A furnace containing a burner, supplied by fuel to be ignited at the burner, in the presence of air, is provided with a viewing duct for viewing the flame and means for detecting the radiation intensity at a plurality of different wavelengths of the radiation spectrum. The output from the detector is amplified and integrated and a signal representing the ratio of intensities at the selected wavelengths is applied to an indicator to indicate fuel-air ratio and also is applied to control means for the fuel flow or the air flow, or both, to control the fuel-air ratio at the burner and to maintain it substantially constant at a desired value. This can be done at more than one burner, providing balanced furnace operation. With this balanced furnace, there is improved firing efficiency, resulting in fuel saving and lowered pollution emissions. A computer type master controller can be used to correlate the fuel-air ratios for the individual burners.