1927-01-01

Effect of Wet Roads on Automotive Headlighting 270049

LABORATORY and road tests of headlighting on dry and wet road-surfaces, with various types of head-lamp beam, are described and the effects obtained are shown pictorially and data are given statistically. The test equipment and the conditions of the tests are described. Strength of the beam was controlled and the photographs were made under standard conditions so that results would be comparable.
Results obtained show that depressing the beam of a depressible-beam head-lamp when an asphalt or concrete road surface is wet greatly increases the apparent intensity of the beam above the road, evidently due to reflection from the road surface, and that this intensity extends far above the horizontal height of the head-lamp, thereby defeating the object of depressing the beam.
With a non-symmetrical beam, produced by a two-filament lamp, disposed so that the beam is directed straight ahead from one filament but can be shifted to the right by switching to the other filament, very little light is projected into the region in which the eyes of an approaching driver would be located. This effect can be produced by using an auxiliary driving-light aimed to the right of the axis of the car.
When two facing cars both used the upper beam of a depressible-beam head-lamp, on a wet asphalt road, a target 3 ft. from one car and 3 ft. above the road was visible at the maximum distance of 67 ft. With both cars using the non-symmetrical beam aimed to the right, the target was visible at 159 ft., and when placed near the right-hand curb was visible at 313 ft.
Conclusions deduced from the tests are that (a) wet road-surfaces change the light-distribution materially, resulting in greater glare; (b) more light-intensity is required to reveal an object on a wet road, due to interference with vision and change in light-distribution; (c) increasing the light-intensity in a symmetrical system does not improve visibility; (d) a non-symmetrical system in which the high-intensity portion of the beam is directed to the right of the car axis reduces glare and allows better vision; (e) use of two 21-cp. lamps does not give sufficient light-flux for all requirements; (f) use of a wide-spread low-intensity beam near the car, in combination with a relatively narrow controllable high-intensity driving-beam has advantages for both dry and wet roads; (g) no reason is known why a fixed-focus design should not be used with a somewhat longer focal-length than at present, with suitable reflectors or lenses for the low-intensity beam; and (h) the use of auxiliary driving-lights with the beam directed to the right should be encouraged.

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