Method for measuring the refractive index and extinction coefficient of liquid metals to determine the angular and temperature dependent laser absorptivity

Accurate simulations of laser material processing techniques require realistic values of laser absorptivity as a function of angle of incidence and material temperature. The laser absorptivity can be calculated with the Fresnel equations, for which one needs the temperature dependent values of the refractive index n and extinction coefficient k of the material for the laser wavelength in question. However, there are hardly any published values for the n and k of metals at high temperatures, especially in the liquid state. This proof-of-principle paper presents a novel and simple experimental method for measuring the refractive index and extinction coefficient of liquid metals and thus their absorptivity as a function of angle of incidence and temperature for a short wavelength range. The method has been developed to determine n and k through the thermal emission over the curvature of a spheroidal droplet and is therefore not based on reflection measurements, which require a flat surface. The thermal emission was recorded using a linear polarizer, a bandpass filter and a camera. The method is demonstrated for liquid copper at a wavelength of 850 nm, and the results are in good agreement with published absorptivity values at a similar wavelength (1064 nm) and temperature range. It was found that the accuracy of the measurements in future work could be greatly increased by using spatial resolutions ≥ 1000 pixels/radius.