Temperature measurements of the gas phase surrounding the hot front during material processing at the focus of a high concentration solar furnace can be achieved by coupled emission and absorption optical spectroscopy. The emitted light results from a fluorescence phenomenon of some vapor species from the heated sample excited by absorption of the incident concentrated beam. Absorption measurements were performed on these same species with a reference beam using the sun radiation as source. These coupled techniques led to consistent results and suggested the existence of a nonequilibrium Knudsen layer near the vaporizing surface. In this paper we report measurements performed on YO diatomic molecule issuing from a melted sample of yttrium oxide.