A laboratory study investigated the effects on night vision of windshield transmittance and light-scattering properties in relation to the effects of reduced contrast, glare, night myopia, and age. Three groups of subjects included: (1) ten elder drivers, (2) ten younger drivers who were susceptible to low levels of night myopia, and (3) ten younger drivers who were not susceptible to night myopia. Visual acuity of all subjects was measured for high- and low-contrast targets viewed through five windshields with and without glare that simulated opposing low-beam headlights at 50 m distance. To assess night myopia, visual accommodation of younger subjects was measured objectively under each of the ten windshield-by-glare conditions while they viewed four realistic low-luminance targets. The transmittance of three clean windshields, with different tints and rake angles (ranging from 45 deg to 75 deg from the vertical) varied from 0.86 to 0.43. Two additional conditions were created by mounting a dirty windshield at rake angles of 45 deg and 75 deg, which produced light transmittances similar to two of the clean windshields but now with a higher degree of light scatter. The main results are as follows: (a) Reduced transmittance of clean glass elevated low-contrast resolution thresholds by 0.09 log units, but had no effect on high-contrast thresholds; (b) Light-scatter of dirty windshields elevated low-contrast thresholds to unmeasurable levels; (c) Glare elevated low-contrast thresholds by 0.18 log units; (d) Resolution thresholds were 0.28 log units higher for low-contrast than for high-contrast targets; (e) Thresholds for low-contrast targets were 0.31 log units higher for elder than for younger subjects; and (f) Tests of accommodation showed significant increases of night myopia with glare and reduced target luminance, as well as a tendency toward greater night myopia for higher luminance targets with the dirty windshields as compared with the clean windshields. (The accommodation data were inconclusive, however, because few subjects had substantial levels of night myopia.) It was concluded that low windshield transmittance due to rake angle and tint has a significant detrimental effect on the visibility in nighttime driving, but these effects are small relative to those of light-scatter from high levels of dirt and wear. (A)
Samenvatting