Review of the Mechanisms of Visibility Reduction by Rain and Wet Road. Nicolas Hautière , Eric Dumont, Roland Brémond, Vincent Ledoux Université Paris-Est LEPSiS, INRETS-LCPC, Paris. Outline of the presentation. Objectives Visual effects of the falling rain Visual effects of sprayed water
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Review of the Mechanisms of Visibility Reduction by Rain and Wet Road
Nicolas Hautière, Eric Dumont, Roland Brémond, Vincent Ledoux
LEPSiS, INRETS-LCPC, Paris
where k denotes the extinction coefficient of the atmosphere.
Rain is a population of water droplets falling.
They interact with each other and with the environment.
While falling, a rain drop undergoes rapid shape distortions:
Rain drops come in a wide range of sizes.
The Marshall-Palmer distribution usually binds rain particles density and sizes
shapes of rain drops
Marshall-Palmer rain drop size distribution
Some experiments were conducted in an attempt to relate optical extinction on a long distance to rain density.
The general relation found between the extinction coefficientks (m-1) and rain intensityR (mm.h-1) is:
where a and differ with respect to the location and the optical devices used in the experiments.
Light scattering in rain is rather limited. Using an analogy with fog:
Different experimental curves relating the atmospheric extinction coefficient and the intensity of the rain
To the best of our knowledge, there is no analytic model for the overall reduction of visibility induced by rain on the windshield.
Some works focus on the appearance of rain drops:
Several experimental studies on wiper usage focused on object visibility and seeing distance.
In particular, [Bhise, 1981] investigated the visibility distance of target vehicles under natural downpours.The resulted model is:
where rt characterizes the accumulation of rain water on the windshield and Lb the background luminance.
fish-eye lens effect created by a rain drop
The water sprayed by vehicles has undeniable effects on visibility.
[Fournela, 2003] studied these effects. Unfortunately, no model came out of it, because of several experimental difficulties.
Other studies showed that splash and spray is reduced by 95% on porous asphalt.
The most rigorous works have been conducted for the development of heavy vehicle spray reduction devices.
Even though these researches do not directly address driver visibility, their metering systems might be used to investigate this problem.
The water on a surface makes it specular because of the smooth air-water interface.
Optical interactions on such a surface are governed by Fresnel equation for dielectric materials:
A film of water on a Lambertian surface can also make the surface appear darker.
This is mainly caused by internal reflections at the water-air interface.
Subscattering effects may also exist.
Rain changes the visual aspect of the pavement.
The pavement appears more specular or darker, depending on the observation angle.
This can be dazzling for the driver, especially in daytime with the sun at grazing angles, or at night with opposing headlights.
The visibility of retro-reflective road markings is also particularly impaired.
In daytime, on wet roads, retroreflective materials sometimes appear darker than the pavement.
When the water layer is higher than the size of the beads, markings may disappear.
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