SQM or SQM-L

[Tim Haymes]

I dont see the type of meter defined clearly in recent posts. I use the SQM-L. This has a reduced area of measurement. The SQM would appear to have a wider field and therefore lower readings. When i look at the Institute of Lighting Engineers Guidance Note 1 (01/2021), quoted by many building developments, the ambient light levels are split into Environmental Zones using an “SQM”. I am in category E2: Rural (low district brightness) in relatively dark outer suburban locations.

My zenith SQM-L on moonless nights are between 20.7 and 20.9, and yet the guidance notes give SQM 15-20. Just about strong Moonlight conditions by my measurement. How are developers supposed to understand what a darkness is?

Im grateful to developers doing their best not to make things worse, but the SQM scale they are working within are rather wide. I hope you can see the attached clip from the Guidance notes.

If i were to re-type the table, i would add 1 unit to all the values so E0 becomes 21.5+. It could be that SQM give 1 unit less scale reading compared to SQM-L. In which case i would understand a little better.

[Tim Haymes]

Here is guidance image

[Tim Haymes]

Sorry i can seem to get the image to show up. Try this:
https://theilp.org.uk/publication/guidance-note-1-for-the-reduction-of-obtrusive-light-2021/

[Robin Leadbeater]

I dont get the idea of zones at all. Good lighting practise is the same if your are in a dark sky site or the middle of a city. This guidance effectively says if you are already in a light polluted region go ahead and put up more crap lighting. (tables 4, 6 ,7)

[Grant Privett]

I think its done in recognition of the fact that people who live in cities are frightened of the dark.

They associate it with danger.

Which is weird, as a Home Office study in the 90s found that if you turned down street lighting in an area the crime rate dropped.

Also, the report is odd. Why are the mags referred to as 16:00 and 22:00 and not 16.0 and 22.0? Didn’t anyone proof read it? Almost as if the author confused sky brightness with times. Hopefully, no AIs were involved in the authorship.

Also interesting that the sky brightness value for E0 changed in 2019. In which direction?

[Christopher Curtis]

I think the guidance is reasonably clear. Since they quote SQM, that’s what lighting designers would use, not SQM-L.

I’m OK with zoning in principle, though I think they underestimate the bad effects of poorly designed and implemented lighting. I don’t really have a problem with Weymouth Town Centre having light levels of 25lx before curfew – it’s busy and you need to be able to see well. The trouble is that 25lx at street level is often accompanied by vast amounts of light going upwards and outwards. This means that 10km away in the national landscape there is a gigantic light dome filling about half the sky and at least as bright as a local floodlight.

The real issue is that planning authorities more or less ignore even vague guidance like this. I had correspondence with the New Forest National Park Authority a few years ago when they were consulting on their planning guidelines and managed not to include ANY guidance on lighting (They did add a couple of lines saying that exterior and interior lighting should not be obtrusive). Even then it felt like trying to bolt the stable door after the horse had bolted as the village I then lived in was full of uncurtained skylights, security floodlights and the like. I was particularly annoyed by car parks that had carefully designed downlighting (which worked well) but someone in the pub or similar building would retro-fit an enormous security light and it would take years for the National Park Authority to respond. In the Dorset National Landscape there are numerous examples of such things as floodlighting for equestrian or agricultural purposes which is very poorly designed and which ruins the dark for miles around and the Council is simply not willing to act unless it is shining straight into a residential window.

[Robin Leadbeater]

Christopher Curtis wrote:

The trouble is that 25lx at street level is often accompanied by vast amounts of light going upwards and outwards. This means that 10km away in the national landscape there is a gigantic light dome filling about half the sky and at least as bright as a local floodlight.

Indeed and this guidance specifically condones this. I can perhaps understand why levels of lighting might vary between zones but cannot understand why they consider higher percentages of upward flux acceptable in more light polluted areas. Table 7 for example makes no sense. Why should 35% upward flux be considered acceptable for amenity lighting in zone E4. I see no reason why the fraction of light emitted upward should not be controlled to the same extent everywhere.

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