Experimental study on the visibility in absorbing media

Abstract

An absorbing atmosphere has been simulated by means of a mixture of a hydrosol and a particle free dye. This model allows an easy distinction between absorption and scattering. For this simulated atmosphere the influence of absorption of the medium on the visibility of black and grey objects was determined. The luminance of the horizon and of different grey targets as well as the visibility of the targets has been measured with increasing absorption under monochromatic illumination. The visibility of black targets depends only on the total extinction coefficient. The visibility of non black targets decreases with increasing absorption, and depends not only on the extinction coefficient but also, through the inherent contrast of the target, on the existent absorption. Measurements of the inherent contrast of the object at the existent absorption are necessary in order to determine the visibility of non black objects in absorbing media. The reflectivity of the ground also influences the visibility of non black targets, especially at low concentration- the target beeing mainly illuminated by direct sunlight - and for bright objects. Even if the standard visibility can be kept constant (e.g. due to additional air pollution control) the visibility of non black targets is smaller; i.e. the optical quality of the atmosphere decreases with increasing absorption. Only when the targets are brighter than the horizon, their visibility will increase with increasing absorption.