The thermal radiation for the wavelengths of 3-5, 5, 10.6 and 8-12 mu m emitted from heating rods is measured in this study by a two-dimensional thermal imaging radiometer. The LiNbO3 and LiF rods with both single crystal and polycrystalline structures are measured. The volumetric emission is enhanced by the scattering effect induced by the polycrystalline rod's porous structure. When the phase is changed from solid to liquid, the sharp decrease of the emission in the 3-5 mu m spectrum has been observed for the polycrystalline rods due to the disappearance of the scattering effect of the porous structure. Indeed, the emission in the 3-5 mu m spectrum does not significantly decrease for a LiF single-crystal rod. However, it still exists for a LiNbO3 single-crystal rod except that the difference in the emission levels decreases. As the phase change from solid to liquid occurs, the emission of LiF and LiNbO3 in the 10.6 mu m spectrum continuously increases regardless of whether the structure is a single crystal or polycrystalline.
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