Abstract: A flame is detected by signal analysis for intensity variations in radiation received by a sensor. A low-frequency spectrum of the signal is analyzed for mid- and cut-off frequencies, and the signal is classified as periodic or non-periodic. Periodic signals with a mid-frequency (.omega..sub.mp) above a first frequency value (G.sub.1), and non-periodic signals with a cut-off frequency (.omega..sub.gc) above a second frequency value (G.sub.2) are classified as interfering signals. The first frequency value is determined by the flicker frequency of a stationary flame having a magnitude corresponding to a flame of minimum magnitude to be detected. The second frequency value is selected greater than the first frequency value (G.sub.1).