Citation

Merrill JA, Carter JH. Aust. J. Optom. 1964; 47(1): 3-8.

Copyright

(Copyright © 1964, Australian Optometrical Association, Publisher John Wiley and Sons)

DOI

10.1111/j.1444-0938.1964.tb04143.x

PMID

unavailable

Abstract

Many persons believe that they experience more visual difficulty while driving at dusk than when driving at night with headlights, though the luminance of objects on the road is greater at dusk. Further, the use of headlights at dusk is of little utility except that they tend to make ones' own automobile more readily seen by drivers of approaching cars. Preliminary evidence from this study supports the presumption that twilight visibility is poorer than would be expected on the basis of the luminance of objects on the highway, It is probably not as poor, however, as a purely subjective appraisal might suggest. Factors which appear to contribute to a loss in visibility at twilight are the following:
* 1A general loss in visual acuity occurs as a normal accompaniment of reduetion in the ambient illumination. * 2The state of adaptation of the eyes lags behind the rapid change in illumination which occurs after sunset. * 3The relatively bright skv in the field of view inhibits dark adaptation and serves as a source of disability glare. It also retards pupil dilation, thereby further lowcring retinal illuminance for terrestrial objeets. * 4The change in light distribution as the sun sets may reduee the visability of low contrast objeets. * 5Loss of accommodative control and low ametropia in the presence of a wide pupil are potentially important factors which should be carefully evaluated. * 6The reduction in obscrvable tree-detail, which occurs at twilight due to the disappearance of strong shadows, may cause the motorist to over-estimate the degree of his visual disability.

Language: en