Daytime veiling luminance from windshields: effects of scattering and reflection

Abstract

Light that is reflected or scattered from a vehicle windshield toward the eyes of a driver creates a veiling luminance that the driver must look through in order to see the roadway ahead. At night, scattered light is probably the dominant contributor to this effect, but for daytime conditions most attention has been given to the component of veiling luminance caused by images of the top of the dashboard seen by specular reflection in the windshield. This study was designed to provide data on real-world values of: (1) the overall veiling luminance that drivers are exposed to under sunny daytime conditions, and (2) the relative contributions to that luminance from specularly reflected and diffusely scattered light. We made photometric measurements of the veiling luminance caused by the windshields of 18 vehicles under sunny conditions. Illuminance on the outer surface of the windshield averaged 88,900 lux, and under those conditions the total veiling luminance from the windshield averaged 561 cd/m2. Of that total, about 60% was attributable to reflection and about 40% to scatter. Thus, under the conditions examined here, although the contribution from scatter is not negligible, reflection causes the majority of veiling luminance, and measures that would reduce reflected luminance potentially offer substantial benefits. Directions for further research include sampling wider ranges of vehicles and of sun angles, and better quantification of the overall importance of windshield veiling luminance for driver vision in the daytime.