The most efficient method to reduce nitrogen oxides in a natural gas flame or a coal flame is a correct adjustment of the burners. A newly developed optical method to measure the temperature distribution in the flame is discussed. KEMA (a Dutch Electrotechnical Equipment Testing Agency) developed the measuring method to control the combustion process in each burner of a natural gas- or coal-fired power plant for NOx formation for a constant energy production, and for the stability of the combustion. By means of data from the temperature distribution measurements some important parameters concerning NOx formation can be determined. Attention is paid to several active and passive spectroscopic methods to measure temperatures in flames. Passive spectroscopy is considered to be the best measuring method. Concentrations of radicals (CH, CN, NH) and temperature distribution are determined by means of a spectroheliograph and a camera, flame temperatures are measured by means of metal tracers. Experimental measurements carried out in the Flevo plant (EPON) are discussed. 8 figs.