There has recently been a UK select committee looking at light pollution in the UK. The full text of their report is online at
Science and Technology Select Committee Reports
I have asked for and been given permission to put a copy of my written evidence submitted to the enquiry on the web. I believe that if asked they will allow others to do the same. All written evidence is also available collectively in the printed document HC 747-II published by HMSO and the text of all submissions is online as an HTML linked document but unfortunately the diagrams and illustrations are not.
My submission on Light Pollution
Illustrates the case in favour of low pressure sodium lights from an astronomers perspective. This was a minority viewpoint.
Low pressure sodium light is very obvious from its yellow colour. But it is also astronomer friendly because it is nearly monochromatic and can easily be filtered out. It is no coincidence that low pressure sodium lights in full cut off fixtures are mandated for regions around professional optical observatories.
Unfortunately, low pressure sodium lighting in the UK is mostly installed in dreadful inefficient luminaires (left) that send about 30% of the light directly skywards. This is an abomination and there is no excuse. It is these badly designed enclosures that deserve to be demonised and not the light source inside. There are plenty of well designed modern full cut off low pressure sodium lamps used in mainland Europe (right). Retro fitting simple reflective shields into our existing low pressure sodium lamps in the UK would obtain the benefits of full cut off without the huge expense of total replacement.
I do not agree with the report's conclusion that replacing the existing 45% of low pressure sodium street lighting in the UK with new high pressure sodium lighting is the best option for astronomers. Full cut off luminaires will always benefit astronomers irrespective of the type of lamp inside. But the combination of full cutoff with a nearly monochromatic light source is preferable to full cut off with white light since about 10% of all light emitted will go up into the sky after reflection from the ground.
I believe that if the report is adopted in its present form and all street lighting is all replaced with peachy white high pressure sodium we will have opted for perpetual moonlight instead of nasty orange night skies. This may be cosmetically more appealing but it an expensive way to trade one form of light pollution for another. Worse still you cannot filter out white light pollution - there is no way to distinguish natural starlight from artificial white light.
This point in favour of low pressure sodium light was also made by the Royal Astronomical Society in their submission:
http://www.parliament.the-stationery-office.co.uk/pa/cm200203/cmselect/cmsctech/747/747we95.htm
Several people have asked me how it is that the committee concluded that there will be little or no energy saving by following the recommendations of the report. The answer is simple. Low pressure sodium lamps are approximately 2x more energy efficient than high pressure sodium lamps. This means that even with all the light from high pressure sodium lamps going downwards it takes slightly more power to put the same amount of light onto the road.
A few extra notes and comments on the report showing rough figures for energy efficiency, percentage contribution to light pollution with and without filters and a sketch showing the shapes of reflective baffles that are used in continental low pressure sodium luminaires.
Light Pollution Additional Notes - some quick comments on the report
I cannot claim to be completely unbiased on this issue. I sell a neodymium doped glass filter for astro photography, Nonad, that blocks low pressure sodium light. But I am well informed and the evidence I have presented here and to the committee should stand or fall on its own merits.