The radiosity from a rod of lithium niobate that was heated uniformly by a CO2 laser with an axisymmetric ring-shaped beam was measured by a two-dimensional infrared imaging radiometer with bandpass filters chosen to be 3-5, 8-12, and 10.6 mu m. The radiative characteristics of the volumetric emission and the significant difference of the emitted thermal radiation between the solid or melt and background for wavelengths of 3-5 mu m was used to determine the interface shapes of the float zone during the CO2 laser melting process. The trend of the power distribution impinging on the rod surface may be deduced from the reflection of the CO2 laser. The gas bubbles existing in the melt may also be monitored. The surface temperature was determined from the distribution of the radiosity for wavelengths of 8-12 mu m. The surface temperature distribution is modified significantly be thermocapillary convection. With an increase of the input power, the gas-melt interface changes from concave towards the melt to convex at the upper zone and concave at the lower zone due to the effect of gravity. Copyright (C) 1996 Elsevier Science Ltd.