,694 



NATURE 



[May 27, 192 



Motor Headlights. 



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 191 1 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 

 Umit " for a field of a given brightness, and it was 

 generally recognised that glare was largely a matter 

 of contrast. The Hmitation 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. 



In the ensuing discussion Commdr. T. Y. Baker 

 (Admiralty Research Dept.) described a simple 

 apparatus for measuring brightness, the object to be 

 tested being viewed through a tube, and its brightness 

 compared with that of a lamp emitting light down a 

 side-tube. 



Mr. Leon Gaster (Hon. Secretary of the Illuminating 

 Engineering Society) remarked that the subject had 

 been much discussed in various countries, not only 

 by the Illuminating Engineering Society in the United 

 States (as Mr. Walsh had mentioned) but in recent 

 proceedings of the German Illuminating Engineering 

 Society. At the first technical session of the Inter- 

 national Illumination Commission in Paris last 

 year it was resolved to appoint an international 

 technical committee to deal with the problem. The 

 Illuminating Engineering Society in this country 

 had formed a joint committee on motor-headlights, 

 and he gladly took the opportunity of inviting the 

 Optical Society to nominate a representative. Mr. 

 Gaster also pointed out that regulations prescribing 

 a certain illumination at a specified distance ahead 



NO. 2743, VOL. 109] 



of the car depended essentially on the speed of 

 driving. The distance was presumably based on the 

 length of road in which a car driving at 20 miles an 

 hour could pull up. With the removal of the speed 

 limit such requirements might need revision. 



The Chairman (Mr. Whipple), having thanked Mr. 

 Gaster for the invitation conveyed to the Optical 

 Society to appoint a representative on the Illumin- 

 ating Engineering Society joint committee, called 

 upon Mr. J. S. Dow to continue the discussion. 

 Mr. Dow pointed out that glare was mainly a 

 matter of contrast and should therefore be con- 

 sidered in relation to street lighting. He believed 

 that in certain cases a device had been employed to 

 direct light on the front of a car, enabling its outline 

 to be seen more clearly and diminishing the contrast 

 between the brightness of the headlight and its 

 surroundings. It had also been suggested that glare 

 depended to some extent on colour, and that a 

 slight yellow tint, though involving some loss of 

 light, was preferable in this respect, as well as 

 giving somewhat better definition of distant illumin- 

 ated objects. 



Sir Wm. Barrett, in the absence of Sir Howard 

 Grubb, described and demonstrated the Grubb head- 

 light, of which he had had favourable personal experi- 

 ence. The primary principle now usually aimed at 

 in modern headlights, is that the main beam should 

 be confined below a height of 3 feet above the road- 

 way, so that the beam did not shine in the eyes of 

 persons approaching, a milder and more diffused light 

 being distributed outside this limit. The original 

 headhght utilised two " D "-shaped lenses separated 

 by a strip of grooved glass to effect this division, but 

 more recently the design has been improved, two 

 horizontal strips being inserted, and the arrangement 

 of the lenses has also been modified. Sir Wm. 

 Barrett presented a table summarising the require- 

 ments of the code for headlights adopted in Massa- 

 chusetts, including a maximum beam-candlepower of 

 5000, and showed that these had been complied with 

 in this form of headlight. 



In the ensuing discussion a great variety of head- 

 lights was demonstrated. Generally speaking, the 

 aim was to provide a powerful beam, but to restrict 

 the candlepower in directions above a certain hori- 

 zontal plane. It was evident, however, that the dis- 

 tribution of light effected by the various devices 

 differed considerably. One ingenious device com- 

 prised the use of a supplementary concave reflector 

 covering the upper half of the headhght whereby 

 light-rays in the upper hemisphere were directed back 

 on to the main reflector and added to the main beam. 

 Various devices to assist the provision of suitable 

 side-illumination, in addition to the main beam, were 

 also described. Thus in the Zeiss headlight there is 

 a special annular reflecting surface which furnishes 

 diffused illumination on either side of the car. An- 

 other feature is the use of a Bowden wire arrangement 

 to enable the motorist to cut out the dazzling effect 

 of the headlight while retaining sufficient light for a 

 shortened track. This effect can be produced at any . 

 time, and is recommended for use in the well-lighted 

 streets of large towns, where there -is considerable 

 traffic. 



One other device that may be mentioned is the 

 Kent glare screen, which consists of a small plate of 

 coloured glass, which can be attached to the wind- 

 screen so that the driver, by slightly moving his head, 

 can bring an opposing headlight within the field 

 covered by this screen and thus reduce its dazzling 

 effect. 



