Motor Headlights

Abstract

A DISCUSSION took place on the above subject at a meeting of the Optical Society on May 11. Mr. J. W. T. Walsh, of the National Physical Labora tory, in his opening paper, recalled that the design of headlights had been much discussed recently, for example, at meetings of the Illuminating Engineering Society from 1911 onwards. The problem resolved itself into a compromise between the needs of the motorist, who required a sufficiently powerful beam to distinguish objects in time to pull up or slacken speed, and the desire of the pedestrian or driver of approaching vehicles not to be dazzled by glare. In discussing the nature of glare, essentially a physio logical problem, Mr. Walsh showed diagrams relating contrast sensibility of the eye and brightness, and the effect of obliquity of the bright source in the field of view. Attempts had been made to fix a “glare limit” for a field of a given brightness, and it was generally recognised that glare was largely a matter of contrast. The limitation of the powerful driving beam below a certain plane, so as to obviate intense light striking direct into the eyes of approaching persons, had been advocated and embodied in various codes, but it is recognised that, in addition to the main beam, moderate diffused general illumination is desirable. Requirements for headlights had been somewhat fully dealt with in American regulations. For instance, it had been prescribed that the illumina tion measured 100 yards away should not fall below a certain value, and in the latest specifications drafted by the American Illuminating Engineering Society minimum values for the main beam-candlepower, and maximum candlepower values at other angles (with the view of limiting glare), had been stated. Some polar curves, showing the distribution of light from typical modern headlights, were shown, a maximum candlepower of 5000 being attained in some cases. Another device for testing the power of the beam, intended to be applied on the road and where photo metric measurements were impracticable, was the Royal Automobile Club standard disc, which com prised patterns of white lines on a black background, the requirement being that the patterns should be distinguishable by the available illumination at a specified distance from the car. The disc was referred to in the latest report of the Ministry of Transport Committee on Lights on Vehicles. It was generally agreed that legislation was a difficult matter but much could be done to improve conditions by making the fundamental principles of design well known.