![]() ![]() This may have influenced the measurement results. For the background luminance, the light traveled through the rain twice, but the background luminance is also influenced by backscatter, which is the light that reflects off the rain. This would result in the effective transmittance being less for the object luminance than the illuminance. It should be remembered that the illuminance measurement was made at the object, meaning that the measured light traveled through the rain only once, whereas the object luminance is effectively attenuated twice as the light travels from the headlamps through the rain to the object and then back through the rain to the photometer. It was expected that the transmittance calculated from the illuminance, the object luminance, and the background luminance would be the same however, this was not the case. ![]() Object luminance, and background luminance in the rain. Transmissivity of the atmosphere for the illuminance, The 200 ft distance was chosen as it represented close to the mean of the visibility distances found in the other ENV studies (ENV Volume IV).įigure 71. ![]() In a typical analysis, transmittance would be measured at more than one distance however, due to difficulties with rain consistency during the characterization process, this was not possible. It should be noted that the rain transmittance was only measured at one distance (200 ft). The transmittance of the atmosphere in the rain was then used to scale the object luminance and the background luminance to achieve the resulting contrast and VL in the rain event. In these equations, is the atmospheric transmittance, L is object luminance, and E is illuminance on an object. The transmittance was calculated using ratios of both the luminance and the illuminance under clear and rain conditions, which are shown in figure 69 and figure 70. The rain rate for the object characterization was matched to that of the rain condition experiment.Īs mentioned, to investigate the effect of the rain on the object visibility, the transmittance of light through the rain was measured. The system is operated from a pumping station where the pressure and the rain rate can be controlled to achieve the rain condition desired for the experiment. The capabilities consist of 75 weather-making towers located along the side of the road. The rain condition was simulated on the Smart Road using the all-weather testing capabilities at the facility. The effect on the photometric characteristics of each of these conditions is considered individually along with their relationship to the visual performance study data. The method used for calculating the transmittance of the rain, snow, and fog used during the visual performance experiments varies by the weather type. From the transmittance, a factor is then used to scale the measured illuminance and luminance values used in the analysis. To account for the effect of the weather conditions on the photometric measures, the transmittance of the atmosphere must be accounted for. The characterization of the objects in adverse weather conditions was considered as part of this investigation. The results of these investigations are documented in Volumes IV, V, and VI of this report series. During the ENV visual performance experiments, the performance of the VESs in adverse weather conditions was tested for rain, snow, and fog conditions. For example, when light shines through a rainstorm, the raindrops both absorb and scatter the light, thereby reducing the illuminance on an object. PDF files can be viewed with the Acrobat® Reader® CHAPTER 5-EFFECT OF ADVERSE WEATHERĪdverse weather conditions affect the observer by reducing the transmittance of the atmosphere. ![]()
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