Streets Ahead

STREETS AHEAD

The use of fluorescent lighting had matured during the Second World War but, by the end of hostilities, it was still primarily thought of as an interior light source. Could it, however, work for exterior use and, especially, street lighting? The race to find out was on

By Simon Cornwell

The end of the Second World War saw a resumption of the optimism and strive that characterised the preceding decade of street lighting theory, technology and practice.

There was a groundswell of opinion that innovations would continue and accelerate; that new concepts in light sources, materials and manufacturing techniques would quickly be utilised; and that the future of the profession was – if you excuse the tired pun – bright.

One new lamp technology had already reached maturity during the war and looked poised to elbow into the industry. So, immediately after the war, several manufacturers started looking at the fluorescent tube and determining if it could be used for street lighting.

The fluorescent tube was developed just before the Second World War, but really came of age during the hostilities.

Its development was accelerated by the war effort, as new cheap and efficient lighting was an absolute necessity for the concept of ‘maximum production’, in other words running factories round the clock to keep up with their tight schedules.

SCALE AND FLEXIBILITY

By the end of the war, the fluorescent tube had matured as a light source: it was available in different sizes (from the 5ft 80W tube down to the 2ft 20W tube), which offered scale and flexibility. It was available in several colours, two of which were hues of white.

But it was still considered an interior lamp, and no one had really tested its full

capabilities. Could the fluorescent tube be used for exterior use and, in particular, the specialised field of street lighting?

Pre-war thinking had preferred small lamps: lanterns could be made compact and light, could be precision built to give close and accurate optical control, and this in turn would create good brightness patterns on the road surface.

But these point sources could also suffer from inherent discomfort glare (which could be mitigated if correct siting was adhered to) and, on wet surfaces, the reflections from the road surface could collapse to thin streaks, which could become almost blinding.

But what of a large-sized lamp, such as a fluorescent tube? This would be very difficult to control optically (without resorting to large reflectors and enormous lanterns), but would give more tolerance in siting and mounting, and the discomfort glare would be reduced thanks to the low, intrinsic brightness of the lamp.

With these elongated lamps placed across the road, the surface reflection would be considerably broadened and, under wet conditions, the streak would never become excessively narrow or excessively bright.

This was the thinking of L J Davies and W D Sinclair, two designers at British Thomson-Houston (BTH), who began experiments with fluorescent tubes for exterior use in late 1945.

Their most immediate concern was the cold weather behaviour of the standard tube; would it work in the extremes of the British climate?

The bright streaks created on the road surface for a point light source (left) and an elongated light source (right). This diagrammatically showed the advantages of the broader light source

Main framework of the ‘3x80’ lantern showing the 5ft 80W tubes in-situ

ON-THE-GROUND TRIALS

Fluorescent tubes were installed outside their Rugby factory and left operating uninterrupted for months without any ill effects. A further 16 80W tubes were studied in a cold store, left running continuously for 4,500 hours at temperatures of -10 to -5 degrees Celsius. They, again, showed no ill effects, apart from only giving 80% of their normal light output.

These same lamps were then operated for a further 3,000 hours, being turned on for periods of one to eight hours, with the starting and run-up conditions closely studied.

The results of the experiments were positive, leading BTH’s engineers to agree that no troubles would be experienced with the use of fluorescent tubes outside (unless the voltages and temperatures were abnormally low).

The engineers then turned their attention to lantern design, particularly for traffic routes, using the Ministry of Transport Report(1937) as their guide.

This called for lanterns to be mounted at spacings of between 120ft to 180ft (150ft

preferable), to be mounted at heights of 25ft, and to give a lumen output per 100ft linear spacing of between 3,000 to 8,000 lumens.

Peak intensities of existing high-pressure mercury, low-pressure sodium and tungsten lanterns were typically around 5,000 candles, which gave the uniform road brightness the researchers were aiming for.

But the best a single 80W 5ft fluorescent tube could achieve, using reflectors of practical dimensions, was around 1,500-1,800 candles.

