Necessary Values of Brightness Contrast in Artificially Lighted Streets

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Abstract
Experiments conducted in a street lighting installation in which the road surface was uniform in brightness have given a curve relating the brightness of the background to the difference in brightness between an object and the background which it is necessary to have for safe driving at 30 m.p.h. This standard curve applies only to the case of installations in which there is no glare from the street lamps. Although the background in most street lighting installations is not uniform in brightness, the results of other experiments have been used to show that, in practice, no modification of the standard curve is necessary on this account provided that the road is dry. If an installation is not free from glare, the necessary brightness difference between an object and its background will be greater than that given by the standard curve. The curve can, however, still be used if, in deriving this brightness difference, all brightnesses of the background are assumed to be increased by a certain amount. Methods of evaluating this correction for glare have been described. The results of this work can be applied to the problem of determining what proportion of practical objects will be revealed to the driver of a car by any region of the background in a street lighting installation.

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