The influence of the color of visibility targets on the visibility

Reduced visibilities were simulated in the laboratory by using a hydrosol. It was illuminated homogeneously by iodine quartz lamps, producing an illuminance similar to a bright day. In the hydrosol, several visibility targets were suspended at different distances, and the visibility observation was performed by counting the number of visible objects. By this method, an accurate and fast determination of the visibility was possible. The extinction coefficient of the hydrosol was determined with a long path photometer: the intrinsic brightness of the objects in the illuminated hydrosol was determined for several wavelengths by using a fiber optic. The correlation between the measured visibility and the extinction coefficient shows that the frequently used visual range underestimates the visibility, since the highest perception for an atmospheric aerosol is not at 550- but at 580-nm wavelength. Therefore it is suggested to use the atmospheric extinction coefficient at 580 nm for the calculation of the visual range. All colored objects could be seen less far, since they had a smaller contrast at the wavelength of maximum perception than the black object. Dark colored objects can be seen farther than bright ones; especially, objects that have a high reflection in the yellow and orange have a small visibility. If the intrinsic brightness of the objects and the extinction coefficient (both as a function of wavelength) are known, the wavelength of maximum perception can be calculated; and from this, the visibility. The visibility, thus calculated, agrees with the observed visibility. If visibility observations in the atmosphere have to be performed with nonblack objects, methods for corrections to the visibility of a black object are given.