Visual range monitors

The determination of the atmospheric extinction coefficient is necessary to allow visual range to be calculated. The limitations of nephelometers (devices which measure light scattered in the atmosphere) are discussed. These limitations lead to the conclusion that optical transmissometers are the only valid devices for assessing extinction coefficients. New techniques for maintaining the accuracy of transmissometers over many months of use are described. The high accuracies achieved allow a wide dynamic range of visibility to be measured with a single base line instrument.