TESTING HEADLIGHTS.
Page 15
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A Description of an Interesting Method of Testing Headlights for Range and Dazzle, Adopted as Standard by the R.A.C.
IN OUR ISSUE for December 21st, we referred briefly to, an interesting method of testing headlights, particularly as regards their dazzling effect, originated and perfected by Mr, Hugh H. Gregory, consulting engineer to the
and technical adviser to the Metropolitan Commissioner of Police.
Pressure on spat* has prevented an earlier detailed reference to the matter,
• This method has been adopted by the R.A.C. as standard ; the tests aim at providing only such information as is really useful, and are based entirely on results. To the average man, the fact that. a lamp ie classified as givinne so many candle-power conveys•but hale: and he would be still less wise if 'told that a particular lamp gave kr0 many units of dazzle. Therefore, the tests are based soil the distances from the headlamps at which a standard object can be seen from the driver'sseat.
.1f a. man in khaki were walking along the road at. nigkt, certain particulars about hint would show up more than Others, and the standard object, which . has been devised for the test is designed on these lines. In practice it consists of a disc painted a 'dead bla.a, hut bearing five irregularly-shaped figures formed by parallel bats of white paint; all-white figures would be, too glaring, and could be seen from too great a distant*. The coiour effect aimed at is grey, but it being almost impossible to standardize such a colour, the same effect is obtained by mechanical means.
In practice, the test disc is carried on a small trolley., and can be held at various heights on a lung vertical arm, which itself is capable ofibeing.fastened to various parts of the trolley so that the disc can face the observer in the car either when the trolley, is being moved towards hist along the centre of the beam from the lamp or across the -beam. .
It will readily be understood that to make the tests comparative, before they are commenced on any night the atmospheric and light conditions must be taken into account as well as any differences between the eyesight of the observers, but this, although it sounds complicated, is achieved in a. very simple manner. The disc is arranged so that it faces a; small shielded electric light of known candle-power. • The observer takes up his gland some distance away, and the trolley is wheeled towards him -until he can define the figures on the disc. The distance be
tween him and the disc is then measured, and this gives the factor on which the results of the tests for. that particular night are based. To take a hypothetical example : If-the factor is 100 on one night and 150 on the next, all the resulting figures of the test on the second night must be reduced by 50 per cent.' in order correctly to compare them with those obtained on the first night. To,obtain the range of the beans from a headlight, the trolley with the disc fixed 1 ft. from the ground is -wheeled down the centre of the beam until the observer in the car can just distinguish the figures on it. He then fleshes an electric hand lamp, the trolley is stopped, and the distance from the headlight measured. The disc is then raised to heights of 4& ft., 6 ft., and 8 ft. from the ground, the procedure being repeated in each case.
To obtain the width of a beam at full range, the distance at either side of the forwardly projected central axis of the car at which the disc is discernible -in detail whe,n its centre is i ft., 4 ft., 6 ft., and 8 ft. 'respectively from the .ground is ascertained, and the side illumination is obtained by measuring the ranges at which the disc is discernible at Ell ft. and. 8 ft, from the ground at (a) 15 ft. and (b) 20 ft. to the side of the forwardly projected central axis of the car.
One of the most important tests is that of the dazzling effect. To obtain this, the disci is placed at a height of q ft. from the ground, 6 ft. to the rear, and 6 ft. to the side of the nearer headlamp, and is illuminated by the small lamp already described as being employed for obtaining the factor of visibility. The observer advances towards the oar until he can. discern the disc, and the distance from him to the headlights gives the comparative dazzling effect; the smaller this distance, the greater this effect is.
'Ile conditions for entering hea,dlight■ for these tests are the same as for the R. .4.0. Certified Trials, and the lamps submitted nin.qt be fitted on a car in the position they are intended to occupy.