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Electric Discharge Lamps for Road Lighting

Nature volume 135, page 262 (16 February 1935) | Download Citation



IN many of the long stretches of main roads between towns and villages the only practical way of lighting at present seems to be by means of the lamps on the vehicles. In a paper on “Electric Discharge Lamps and Road Lighting“read to the Institution of Automobile Engineers on December 11, Mr. H. Warren and Mr. L. J. Davies show how the length of the permanent illumination of roads can be extended, with acceptable economy, by means of the new discharge lamps, when care is taken to distribute the light scientifically by means of suitable lanterns. Controlling the reflective properties of the road surfaces has also to be taken into consideration. With mercury and sodium vapours we have two substances which, when excited in the correct way, produce a sufficient proportion of energy in the visible spectrum to give a two- or three-fold increase in efficiency over incandescent lamps. A fifty-fold efficiency is theoretically possible, but the practical utilisation of electrical discharges has just made a beginning. At present, electric discharge lamps for street lighting are of two main types, high-pressure mercury vapour and sodium vapour. The former type of lamp is most favoured in Great Britain, while on the Continent and in America the sodium vapour lamp is most used. Mercury discharge lamps have an excellent ‘luminous output’ during their lives, which are longer than those of other forms of lamp. The colour of the lamp, when viewed directly, is greenish-white. In the colours radiated, blue, green and yellow predominate, but red is practically lacking. By incorporating cadmium with the mercury a satisfactory red tinge can be introduced, but at a slightly lower efficiency. The colour correction of these lamps is receiving a great deal of attention in commercial research laboratories at present.

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