Instruments for the Measurement of the Visual Range

Abstract

It is often necessary to know the visual range or `visibility' at times when a direct observation on `visibility marks' is not possible. This note describes a series of instruments of small size in which the visibility is measured by measuring directly the scattering coefficient of a relatively small volume of the air. The instruments take different forms according to what is most convenient for the conditions of use but all give the true scattering coefficient irrespective (within wide limits) of the nature of the polar distribution of the scattered light. This is achieved because the polar curve is effectively integrated in the measurement.