Highway Lighting in the Limelight
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LAST week we published the programme of the Conference of the Association of Public Lighting Engineers, which took place at Folkestone this week.
One of the most interesting features was the excellent exhibition of methods of street lighting, etc., covering a square mile of the town; members were thus able to observe an effective display of the subject at their meeting.
In his presidential address, Mr. C. I. Winstone referred to the amazing achievements in street lighting during the past few years. He believes that there are greater ones in store. When gas was introduced, its first use was to give light; the same applied to electricity; thus lighting engineers are members of a time-honoured profession.
Thirty-eight years ago, 1 cubic ft. of gas, as a flat flame, gave 25 lumens, with electricity a watt in a carbon filament lamp produced 3.5 lumens. Now the latest low-pressure gas lamp will give 300 lumens, and the high-pressure 364, whilst a tungsten-filament lamp gives 16.8, and the electric-discharge lamp 55.
Some methods of light distribution for public lighting are more spectacular than useful. On certain main roads one may 'see a bright halo over a town and, on arriving, notice that the installation is of lamps from which a percentage of light goes above the horizontal. This seems wasteful. Lamps should have a cut-off to prevent this loss, and reflect more light on to the road surface. The cost for the same illumination would thus be materially reduced.
The Two Schools of Thought.
There are two schools of thought—the complete cut-off and the seeing of a bright source. There is a happy medium to pre.vent the waste of light occurring in some installations, and the dull uninteresting appearance common to others where the source is obscured until one is almost under the lamp. There are eight classes in the British Standards Institution Specification for Street Lighting, and all experts agree that 0, E and F are those suitable for main-road lighting. To people with normal eyesight a road lighted to D classification gives ample illumination for driving without headlamps.
Every driver should be compelled to pass what may be described as a " night-seeing " test. The author is afraid that there are many accidents where the driver's eyesight, and not the light, is the cause.
Illumination on the ground on a sunny day may be 10,000 ft. candles, but on the average lighted main road it may be only .2 ft. candle on the test points, with an average illumination of .5. Yet the author states that he can see 300 yds. ahead through a glass windscreen with a degree of certainty that would prevent him running a person down, whilst a road lighted to a generous F classification with .07 ft. candle on test points and good visibility always seems well lighted.
What can we consider safe illumination for various speeds? Many experiments have been carried out, and the author believes that a generous F classification of .07 ft. candle is safe up to 40 m.p.h.; an E classification of ,1 ft. candle up to 50 m.p.h., and a D classification of .2 ft. candle up to u20 60 m.p.h., with good visibility each case.
Certain roads brilliantly lighted from dusk to dawn do not carry 20 vehicles an hour after 1 a.m. Money would be saved by reducing the lighting after this time.
Lighting should be economic. Would it not be possible to light effectively many miles of main road, at present unsatisfactory in this respect, by cutting down to a sufficiency the illumination of roads at present extravagantly lighted? It is often impossible for a poor parish to light a main road where 90 per cent. of traffic may be foreign. Only a central authority could bring this about. May we ask the Minister of Transport, remarks the author, to keep this matter before Parliament and see what can be done towards an improvement on the lines suggested? There has been much controversy in respect of road brightness and test-point illumination. The B.S.I. Specification requires that the minimum illumination must be on the test point, which is equidistant between two lamps, but it is possible to focus the light so that it will give the illumination asked for in a tender, to the detriment of the general brightness of the road. The eye is the final judge of lighting. There is no scientific instrument to register and convey the feeling that the illumination is satisfactory so adequate as the human eye. You can place graphs, iso-foot-candle diagrams and photographs before a lighting committee, but before deciding the members go out and look at the illumination.
The author then refers to the 50 per cent, depreciation allowed in the standard specification, and asks whether it be right that such a depreciation should be permitted. He knows of installations, both gas and electric, where the illumination is between 40 per cent. and 50 per cent, less than that quoted for in the tender, and he is tempted to ask whether this be playing the game. Such a large depreciation tends to allow maintenance to be neglected. In the author's opinion not more than 25 per cent., or, at the most, 30 per cent, should be allowed.
Use of Modern Electric Lamps.
SIX other papers were read and discussed, and we now ideal with the most interesting features in these. The first, by Mr. G. H. Wilson, B.Sc., was entitled "The Application of Modern Electric Lamps to Street Lighting." This mainly records the results of new research work on the design of installations. The author points out that if artificial lighting in any application can be so arranged that a scene and the objects in it appear in their natural form, then, nearly always, surer visibility is obtained. A frequent criticism of poor lighting is that things appear unnatural.
As regards spacing of light units, it has been found by experience that if spaces do not exceed 300 ft. the highway will have a natural appearance. With any but very short spacings, most satisfactory results are obtained with a diagonal arrangement, the units on one side being mounted midway between those on the other. The maximum recommended spacing per row then becomes 150 ft.
For general planning the width between the rows of sources should not exceed 30 ft., when kerb mounting should be satisfactory. On wider roads an overhang is desirable, so that the width between rows does not exceed 30 ft. For thoroughfares exceeding 40 ft. to 45 ft., a central third row of sources is desirable.
It might be better for wet roads to adopt a complete change in technique, e.g., the use of lighting sources extending across the roadway.
The mounting height should not be less than 25 ft., as the effect of reducing height is to increase the tendency towards glare.
New units have been designed by the General Electric Co., Ltd., foruse with 100-watt and 150-watt -sodium lamps. Amongst new lamps is the horizontal H.P.M.V. At first, this lamp was burned vertically, the cord-like arc lying closely along the axis, but if the lamp were turned over the arc tended to bow upward and lie mainly near the upper wall of the inner bulb. An electro-magnetic deflector is now used to force the arc towards the lamp axis. Further research enabled glass with a higher softening point to be developed, and with a slight reduction in the mercury vapour pressure special lamps can be operated hori zontally without a deflector. Operating a lamp in the horizontal position results in increased loss of energy from the discharge to the wall of the inner bulb, and, consea quently, a small reduction in efficiency. This facility for using lamps horizontally without a deflector has led to the development of a number of new street-lighting units.
