A robust, non-contact sensor that measures BOF endpoint bath temperature has been developed and evaluated over a large number of industry heats. The optical hardware consists of a radiation collection head and a multi-wavelength detection unit. The radiation collection head aimed at the furnace mouth is used to collect, and thus enhance, the effective radiant energy emitted from the hostile environment adjacent a furnace as well as to reduce spurious radiations from other nearby furnaces. The detection unit separates the collected radiations into spectroscopy and then converts this spectroscopy into digital signals. Finally, the signal information is processed to determine the carbon content of the steel contained in the metallurgical furnace. The Preliminary results after online carbon content sensing are discussed.