Left:
A “quirky little number” in Leek, Staffordshire (2004) — an Eleco HW846.Left:
A “quirky little number” in Leek, Staffordshire (2004) — an Eleco HW846.
So what are the benefits of low-pressure sodium lighting? Below are several:
1. Energy efficiency: SOX light gives you more light output for the energy or power (wattage) you put in than any other lighting source. In a time when many people are demanding more energy efficiency for environmental reasons, an installation of SOX lighting will consume significantly less energy than the more modern SON and metal halide lamps which run on a higher wattage and have less light output.
2. It can be disposed of more easily: The SOX lamp contains zero mercury, and can therefore be disposed of more easily at its end of life. Most high-pressure sodium and all other light sources employed in street lighting contain mercury, which is poisonous, and special restrictions apply to the disposal of used lamps. (ref: James Hooker)
3. More effective in rain, fog and snow: Low-pressure sodium light is safer for driving in rain as the light remains sharp and unblurred. This is because white light consists of various colours, which undergo varying degrees of refraction and diffusion as they pass through water vapour or ice crystals; in contrast, SOX light is monochromatic.
4. High contrast recognition: Although the colour rendering of a low-pressure sodium lamp is poor, the contrast between light and dark is very good.
5. The light pollution it causes is easier for astronomers to filter out. Because the light is monochromatic, it can be filtered out by removing its single wavelength and a dark sky is then restored totally.
6. It has a relatively low operating temperature: This means that it can be used with lightweight plastic components. (ref: James Hooker)
7. It is good for tunnel lighting: The long lamps may be aligned end-to-end to produce a continuous line of light and this almost totally eliminates the stroboscopic effect of driving past high-brightness lights at speed. (ref: James Hooker)
8. The SOX lamp can be operated on low-cost electrical control gear. (ref: James Hooker)
9. SOX lamps do not require a cooling-down time before they switch back on.
10. They are easier to service than other lamp types.
11. Yellow is a good colour to combat the winter blues: According to a survey by the Jamaica Tourist Board shortly before the time of writing, the colour yellow can drive away the winter blues and actually make you feel mellow.
Right:
Are SOX lamps still wanted on our streets? (Clitheroe, Lancashire, U.K., 2002)
But some disadvantages are discussed below.
1. The unnatural deep yellow/orange colour and the lack of colour rendering — all objects appear yellow, grey or black.
2. It is difficult to control the distribution of light emitted from a SOX lamp. (I'm not too convinced by this - a 'cut-off' design would at least eliminate sky glow, and surely a road/street should be illuminated in a uniform way)
3. Highway engineers, especially nowadays, use colour-coded road features (e.g. red bus lanes) which do not stand out too well under SOX lighting. Creating road features in different colours seems to be fashionable at the moment.
4. The rated life of SOX can be shorter than for other types of discharge lamp. However, the SOX-PSG lamp (introduced in 2003) is a new variation of SOX lamp which almost eliminates failure of lamps in early life.
5. SOX lanterns can be bulky in size, and the high-wattage SOX lamps also have a high “windage”. Windage is the force of the wind blowing on the lantern, and a large SOX lantern on a long side-arm bracket can move about noticeably in the wind.
6. The fact that SOX lanterns are large in size means that more material is required to make them and so this increases the cost of manufacture.
Therefore, I would not necessarily suggest that SOX lighting be used for everything, but I believe it is the best all-round solution for general road lighting. SOX lighting is good for illuminating areas, it is good for light/dark contrast, it is energy-efficient, and it is particularly effective in rain, fog and snow. Some areas such as crime hotspots, shopping centres, major road junctions etc. (where colour rendering is more important) may best be served by white-light streetlights such as SON (high-pressure sodium) or metal halide lamps.
I have never understood why 'cut-off' SOX streetlights have never been used on a large scale. If you include a cover that only allows light to escape underneath the lantern, surely this would reduce the 'orange glow' night-time sky pollution that has earned SOX a bad reputation! If anyone has any comments on any of this, please feel free to e-mail me:
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© Matthew Eagles 2005. Last updated 15th May 2009.