The pyroelectric detector of the flame sensor detects the typical spectral radiance of burning organic materials such as wood, natural gas, oil or plastic. In order to prevent a false alarm due to sunlight or other intense light sources, such as light from arc welding, two independent criteria of a flame are analyzed: First a typical flame is characterized by a flicker frequency of 1 to 5 Hz. Secondly, a hydrocarbon flame contains the combustion gases carbon monoxide (CO) and carbon dioxide (CO2). Their emission bands lie in the infrared spectral range of 4.0 to 4.8 µm. In order to obtain a high signal, one uses wide bandpass filters for the detector window, which include both the radiation emission of CO and of CO2. Optionally, a further channel can be used to recognize a further combustion by product, water.