Abstract: The apparatus has spaced light sources and light sensors on opposite walls of a conduit positioned to provide a pair of equal shorter path lengths and a pair of equal longer path lengths for narrow collimated beams between the sources and sensors. Each sensor is exposed to beams from the sources alternately during measuring periods. Computing means computes the quotient of the products of the sensor outputs corresponding to light transmitted over the longer and shorter paths, and an output is produced from the quotient corresponding to the transmittance or opacity over a predetermined distance of smoke, etc. Test means are provided to monitor the operation continually. Beam splitters for the sources, and disks having transparent and mirror sectors for the sensors, are described. Mirror sectors of different reflectivity operate during measuring and test periods.