Influence of windshield tint and tilt on recognition distance under mesopic conditions

The influence of the angle of inclination and tint of windshields on recognition distance was tested in 94 subjects aged 20-83 years. Both clear and tinted (heat-absorbing) windshields were tested. Light transmission (in the vertical position) was 90.3% for the clear windshield and 81.0% for the tinted windshield. The windshields could be set at angles of 0 degree (vertical), 30 degrees, 50 degrees and 70 degrees of inclination. The tests were performed at night with low-beam headlights (mesopic conditions). A Landolt ring (diameter 43.5 cm; contrast 1:1.5) served as a test stimulus; it was painted on a disc 87 cm in diameter that could be rotated in steps of 45 degrees. The mean recognition distance without the windshield was 32.2 m +/- 5.5 m (mean +/- SD). It decreased to 31.0 m +/- 5.6 m (29.0 m +/- 5.3 m) by using a clear windshield in the vertical position (values for the tinted windshield are shown in parentheses): 30.1 m +/- 5.6 m (27.9 m +/- 5.3 m) with a 50 degrees angle of tilt, and 28.5 m +/- 5.4 m (26.9 m +/- 5.1 m) with a 70 degrees angle. The differences between the recognition distances obtained using the various tilts and tints were small but statistically significant (P < 0.01 in all cases: Student's paired t-test). Since the performance of the human eye is stressed to its limits during nighttime driving, our study suggests that extreme tilt of the windshield should be avoided and that the option of delivering cars equipped with a heat-absorbing but clear front windshield should be provided.