BTH theorised that a seven-tube lantern would be required to fulfil the requirements of the specification. This lantern would house three tubes either side of the road to create the main beams with one tube situated beneath to ‘fill in the distribution’.

This seven-tube monster lantern would have consumed around 700W, been extremely heavy and large, and so would have a high capital cost, high running costs and be visually unacceptable. It was clearly a

non-starter for BTH. So, with traffic route lighting off the table, BTH turned its attention to the lighting of civic areas and shopping centres. The suggestion was that good colour and the absence of glare would be more important in these places and the capital value of the busy town centre permitted a ‘deluxe’ system.

To that end, a smaller lantern, mounted at a closer spacing of 80ft to 100ft, would be advantageous and useful in these areas. It was along these lines that BTH worked on its first design, a three-tube lantern it called the ‘3x80’.

Two tubes were used to fashion the main beams with the third tube positioned underneath. As the lanterns were mounted so close together, a controlled cut-off (semi-cut-off) distribution was created with ‘anti-glare’ shields mounted above the parabolic reflectors.

The lantern was self-contained and included all the chokes, starters and power correction capacitors for operation. This was a necessary decision, as the lanterns were to be mounted on double span-wires directly above the centre of the roadway. As there was little control of the wide main beams, then a central mounting was more advantageous.

Pulleys were also fitted, along with a simple tractive wire device, which allowed the lanterns to be pulled to the kerbside for servicing.

Night view of the Rugby trial installation. The street name was never given in the literature but it’s Rugby High Street

Day view of the Old Bond Street, London installation. This clearly shows the 3x80 lanterns positioned over the centre of the carriageway by span wires

Night view of the Old Bond Street installation showing how evenly and brightly the road was lit. It is clear that BTH’s engineers selected narrow streets with narrow pavements and tall buildings so as to give the best results for the trials

RUGBY INSTALLATION

The first trial installation of the 3x80 was along Rugby’s High Street in 1946. This proved to be highly successful: the wide beam spread of the 5ft tubes was seen as a distinct advantage. Kerbs, pavements and the faces of the adjoining buildings were well illuminated and this all gave good visibility.

An added benefit of the tubes was their white colour (‘warm white’ was used over ‘daylight’) which preserved the natural colours of the buildings.

BTH was also slightly sneaky: selecting a narrow road that was built up with high vertical façades on either side. This provided ample opportunity for all the light from the lanterns to be utilised.

With an eye on the Association of Public Lighting Engineers (APLE) next conference that September in London, BTH then arranged with local energy supplier, Central London Electricity Limited, a second trial installation along Old Bond Street in the capital.

The same lanterns and span-wire systems were installed, and the energy authority reported that the new system of lighting consumed about half the power of the pre-war installation.

‘REVOLUTIONISE OUR IDEAS’

As the Public Lighting journal (No #42, July-September 1946) mentioned ‘… [this] forms an experiment in street lighting of great originality which may revolutionise our ideas and methods and do much to reduce the risk of street accidents after dark.’

Like the high street in Rugby, the London location was a narrow street with high, vertical buildings providing a convenient façade, and showed off the new lighting to its best.

BTH was upbeat and enthusiastic about the trials.

It anticipated that the capital expenditure would be higher than alternative systems (namely high-pressure mercury or low-pressure sodium) but the costs were not necessarily prohibitive, and the light quality was a definite benefit for these high-prestige areas.

The firm stated that further tests, including designing and installing smaller residential or side-street lanterns, would be next, and that it would be energetically pursuing the use of fluorescent tubes in the future.

But BTH was not alone in its interest. Davies and Sinclair gave a paper at the London conference (‘ExperimentalApplications of Tubular Fluorescent Lamps to Street Lighting’) in which they detailed their research and showed off pictures of the Rugby and London installations.

The discussion that followed revealed that the General Electric Company (GEC) had also been developing its own fluorescent street lighting lantern.

An open invitation was made for anyone to travel to its research laboratories in Wembley, north London, to see it – because GEC had made the seven-tube monster and was intending to install it.

Simon Cornwell BSc (Hons) is an R&D development senior manager at Dassault Systems