System for Measuring the Spectral Distribution of Normal Emissivity of Metals with Direct Current Heating

A system for measuring time variations of the normal spectral emissivity at wavelengths ranging from 0.55 to 5.3 μm was developed and applied to metal specimens in vacuum and oxidizing environments in the temperature range from 780 to 1200° C. The specimen was heated to high temperatures by passing a direct current in a vacuum chamber, and the surface oxidation was controlled by a low-pressure oxidizing gas. The specimen temperature was measured by a single-band (0.9-μm) radiation thermometer viewing at a cavity formed in the specimen from the rear side. The front surface of the specimen was observed by a multiband (112-wavelength) radiation thermometer to measure the normal spectral emissivity. The effective normal spectral emissivity of the specimen cavity was evaluated to be 0.94±0.05 at a wavelength of 0.9 μm in comparison with a metal tube having a small blackbody hole on the rear. The measurement uncertainty of the normal spectral emissivitiy by the system was estimated to be 5 to 10% of the emissivity value in most of the interesting ranges of emissivities, temperatures, and wavelengths.