Observers’ Judgments of the Effects of Glare on their Visual Acuity

Abstract

The design of headlamps must balance two conflicting goals: providing sufficient forward illumination to assure adequate visibility while not being a glare source for other road users. By projecting illumination downward onto the nearer roadway, low beam headlamps emphasize the latter goal at the expense of the former. Yet, the illumination provided by low beams is insufficient to assure safety. When drivers use high beam headlamps their ability to see and respond to hazards improves substantially. While it is common for drivers to report that they have been “blinded” by oncoming headlights, it is unclear whether observers can accurately judge when they are visually disabled by glare.

This experiment sought to use lab-based psychophysical methods to quantify the accuracy with which observers can judge when a glare source reduces their visual acuity. Seventeen observers estimated their disability glare threshold - the luminance of a glare source that would be just sufficient to impair their ability to discern the orientation of a Landolt C (high and low contrast and of varying sizes). These estimated disability glare thresholds were compared to the participant’s actual disability glare thresholds.

Across all stimuli, participants consistently underestimated their ability to recognize the Landolt C stimuli through the veiling luminance of the glare source. On average, estimates of glare threshold were 88% lower than actual glare threshold intensities. Additionally, participants’ judgments were significantly affected by the size of the stimulus but not stimulus contrast.

These results confirm that observers can exaggerate the debilitating effects of glare. This work may be useful in understanding why typical drivers may under use their high beam headlamps. A driver that overestimates the extent to which light from their own headlamps visually disables other road users may choose not to use these higher intensity headlamps, despite the significant visibility benefits provided by high beam headlamps. As long as drivers have the ability to decide when the use of high beams is appropriate, it seems worthwhile to understand the factors that influence their usage of high beams and, simultaneously, to explore ways to encourage appropriate reliance on high beams.