The invention relates to an optical fire-detector in which a radiation-emitting means is arranged to emit a beam of radiation and which has modulator means for the modulation of the beam of radiation in a phase-inverted relationship within a first and a second wavelength band, and a radiation-detecting means is arranged to receive the beam of radiation after it has passed through an intermediate air medium and includes first means for an individual measurement of intensity in the two wavelength bands and second means for the detection of such variations in the measured intensities which are representative for a fire.