Fog usually occurs by radiational or advectional cooling of moist air, resulting in a high concentration of microscopic airborne water droplets. When visible light interacts with one of these droplets, it is scattered in different directions with different intensities, following the so-called phase function, which depends on wavelength and droplet size. Scattered light does not simply disappear as if it was absorbed by smoke or dust. It simply propagates along different paths, some of which end up into the driver's eyes. It results in an added luminance which impairs visibility by reducing contrasts. Paradoxically, the driver's own headlamps, which are designed to increase visual range in night-time, may actually add to this problem by illuminating the fog ahead of the vehicle, causing a veil of backscattered luminance. The only way to minimize this phenomenon is to work on the beam pattern of the headlamps, in order to avoid stray light above the cut-off line. Simulation is useful when it comes to the design of automotive front-lighting systems, or traffic control devices. However, the simulation of complex optical phenomena, in driving simulators for instance, involves some approximations which need to be assessed. In this paper, the authors address the backscattered veil caused by low beam headlamps in fog. A computational approach is used, based on a Monte Carlo light tracing technique which allows the computation of the backscattered luminance distribution in the driver's field of vision. Several hypotheses were investigated (far field photometry, single scattering, and absence of volume shadows). The influence of these approximations on the average backscattered luminance in a 20deg solid angle for various fog conditions is examined, and conclusions concerning their validity for a particular set of headlamps are drawn.
Assessment of the simulation of low beams backscattering in fog
Bewertung der Simulation der Abblendlicht-Rückwärtsstreuung im Nebel
2005
10 Seiten, 9 Bilder, 6 Quellen
Conference paper
English
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