from the water in hot-rolling processes is presented. When a radiation thermometer detects the radiant flux emitted from a steel surface through unstable flowing water and evaporated water, the apparent transmissivity of these obstacles is unknown and it is extremely difficult to perform precise measurements. Thus, we propose a two-color thermometry technique that use specific wavelengths. A combination of wavelengths unaffected by the presence or absence of water in the optical path was studied both theoretically and experimentally. This two- color method enables simultaneous measurements of the water thickness. A prototype two-color thermometer was designed using detection wavelengths of 1 200 nm and 1 300 nm. The experimental results indicated that the temperature and apparent water layer thickness of heated steel beneath a water layer can be measured. It was also confirmed that two-color thermometry is useful for the obstruction of foggy steam.
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