Reflections of the top of the dashboard seen in the windshield can result in disability glare because these reflections reduce the contrast of objects in the road scene. This phenomenon, which occurs especially during direct sunlight, is due to the veiling luminance of the reflected sunlight being superimposed on the image of the road scene. The amount of veiling glare is influenced by the windshield rake angle and the dashboard reflectance. A field experiment under particular sunlight conditions was performed. The independent variables included windshield rake angle (45, 60, and 70 degrees), reflectance of the top of the dashboard (13%, 20%, and 34%), and subject age (eight younger with a mean age of 24, and eight older with a mean age of 72). The subjects were asked to detect pedestrian dummies having either high or low contrast against the background. Reaction times to the high-contrast pedestrian and misses of the low-contrast pedestrian were used as the main dependent variables. Subjects also had to rate their own ability to see in the different experimental conditions. The results showed that both windshield rake angle and dashboard reflectance affected visual performance. Visual performance decreased with larger windshield rake angles, especially with rake angles larger than 60 degrees. During those conditions, subjects needed more time to detect objects, and they had more misses in detecting low-contrast objects. The effect was particularly pronounced if a large rake angle was combined with a high dashboard reflectance. The reduced contrast produced by the experimental conditions had a larger effect for older than younger subjects. The perceptual consequences of decreased contrast sensitivity of older persons became evident when older subjects had to detect low-contrast objects or when extreme veiling glare conditions reduced the contrast of otherwise high-contrast objects. Although older subjects tended to overestimate their ability to see during conditions with high veiling glare, overall the ratings showed that subjects are reasonably well aware of the veiling glare problem, because the rating data corresponded well with the objective reaction time data. Both subject groups gave their lowest visibility rating for the condition involving the largest rake angle and the highest dashboard reflectance. (Author/publisher)
Abstract