The influence of the aerosol on the color and luminance differences of distant targets

Abstract

An object is visible due to its color and luminance difference to the background. When seen through an aerosol generally both the luminance and the color difference (contrast) decrease with increasing distance of the object. Upon reaching the threshold value, the object becomes invisible. The distance at which the threshold is reached is called the visibility. The standard visibility is the distance where the contrast of a perfectly black target seen against the horizon reaches a luminance contrast of 0.02 (absolute value) at a wavelength of 550 nm. The distance where the luminance contrast of normally occurring targets reaches the threshold of 0.02 generally is smaller than the standard visibility. When considering aerosols with different wavelength dependences of the extinction coefficient (described by the Angström exponent) but the same standard visibility, a higher visibility is attained through aerosols with large Angström exponents. This is caused by the lower extinction coefficient in the red, giving larger constrast at the longer part of the spectrum and thus a larger average contrast. Due to the aerosol between the target and the observer, the color of the target as well as its background is altered, the color difference normally decreases with increasing distance (the color difference can be described as the distance in the Mac Adam color plot). Even a black target has a slightly bluish color when seen through aerosols with positive Angström exponents, leading to a slight increase in visibility. The same is true for other targets, although they may appear in a different color than originally. The color effect is largest at high Angström exponents, and in all considered cases causes an additional increase in visibility. In some cases an increase in color difference over the original color difference between target and background has been found at distances smaller than one tenth of the visibility. If targets are considered at distances much shorter than the visibility (e.g. one third) both the color difference and the luminance difference is large making the recognition, by far, easier. In general then the color difference is more important. Again the color difference increases with increasing Angström exponent. From this it can be concluded that the color difference is an important factor when considering vision through the atmosphere. The color difference is highly influenced by the wavelength dependence of the aerosol, the stronger the wavelength dependence the larger the color difference and thus the visibility.