Interesting Experiments in Street Lighting.
I N their paper, " Some Further Experiments in Street Lighting," Dr. S. English, D.Sc., F.I.C., and Mr. E. Stroud suggest that the real reason for lighting is to enable road users of every class to see easily whatever they need to see, and more particularly obstacles or conditions that may lead to danger. It is necessary for lighting engineers to combine the factors to produce the best visibility, combined with installation, running and maintenance costs. It is necessary to have some means for measuring visibility. Various instruments have been suggested, but, in the authors' opinion, none of these actually gives the required measurement, and they believe that visibility must be determined by the unaided eye in the same way as obstructions are seen on lighted roadways. Experiments were carried out with a model road on a one-fifth scale. The visibility was measured by utilizing a brass disc 5 ins, in diameter with a projection 1 in. square at a point on its periphery. This was spun, the projection coming to rest in one of the 12 positions represented by hours on a clock. • The disc was placed in turn at each of 15 spots. If the observer was able to see the projection after spinning, he noted the clock number, which was checked by the operator. It was found that visibility with the screened sources of light continued to rise as the angle of the main beam was increased. In each case, the light sources were screened from view, but without the screen the percentage visibility rose a little on 70-degree to 75-degree beams, but fell on passing to 80 degrees, and still farther at 85 degrees.
For main beams at 70-75 degrees, visibility was unaffected by the shield, but with main beams at 80 degrees there was a reduction of 9 per cent, due to glare, and at 85 degrees a reduction of 21 per cent.; thus the effect of the high-angle beams in .building up strong contrasts was more than coun
terbalanced by glare. The best compromise was obtained when main beams were directed along at 75 degrees to the vertical.
It would be worth while to try a beam at one to two degrees below the horizontal, and a zone of reduced intensity from 88 degrees to about 77 degrees, followed by a beam maximum at 75 degrees. This might combine good general illumination, high road-surface brightness and comparative freedom from glare.
In lighting unidirectional motor roads it appears that as much light as possible should be directed towards the oncoming driver, and as little as possible in the direction of travel. This scheme is just the opposite to that recommended on the Continent. The greatest care, however, must he exercised to avoid glare. High-intensity Gas Lamps.
DEALING with high-intensity gas lamps, Mr. G. Keith said that modern incandescence gas lamps of the inverted-mantle type operate on two distinct principles, the light from a given amount of heat largely depending upon which of these is used.
In one case, the air-gas mixture contains only a proportion of the oxygen required, a secondary supply being necessary to complete the combustion on the outer surface of the mantle. If the oxygen be mixed with the gas the heat produced is liberated wholly within the mantle, thus a secondary supply of air is unnecessary and may be detrimental owing to its cooling effect. The author's paper deals, therefore, only with the second system.
It has been found that when the whole products of combustion are passed through the meshes, particularly at a high velocity, the light intensity per unit area of mantle surface is greater, and that produced from a given volume of gas increased.
The considerations demand energy in addition to that available in low-pressure gas mains. The method devised is known as the High-pressure Gas System, a compressor being used to increase the pressure to about 80 ins. eater column above that of the mains.
The remainder of this paper deals with descriptions of various forms of lamp to work on this system.
High-pressure Gas System in South London.
THEpaper, " Street Lighting by Gas with Special Reference to the High-pressure System," by Mr. D. Chandler, is practically a historical survey of the subject, with special reference to the equipment designed by Mr. G. Keith. Two years ago Mr. Chandler, with a colleague, brought to the notice of public-lighting engineers the development of a high-pressure lamp which possessed new and distinctive features. With the cylindrical mantle, some of the light must be redirected, but these experts took the view that much of the redistribution could be effected by altering the shape of the mantle, so that more of the output could be radiated in the direction in which the greatest intensity was required. This expedient was adopted in the Supervia lamp, and in South London there are in use 1,800 of these out of the 2,700 high-pressure lamps lighting nearly 73 miles of roads.
The author -mentions one point about high-pressure gas lamps which is a distinct advantage and, in certain circumstances, of great importance—al the lamps may be turned on or off rapidly by the simple process of starting or stopping the gas compressor. The high-pressure system is, of course, independent, and any interruption in it does not interfere with the supply of low-pressure gas The high-pressure gas lamp is a self-contained unit which needs only to be properly connected to the high-pressure mains. The fittings can be so arranged that the lamp can be brought dawn to street level for attention.
In South London the system gives a gas pressure of 81 ins, water column, or approximately 3 lb. per sq, in: at the lamps. There is a large compressor station at Kennington, which, with another, will eventually supersede the ' numerous small stations at present in use. All the compressors are driven by gas engines giving a total of 237 h.p.
The medium chosen for the presentation of the main body of the paper. " Planned Street Lighting," by Messrs. T. Cotten. R. Maxted and G. S. C. Lucas, was the cinematograph, two films being shown.
The first emphasized the difference between indoor and road lighting in respect of the intensity of light available. For indoor lighting intensities of 10 ft. to 20 ft. candles are used; in road lighting, there is only one-tenth to one-hundredth of this amount. Where the actual light is small, it is better to use it to illuminate the backgrounds, against which objects can be seen in silhouette.
It showed also the difference between the plan view of a lighted road and the appearance to an observer on the road surface, as well as how the bright backgrounds are produced, and the importance of correct placing of the lanterns.
The second film indicated how the characteristics of the lanterns are linked with road-surface conditions and the layout of the installations.