Extended Photometric Model of Fog Effects on Road Vision

The road environment projects a luminance distribution into the eyes of the driver. In the presence of fog, the resulting visual signal is disturbed because of light scattering by airborne water droplets. This phenomenon has three major effects, which cause an overall visibility loss: attenuation, halo, and veiling. Koschmieder's law is widely used to describe how fog modifies the visual signal, but it applies only when nonluminous objects are being observed in daytime. In nighttime and with self-luminous objects, luminance from scattered light can no longer be neglected. The optical mechanisms underlying the visual effects of fog have been analyzed. Koschmieder's law was then extended to account for halos and backscattered veil in the disturbed visual signal. The utility and validity of the resulting photometric model were discussed, and an example was provided that illustrates how it can be used to predict the image of a foggy road environment as seen by a driver. Despite its complexity, the model should find many applications, such as driving simulation and aiding device design.