Emissivity Compensated Radiation Thermometry Using Directional Radiances

Emissivity of a metal is fairly low in general. Due to an oxidation film formed on the metal surface, however, its emissivity changes rapidly, and large errors in the temperature measurement can be caused. This is a serious problem for the radiation thermometry of metals. In order to eliminate the error due to surface conditions of the targets, the authors propose a new method based on their findings that the apparent emissivity of a metal surface depends on the direction from which the target is looked at. This means that the radiance of the target does not obey Lambert law. The proposed method uses the ratio of radiance measurements taken at two different angles to compensate the emissivity or the emissivity ratio. This method is promising for the in-line use at metal manufacturing processes such as steels or aluminums.