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The problem of street lighting is fundamentally different from the problem of interior lighting, not only owing to the very much lower intensity, even of present day up-to-date street lighting installations, as compared with the illumination of rooms, but also in view of the great importance of obtaining distance visibility in the former. An experiment in daylight will illustrate the latter point: a person standing or driving on a street observing cars in front of him, or pedestrians crossing the road, say, 100 yards ahead, will not notice the colour of these objects, but will be aware of them only as dark silhouettes against the light background of even the blackest road surface imaginable.

This phenomenon is of the utmost importance in street lighting. The criterion of a good installation should not be the illumination measued on a horizontal test plate as described by the "B.S.I. Specification 307-1931 for Street Lighting," but rather the achievement of clear visibility by making the road and any object on it appear in distinct contrast.

Theoretically, there are two possible ways of attaining this end, namely, by lighting the object and leaving the road dark, or by lighting the road and showing the objects as dark silhouettes. It is obvious that the road is more capable of being satisfactorily lighted than the objects which vary in size, shape, colour and are continually moving. Satisfactory lighting does not necessarily result from the provision of a high order of horizontal illumination, but rather from the production of a uniform road surface brightness, so that any object at any point on the road may seen immediately as a dark silhouette. It follows that the light reflected from the road surface is the most imporant factor contributing to road brightness, and it has been found that light emitted at high angles to the vertical (at about 80° or more) produces specular reflection on most road surfaces and contributes very largely to the achievement of road brightness.

There are three important factors contributing to the quality of a street lighting installation :-

(1) The efficiency of the light source which will determine the degree of brightness of the road. The OSIRA high pressure mercury vapour electric discharge lamps, with a lumen output three times that of the gasfilled filament lamps, have proved the most economical and satisfactory light source wherever installed.

(2) A scientifically designed lantern. If a bare lamp, and especially a bare discharge lamp, were used, a very large proportion of the light would be completely wasted. The G.E.C. lanterns are fitted with prismatic glass refractors constructed to direct a maximum of light at the correct angles on to the road.

It has been shown above that light emitted at high angles is the most important for attaining good visibility, and the G.E.C. lanterns are constructed to direct an adequate proportion of light at these angles. At the same time, great care has been taken to avoid the occurrence of glare.

It is very difficult to find any satisfactory definition of glare but the problem can be best be understood if we compare the effect of a car headlight in a dark country road with the same headlight in broad daylight. In the first case there is such an extreme contrast of illumination between the very powerful light source and the surrounding darkness that the eye, adapting itself to the strongest stimulation, becomes insensitive to the weaker stimuli and incapable of perceiving constrasts of low intenstiy. The same headlight when viewed under daylight conditions would cause not the slightest strain to the eye and is hardly visible at all.

Glare is a function of both the apparent brightness of the lantern and the brightness of the street surface. The apparent brightness of the G.E.C. lanterns is considerably reduced by the use of diffusing glasses having flutes which result in the whole lantern appearing evenly flashed. At the same time, with this form of distribution a high order of brightness on the road surface and abutments is obtained, thereby still further reducing the contrast between the light source and the surroundings, with the result that glare is entirely eliminated, and maximum visibility over long distances produced. It will be readily appreciated that only installations producing high and even road brightness can satisfactorily counteract the danger of dazzle from oncoming cars.

(3) The arrangement of lanterns in the installation. When a light source is suspending from a column at a certain height the road surface will reflect the light, and the reflection will take the form of a streak of brightness of a particular shape. It is the task of a lighting engineer to plan the installation in such a way that these streaks from the different light sources overlap and produce a lane of even road brightness which is essential for "accident-proof lighting."

Specially attention has to paid to the problem of bends; it is possible and advisable in the interests of safety to obtain the same degree of visibility on bends as on straight sections of road, and this is very clearly illustrated in photographs of bends in the recently completed Fulham and Birmingham installations. Special problems are encountered when the lighting of road junctions, squares, roundabouts, etc., is considered, and the G.E.C. will be very pleased to assist in designing the most appropriate lighting layout.

It is obvious that the lighting intensity of a main traffic artery has to be higher than that of a purely residential road, but it must be kept in mind that great danger of accident exists if a driver turns from a well-lighted main road into the darkness of a side street. This is because he will be unable to see anything until his eyes become adapted to the very low intensity of lighting. It is, therefore, most imporant to plan a suitable grading down of the lighting intensity.

A revolutionary development in side street lighting - similar to that in main road lighting with the introduction of the mercury discharge lamp a few years ago - is to be expected with the adoption of the recently introduced 80 and 125-watt mercury vapour discharge lamps.

The services of the G.E.C. lighting engineers with their unique experience in the design of complete schemes are at the disposal of all those interested in the improvement of the existing street lighting, and enquires are invited.

The following are a few outstanding installation which have been planned in this comprehensive and scientific manner :-

(1) The main road lighting of Fulham, in which the lanterns are supported on an entirely new design of concrete column, with overhanging bracket.

(2)The main arteries of Birmingham, where specially designed lanterns have been employed.

(3) A big proportion of the main and some of the side street lighting of Wandsworth.

(4) The whole public lighting of the Borough of Camberwell, where the area was divided into six different grades of lighting to conform to the various B.S.I. classiciations.

(5)The main through roads of the Romford and Hornchurch districts, and many miles of secondary roads and side roads in housing estates.

(6)Newport (Mon), where B.S.I. Class B lighting has been installed in the main thoroughfare.

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