Abstract: A photoelectric detector in which partial light beam obfuscation caused by dirty windows is balanced out by providing two beams of light passed through the gaseous medium in which smoke or the like is to be detected, the beam windows being subject to substantially equal accumulation thereon over a period of time of soot or the like which causes the windows to become dirty or clouded. The windows are so spaced that the respective beams pass different distances through the smoke. The relative beam intensities as received are compared by connecting photoresponsive devices in a bridge circuit. A meter is connected to the bridge and preferably calibrated to read directly in Ringlemann numbers.