Samenvatting
This report describes a fog detector of the split beam transmissometer. Two light beams, derived from a single modulated solid state source, are detected at the ends of two separated paths, one enclosed and one through the atmosphere. The outputs from the detector are switched and combined in such a way that the final output is a direct measure of the attenuation undergone by the fog path beam. The prototype has established that it should be possible to produce an instrument that will determine meteorological visible ranges between 10 m and 200 m within an accuracy of 10 per cent.