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Ideal Lighting System
By Capt. J. B. Walton, M.I.Mech.E., M.II.R.T.E.
Chief Engineer, S.P.D. Ltd.
THE lighting system of a road vehicle should provide adequate illumination where required, be simple to operate, and should not be a cause of annoyance to other drivers. Adequate means are at hand for producing the amount of light required, but its application to the point where it will do most good is not so developed, and means for preventing inconvenience to other road users are still less adequate. These requirements are enumerated in their order of importance to the individual, but the third item is the one which, in its effects, concerns the general public the most.
The method of generating light is customarily by a current of electricity through a filament bulb, and the measurement of intensity is watts. However, this unit refers only to the capacity of the lamp to absorb current and not to the output of light. The rating should therefore be in lumens, for there is not necessarily a relationship between the wattage of a source of light and its output.
To accept a standard dissipator, we could take the 12-volt 36-watt bulb, which gives a light output of about 650 lumens. This amount, if properly applied, is adequate for any illumination from the head lamps of a vehicle. Usually, the beam from a lamp is in the shape of a cone surrounded by scattered rays.
"All Out" for Range In general, the aim of makers is to provide a concentrated beam with the largest possible range, regardless of whether this is actually what is required and of the effect on other people. There are exceptions, however, in which manufacturers aim at producing an even source of light as near to daylight as possible, which would give comfort to the user and constitute a lessened source of irritation and confusion to others.
The mechanics of lighting systems are simple, and, including a means for dipping, only two head-lamp units are necessary, apart from side lights. Dipping systems are slightly complicated, there being three different methods in use on the road. In the first, one lamp is extinguished and the other dips and swivels. In the second, both lamps dip, but do not swivel; and in the third, which is not so general, both lights are cut out and a separate pass light comes on. There are no standards of lamp size, length of foci, or position on the vehicle, neither is there any standard of beam or illumination value of the light source.
It will, therefore, be gathered that no determined effort has been made to standardize vehicle lighting, and thus accustom the public to one degree of illumination. There is no logical argument why a high-built lorry should have lamps higher than a car, or why one reflector should deliver a narrow intense beam, whilst the be,am of .another should be wide and diffused. With a standardized form of illumination there would
be a chance for the public to become accustomed to one set of conditions and so, in time, obtain an automatic reaction to it, which would not cause more than a minimum of discomfort.
Until a new form of lighting technique is evolved, there should be one size of standard pre-set, fixed-focus head lamp, with a light output of not more than 650 lumens. Two of these could be fitted to a vehicle, irrespective of its size and type, and a system incorporated to enable the off-side lamp to be put out and the other to be dipped and swivelled to the near side. The reflectors should give a beam with a flat top, with an illumination value of 1 ft.-candle at 50 ft., and not less than ft.-candle at a 10-ft. spread of the centre line.
It is possible to design a reflector giving an evenly distributed light with an effective range of 180 ft. to 200 ft. With two such lamps, an adequate light is provided. The height of a head lamp should not exceed 2 ft. 6 ins, from the ground and a central beam should be eliminated from the reflection system.
Two Directions for Progress The future holds promise in two directions. One is to polarize the emitted light. This has limitations, equipment is costly, the amount of emitted light required is greatly increased to give adequate illumination, and polarizing screens are subject to relatively quick deterioration. Whilst being suitable for motor vehicles, of course, it would be out of the question for adoption by cyclists or operators of horse-drawn vehicles. These drawbacks will. I fed, restrict the general popularity of this system, and to accept it as a solution is, in my view, to take the line of least resistance, and not tackle the basic problem.
Adequate street lighting could be provided so as to flood roads with light in city and urban areas. Such districts are to-day badly served with lighting and much has to be done to improve night-driving conditions.
To drive within a pool of light provided by the vehicle itself and with the light source invisible to the onlooker is another ideal capable of research. To a certain extent it is practical to apply this idea to-day, but the average driver must cultivate an outlook different from that which he has at present. He must rid himself of the searchlight mentality.
Dazzle can be greatly ameliorated by devising new positions for driving lamps. It might be possible to have the off-side lamp " off " whilst the near-side lamp remained "on." but invisible to oncoming traffic.
Fluorescent lighting, with appropriate reflectors, is another practical line of development. It would emit light without glare and the light source could be obscured. It should be the aim of all lamp designers to eliminate the head light beam and to hide the effective source of light from other road users,