Microsecond laser polarimetry for emissivity measurements on liquid metals at high temperatures - Application to niobium

Prior to now, all existing pulse heating systems capable of investigations on liquid samples could measure the radiance temperature only during the experiment. To determine the true temperature, it was necessary to assume that the emissivity is constant over the entire temperature range, with a value equal to that at the melting point. This assumption can cause large uncertainties in temperature measurements, especially at elevated temperatures. To avoid these uncertainties, a microsecond-division of amplitude photopolarimeter (mus-DOAP) was added to our experiment. The mus-DOAP detects changes in the polarization state of a laser beam (lambda = 684.5 nm) reflected by the sample surface during the pulse-heating experiments. This change in the polarization state is used to determine the index of refraction n and the extinction coefficient, k. This leads to the reflectivity R of the sample, and by means of Kirchhoffs law for opaque materials, the normal spectral emissivity is obtained. The application of simultaneous laser polarimetry and spectral radiometry leads to the true temperature and is demonstrated on liquid niobium.