[Visual compromise of automobile drivers by frontal photo-flash in mesoptic conditions]

The time for recovery of recognition of an optotype presented at a distance of 40 m subsequent to exposure to a flash gun (Eso company, Tettnang; flash energy 200 Ws, duration 1/1000 s; distance 10 m) was studied on 97 subjects aged 20 to 83 years. A Landolt-ring (contrast 1:5, diameter 29 cm), mounted on a disk (diameter 58 cm) which could be rotated in steps of 45 degrees served as a test target. The flash gun could be dimmed down by a red filter (Schott RG 665). The tests were performed at night without additional illumination except the low beam of the car. The flash gun was foveally fixated. After a white flash, a period of 93.1 s +/- 48.6 s (MEAN +/- SD) passed before the optotype could be recognized correctly again. By adding the red filter in front of the flash gun the recovery time could be shortened significantly (p < 0.0001; Wilcoxon signed-rank test) to 4.9 s +/- 1.7 s. "Time for re-adaptation" after dazzling by red flash is comparatively short and therefore strongly influenced by many variables (e.g. time for recognition and for verbalization). Because of that a final evaluation of dazzling by red flash seems to be impossible with this method. Subjects of advanced age and opacities of the ocular media showed an increase in the "time for re-adaptation" for the white as well as the red flash. In conclusion, a speed control using white flash guns at night can be considered as critical.