From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Authoritative article discussing the power of motor lamps, anti-dazzle devices, and suggested regulations.
| Identifier | ExFiles\Box 61a\1\ scan0039 | |
| Date | 4th January 1919 | |
| Reprinted from “The Light Car and Cyclecar,” 4th January, 1919. THE POWER OF MOTOR LAMPS. An Authoritative Article by Mr. D.{John DeLooze - Company Secretary} Elyard Brown, the Technical Expert of Messrs. C. A.{Mr Adams} Vandervell and Co., Ltd. IN regard to the discussion on the power of motor lamps, the whole argument turns upon the accuracy or otherwise of the statement made by the contributor in THE LIGHT CAR and CYCLECAR who said that “before the war I drove behind a set of lamps which it is no exaggeration to say illuminated the road ahead for at least a mile.” Personally, I am inclined to think that his recollection is at fault after this lapse of time, or else that he was deceived as to what a mile of road looks like at night. As a matter of fact, 99 people out of a 100 sitting in a car at night, and asked to give the distance of an object only a quarter of a mile off would guess nearer a mile than a quarter. To turn to actual tests: with a pair of 32 c.p. bulbs in 9 in. reflectors, a newspaper can be read fairly easily at 250 yards distance; and by photometric test the illumination at that distance, quoted by Lt. Hartley, 0.1 of a candle foot would be about right. Looking from the car, however, although the newspaper could be plainly seen, the men holding it could not, as they were dressed in dark clothing; but a line of white posts much further down the road was clearly visible. The minimum amount of illumination necessary to discern dark objects is about .25 of a candle foot under average conditions. To be able to do this at 250 yds. would (unless the beams were so concentrated as to be useless as a driving light) entail lamps of a power that will probably never be allowed again in their pre-war form. Anti-dazzle Devices. This brings us to the question of anti-dazzle devices. Obviously, an upward light is not only wasted and a nuisance to other road users, but is an actual hindrance to the vision of the driver. If it were possible to restrict the beam to a certain distance above the ground, there seems no reason why the power of the light itself should be restricted at all; in fact, the more powerful the better, and safer. The suggestion by your contributor of blacking the lower half of the reflector is somewhat crude and not very effective. Most anti-dazzle devices so far known cut off so much of the total light that one might as well be content with a less powerful light with unrestricted beam. The war has taught most of us how little light it is possible (I do not mean safe or comfortable!) to drive with. When the late and unlamented lighting order first came out, I remember going to the trouble of making a lamp exactly to comply with regulations, but at the same time by careful selection of bulb and accurate focussing to get all the light possible. I found I could drive fairly comfortably under 20 m.p.h. but was stopped so many times by zealous police and specials for alleged excessive light, that I decided I could make a better average with less light! The whole problem bristles with difficulties, as the value of light varies so enormously under varying conditions; such factors as the darkness of the night, (the darker the night, the more effective the light), the colour of the road, and the objects on it, the presence or absence of mist all play an important part, whilst perhaps the most important item in the whole optical system—the eye of the driver—is usually left out of the calculation. The human eye being fitted with an iris diaphragm automatically adjusts itself to admit more or less light according to the conditions. Bearing this in mind, it is obvious that a bright light near at hand, by causing the aperture of the eye to contract, will automatically render distant objects less visible. Thus, when meeting another vehicle with bright lights, the driver is, for the moment, unable to see anything behind it unless his own lights are powerful enough to drown the opposing light. No Act of Parliament can alter these conditions, and the thing resolves itself into a matter of degree. When cycling on a dark night with a feeble oil lamp, even the candle lamps on a brougham are trying to meet and impossible to see past. The cyclist has the worst of it every time, because proceeding in comparative darkness, his eyes are working at practically their maximum aperture. Even suppose the car headlights are switched off, the sidelights are just as trying at the moment of passing; in fact, personally I prefer the headlights, as they give one’s eyes a longer period to adjust themselves. Against this there is the problem of the nervous cyclist, who has a longer period in which to get “rattled.” There is also the physiological aspect of “light shock” which has been very little investigated; I refer to the unreasoning feeling of irritation which occurs when a bright light is suddenly flashed in one’s face. I am certain this is at the root of more complaints against powerful headlights than any actual inconvenience or danger, and I am convinced that no reduction in the power of light that is at all likely to be enforced will eliminate it. To reduce the lights on all vehicles to the level of the cyclist’s oil lamp would reduce speed at night to 8 m.p.h., and be dangerous at that. A 12, 20, 32 or 48 c.p. arbitrary limit for headlight bulbs would not minimize the inconvenience caused to other road users in the slightest degree. Suggested Regulations. At the same time the road hog will always be with us, and with no restriction at all he will be a “light hog” as well. My own opinion is that the amount of light necessary for safe driving is given by a pair of 32 c.p. bulbs in 9 in. or 10 in. reflectors. This is for the average reasonable individual at the average speed at which cars are driven at night. Certain individuals may require more light for reasons of eyesight. To reduce the power to 12 (which would be absurd) would not reduce the inconvenience to other road users appreciably, but it would most materially increase the danger to all concerned. The solution, then, must be sought in some other direction. In the first place, now that the law compels sidelights to be in use to show the width of the vehicle, the side lamp fitted to the windscreen is a nuisance, at all events on light coloured cars with long bonnet and wide mudguards, as the effect of the reflected light in the driver’s eyes is to reduce the apparent illumination of the road ahead. To my mind, then, the “regulation” sidelamps should be on the front wings. They should be of streamline form with bull’s-eye lenses, so that those meeting the light will not be for long in the direct beam, and not at all when actually passing. A pair of ordinary wide-angle sidelamps in the conventional position for town use or for seeing the sides of the road on foggy nights would be a useful addition. This entails carrying seven lamps in all, so it would only be adopted on comparatively expensive cars. For the light car or cyclecar the “combined” type | ||