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PostPosted: Fri Mar 22, 2013 9:14 pm 
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Is this actually true?

I remember asking my father when I was young why street lights produced a yellow/orange rather than white light and his response was "it cuts through fog better." Yahoo answers and the like have had many responses to people who've asked similar questions and again this idea of it cutting through fog crops up.

Obviously we know as street light enthusiasts that there are more reasons for using SOX other than fog; it's the most economical light source on the market (hence why it was particularly big in the '70s) and is good for astronomers who can easily filter out yellow light.

The Secret Life of a Motorway (broadcast on BBC FOUR a few years back) showed evidence of public guides, which said SOX was used as it is easier to maintain and softer on the eyes. Now the latter is indeed true but when they say easier to maintain, I assume they mean in terms of long-lasting and thus a lower maintenance frequency?


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PostPosted: Sat Mar 23, 2013 12:06 am 
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I've been told the same.

Heading up to Bacup from Todmorden, a section of the A681 is lit with MA60s? (Calderdale side) but this is because its high ground but also because there is an observatory nearby. Never been along it in fog or a blizzard but the way I see it would be because of the golden glow could easily fllter through deep low cloud where as other colours of the lighting spectrum  may struggle.

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PostPosted: Sat Mar 23, 2013 9:46 am 
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There must be some truth in it, as fog lights on cars and buses used to be yellow years ago.


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PostPosted: Sat Mar 23, 2013 11:56 am 
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Its all to do with the visible light spectrum, lilac has the shortest and wavelength whereas red has the longest wavelength. As fog is water droplets in the air some of the light produced is reflected away but due to red light having a longer wavelength more actually reaches the ground. White light is a mix of all waves of light whereas SOX/SON have a higher percentage of red/orange light in them. This is why a move away from SOX/SON would reduce sky glow as there would be less long range red light waves to be reflected upwards.

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Perhaps they could fit colour changing LEDs that could use white light for normal driving conditions then change to the orange glow of SOX during poor visibility.  ;)


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PostPosted: Mon Apr 01, 2013 11:34 am 
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A search in Hansard for "sodium lighting" revealed the following information. In June 1985 a comparison was made between high and low pressure sodium.

Lamp life: 8000 hours SOX, 12000 hours SON
Lantern cost: £110 SOX, £70 SON
Lamp cost: £19 SOX, £70 SON
Energy cost per year: £29 SOX, £40 SON

Hence, SON lamps lasted longer than SOX and cost less than SOX, and SON lanterns cost less than SOX lanterns, but SOX had a lower operating cost than SON.

The evaluation in June 1985 concluded that whole life costs of SOX and SON were comparable, but that SOX should be favoured on the basis that it saved energy. So in effect, it was not a matter of cost as such, but more a matter of the environment.

That was 1985. Go back earlier than this date and SON lamps were less efficient and more costly, so on cost grounds SOX had an advantage. But fast forward and the cost difference between SOX and SON has grown in favour of SON.

The fact that SOX cuts through fog better due to being a monochromatic light source would have some truth, but I expect this is just an added bonus rather than a reason for using it. Cost would have been the main driver, because for early motorway lighting the choice was between low pressure sodium and mercury, and that was a comparison between 135W SOX/140W SLI and 400W MBFU. Of course, mercury is more glarry than SOX, and due to being white light would not cut through fog quite as well.


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PostPosted: Thu Apr 04, 2013 8:16 pm 
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There seems to be some confusion on this subject.

Many lighting engineers and text books simply state that the water droplets in the air scatter all wavelengths of light equally* - and therefore the orange light of LPS is not able to cut through fog better than any other wavelength.

However, I think there's something in it. I have often driven on dark misty and/or foggy nights towards a HPS street lighting installation and noticed that it looks very "orange" at a distance - almost like LPS - until you get nearer. It would appear that the longer wavelengths are not scattered as much as the shorter ones.

And ask yourself this... why is the sky blue?

All the best,
Simon

*Not quite. Different wavelengths will be refracted at different angles - hence the effect of the prism.


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PostPosted: Fri Apr 05, 2013 9:57 pm 
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Makes you wonder why Medium Pressure Sodium wasn't invented as something like that may cut through fog even better, as it would be a blend of SOX and SON. I do think that it'll be a case of looking further into the orange to yellow part of the spectrum in order to find the actual answers.

It'll be one of those things that we shall never properly find out.

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PostPosted: Sat Apr 06, 2013 11:19 am 
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It is not as simple as that without going into the physics of discharge lamps.


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PostPosted: Mon Jan 05, 2015 1:05 pm 
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The human eye does achieve higher visual acuity under monochromatic light, so they do work well with fog, however, the use of SOX in this country came about with the 1970s oil crisis, that then created an energy crisis!
If not for this, we would still be using the MBF lamp as the Americans did.

Low pressure sodium was used over there, but they favoured the whiter, (albeit greener), colour of the MV lamp.


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