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Light pollution


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 What purpose do street lights serve?

  1. To deter crime
  2. To avoid pedestrians having to carry torches at night

I can think of no others.

 So why do they have to be white?

I can think of no reason at all. The old sodium lamps were perfectly adequate but they must have been more expensive to run, so now we have much broader spectrum lights only as a by-product of the benefits of more efficient lights.

Sodium lamps were much better for seeing the night sky because their two very close spectral lines could easily be supressed by a narrow-band filter, enabling the sky to be seen across practically all of the optical spectrum.

 LEDs - white or narrow band?

I view with alarm the current plans of local councils to use white LED lamps because they really blank out the whole spectrum. My local council is already trying them out in some streets. It is really good that they will be very efficient but why must they be white? Only for aesthetic reasons among those who are not interested in the wonders of the night sky, it seems.

Why not use LEDs emitting single lines, like the old sodium lamps?

 Total costs

Over the past 2 or 3 years my local council has replaced all of the street lights across the borough with broad band flourescent lights. They are patting themselves on the back that the lights are brighter and yet cause less light pollution (!) - according to their web site.

I do not know how many tens of thousands of lamp post replacements that involved. For many months streets near here were a forest of lamp posts with both the old and new ones standing.

I wonder - did they take into account the energy (and pollution) costs of making and installing the new posts and of removing and disposing of (or recycling) the old ones? I guess it will be decades before the energy savings from the lamps cover that. Does the council realise that? I rather doubt it. I think the company that they subcontract street lighting to is likely to be doing very well out of it.

And guess what? I have read that the new LED lamps cannot be fitted to existing posts!