The thermal radiation emitted from a LiNbO3 rod in a CO2 laser-heated float-zone system has been measured by a two-dimensional thermal imaging radiometer with the spectral bandpass selected to be 3-5, 10.6 and 8-12 mu m. When the float zone is formed, the interference of the monochromatic CO2 laser with the thermal emission at the wavelengths of 10.6 and 8-12 mu m can be minimized because the deformable gas-melt interface is optically smooth. The present results show that the emission of thermal radiation at the wavelengths of 10.6 and 8-12 mu m is a surface phenomena, while that of 3-5 mu m is a volumetric phenomena. The emission of thermal radiation in the 3-5 mu m spectrum is used to determine the shape of the float-zone interface by using the method developed by Chen and Hu [J. Crystal Growth 149 (1995) 87]. Because the temperature at the gas-melt-solid interface is the melting point of the material, the emissivities for the wavelengths of 10.6 and 8-12 mu m for the melting temperature can be obtained from the Planck radiation law, and they are around 0.92 and 0.98, respectively. Since the emissivity at these wavelengths is insensitive to the variation of the temperature, the surface temperature can be obtained from the thermal radiation distribution.