The Turbine Inlet Temperature (TIT) is a critical parameter of Gas Turbine systems influencing both material and coating lifetime of turbines as well as their efficiency. On line TIT monitoring systems would allow both the residual life of the most critical hot parts to be foreseen with higher reliability and to maximize the gas temperature thus increasing the Gas Turbine overall efficiency. However, at present there is no method available to accurately measure this parameter on operating plants. For these reasons we have developed a temperature sensor based on spectroscopic photometric measurements of the infrared radiation emitted in a selected wavelength band by the CO2 molecules of the combustion gases. Aim of the sensor is to measure on line and in real time the average gas temperature on a plant section transversal to the gas flow. The developed sensor prototype is mechanically robust and thermally resistant so to withstand typical operating conditions of industrial gas turbines. Experimental results will be presented relevant to tests carried out both at laboratory level in controlled stable conditions and on a full scale combustor test bed simulating the behavior of real gas turbine machine.