A non-intrusive method for monitoring flames in gas burners is presented. The method is based on the optical analysis of the flame by using a Silicon Photodiodes array and a set of interference optical filters. The covered wavelengths evaluate the formation and behaviour of excited CH* and C-2* radicals for confined gas flames. The monitoring of these radicals is carried out in the flame reaction region at the burner exit, where the C-2*/CH* ratio signals provide enough information to identify the optimal fuel-air ratio in order to obtain the boiler maximal efficiency and to prevent an incomplete combustion. The experimental results obtained for two power levels, in a boiler of 150 kW, demonstrate that the C-2*/CH* ratio provides important information concerning the combustion state and gives a clear indication about the burner adjustment when CO emissions begin to increase to the upper the levels accepted by environmental laws. The fast response and its non-intrusive character give to the proposed optical sensor an important potential to be used in advanced control strategies for the on-line optimization of the combustion process.