An Optical Device for the Detection of Clouds and Fog

A simple, low-cost optical fog detector has been developed. The principle of operation lies in the detection of near-forward scattered light. The unconventional in-line arrangement of light source and detector, in combination with the centrally placed light stopper, enables the use of readily available devices in which optics and electronics are integrated, thus reducing manufacturing costs. The performance of the device is studied in the field as well as in a facility in which a well-defined fog can be produced. In addition, the response of the device as a function of fog droplet diameter is calculated. Both the experiments as well as the calculations show that the instrument has a good response to visibility reductions caused by water droplets within the size range encountered in fog and clouds, whereas the instrument is not sensitive to visibility reductions caused by larger droplets (drizzle or rain) or by haze.