The most general approach to visibility is dE=-Eb ext dx  (2.5) where E is the light intensity of a collimated beam of light, b ext is the extinction coefficient, which is the sum of the four separate b values for light scattering and absorption by gas molecules and by particles, and x is the distance [28]. If the air mass through which one views a distant object is uniform, so that bext is constant, this may be integrated to (2.6) The Koschmeider equation [Eq.(2.4)]is based on the visual range corresponding to E/E 0 =0.02 when L V =Δx.