Temperature dependence of the emittance of LiF and LiNbO₃ in the near-infrared spectra

This study investigates the temperature dependence of the normal emittance in 3-5, 5, 10.6 and 8-12 μm spectral range for LiNbO₃ and LiF rods with both crystal and polycrystalline structures. According to the results, the normal emittance in the near-infrared spectra increases with increasing wavelength. The normal emittances at 10.6 and 8-12 μm are not only invariant with temperature, but also exceed 0.9. When the temperature increases, the normal emittances at 3-5 and 5 μm for the LiF single-crystal rod continuously increase regardless of the phase change from the solid state to the liquid state. Before the temperature reaches the Curie point, the normal emittances at 3-5 and 5 μm for the LiNbO₃ single-crystal rod increase with increasing temperature. The normal emittances at 3-5 and 5 μm then decrease as the temperature further increases. A significant decrease in the normal emittance at 3-5 and 5 μm occurs when the emission is only from the melt. In addition, the magnitudes of the normal emittance at 3-5 and 5 μm are enhanced by the scattering effect due to the porous structure of the solid.