Abstract: An electro-optical sensor operative in the middle ultra-violet (UV) region f the electro-magnetic spectrum, and more specifically in the so called "solar blind" region, for the long range detection of flames and artificial light sources. The sensor has a signal sensitivity of a single photon per second and includes an optical filter and an electro-optical detector, such as a photomultiplier, arranged in cascade so that the overall outband response of the sensor satisfies the equation:F*D=K/A,wherein:K is the order of magnitude of 10.sup.-17,F is the average filter transmittance outside the solar blind region;D is the average detector responsitivity outside the solar blind region; andA is the sensor aperture area (in cm.sup.2).