Effects of overall low-beam intensity on seeing distance in the presence of glare.

Previous studies have demonstrated that current low-beam headlamps do not provide adequate seeing distance for safety. One approach for remedying this situation might be to provide more total light from low-beam headlamps, leaving the relative distribution of light unchanged. Increasing the light output of low-beam lamps, in addition to providing more light for seeing, would also cause an increase in glare light for oncoming drivers. This raises the question of whether there would be a net benefit in seeing distance for all drivers if the light output of low beam lamps was generally increased. The authors addressed this question in an empirical study in which we varied the intensity of headlamps between a value typical of current U.S. low beams and a level about 3.8 times higher. They measured seeing distance as a function of headlamp intensity, always varying the intensity of the seeing light and glare light by the same proportion. Increasing intensity by a factor of about 3.8 increased seeing distance by 17%. This result is qualitatively consistent with earlier findings that seeing distance in a glare situation is longer when high beams oppose high beams than when low beams oppose low beams. It is also consistent with predictions from quantitative vision modeling using veiling luminance to represent the disabling effects of glare. The present findings suggest that, if objective visual performance is the only criterion, there is no clear upper limit to how intense low-beam headlamps should be. However, there may be a level at which people simply will not tolerate the subjectively discomforting effects of glare, or at which glare indirectly affects objective performance through its effects on subjective comfort. Because subjective discomfort, rather than objective visual performance, may be the limiting consideration for setting maximum glare levels, more research should be done to understand the nature and consequences of discomfort glare, including possible effects of subjective comfort on objective visual behavior. (Author/publisher)