Abstract:
Reflections from the top of the dashboard are often seen in the windscreen as veiling glare superimposed on the road scene. Veiling glare can interfere with visual function and potentially compromise safe driving. Characterisation of a driver's vision is traditionally based on measurement of visual (letter) acuity and although it has shortcomings, visual acuity is a simple, well-tried measure of visual function. The aim was to determine the reduction in visual acuity and contrast sensitivity caused by veiling glare. Methods: Binocular visual performance was measured with Bailey-Lovie High and Low Contrast Acuity charts and a Pelli-Robson Contrast Sensitivity chart. Twenty subjects, 10 under 40 yrs of age (mean 23 yrs) and 10 over 40 yrs (mean 57 yrs) viewed the charts through a car windscreen with and without the presence of veiling glare. Veiling glare was induced naturally (by sunlight, outdoors) or with artificial light in a laboratory, where eye accommodation was also recorded. Results: visual performance was markedly reduced in the presence of veiling glare induced by sunlight. Visual Acuity Rating (VAR) dropped by 43 4.8 letters (mean SD, n = 20) for low contrast letters and by 27 9.5 letters for high contrast letters (approximately equivalent to reductions from 6/6 to 6/48 and 6/6 to 6/24, Snellen acuity respectively). Contrast Sensitivity decreased from 1.65 to 0.29 (log Contrast Sensitivity). Similar reductions were observed under laboratory conditions. In the younger group veiling glare induced about 0.25D of accommodation while reading the distance charts. Conclusions: the presence of veiling glare from a dashboard can severely compromise static visual acuity and contrast sensitivity. Visual acuity is reduced well below the levels typically required to meet standards for obtaining a driving licence.