This is something that I've been wondering about for awhile and even more so since I joined LG.
What's the purpose of using SOX lamps when HPS was readily available?
Ok I realize that SOX lamps are very energy efficient and provide more lumens per watt than HPS.
But isn't that more than offset by the awful color rendering, lower life of the lamps and the greater expense of both the lamps and the fixtures.
Wether you love SOX lights or hate them, it just seems to me that when it comes to sodium lighting HPS is by far the better way to go.
Admittedly though I do find SOX an interesting light source but I just don't feel that it's benefits can compete with HPS.

Oh you're really asking the question now, aren't you 

You have to remember the history of sodium lighting. When LPS was first introduced in the 1930's, it was all there was - HPS wasn't developed to a usable form until the 1960's/70's, by which time there were lots of LPS installations around (don't get into the bad habit of calling all low pressure sodium SOX - there are other types as well, although nowadays SOX is the only one still in common use).

In terms of street lighting, for which LPS/SOX excels, the key is contrast. Being monochromatic, the light is not split up by mist and fog, making it ideal in those conditions. I don't know how much of that you get in Florida, but it's rather common over here 

People talk of colour rendering. If I'm driving, I don't need colour rendering from the street lighting, I have perfectly good headlamps for that. What I need to see with the street lighting is what's around me outside of the range of headlights. People walking, cars around me. I need to know if there is a car on a collision course with me, not that the car that just hit me was red..!

Also, for people with some visual problems, such as Nyctalopia, LPS is the best form of lighting. Sammi suffers with this, and can see far better for reading when we have a SOX lamp on than under the normal room lights (incandescent or CFL, depending on the room).

Maybe we're biased, we just love LPS. But we love HPS as well, it's just that LPS wins every time with us   

This is something that I've been wondering about for awhile and even more so since I joined LG.
What's the purpose of using SOX lamps when HPS was readily available?
Ok I realize that SOX lamps are very energy efficient and provide more lumens per watt than HPS.
But isn't that more than offset by the awful color rendering, lower life of the lamps and the greater expense of both the lamps and the fixtures.
Wether you love SOX lights or hate them, it just seems to me that when it comes to sodium lighting HPS is by far the better way to go.
Admittedly though I do find SOX an interesting light source but I just don't feel that it's benefits can compete with HPS.

This is only just my opinion- unlike High pressure sodium the Sox
Lights are in a class by themselves, They have a very unique design and look about them and also the fact they require filaments for preheating them although the color rendering is not the best - They are just so cool
To own  - Just my opinion judging from what I’ve seen ,

Whoops My bad - I was referring to collecting,

This is only just my opinion- unlike High pressure sodium the Sox
Lights are in a class by themselves, They have a very unique design and look about them and also the fact they require filaments for preheating them although the color rendering is not the best - They are just so cool
To own  - Just my opinion judging from what I’ve seen ,

LPS is cold start, not pre-heat. There is only one connection to each electrode.

See that's the part that still gets me.
By the 60s there may have been plenty of LPS lighting in service but by then MV was well on its way and by the time the 70s rolled around HPS was rapidly being improved.
I'm surprised that by the time the 80s arrived that SOX just didn't completely disappear altogether as MH was another fierce competitor.

As far as color rendering yeah I guess it doesn't matter to much out on the highways but parking lot lighting well I'm altogether clueless there.
And I sure as hell wouldn't want them along neighborhood streets making the houses look bad.

Yeah I guess they're useful in some parts of Europe with the foggy conditions.

I realize that they may have some benefits but it just seems like it doesn't take much to outweigh them.

Admittedly when it comes to HID lighting I'm completely biased as Mercury Vapor is the only HID source that I want to see even though HPS is superior to MV as well.

I think with SOX hanging around so long is that it worked, and worked good for its intended purpose.
And also before LED came around there was a replace (almost)everything...
But now with LED its replace everything, and its mother, oh and make sure you get the grandmother too .LOL.

Although in production in the 30s, I often wonder how widespread they would have been in the UK without the 70s energy crisis?
Even in our little town, (that was a Mecca for LPS in the 80s), mercury lamps were still the preferred choice for street lighting BEFORE the energy crisis happened. I often wondered whether we would have kept our mercs if it would never have happened?

LPS is an interesting technology...kinda similar to fluorescent. While LPS never really caught on here in the US, except for Southern California, I find it just to monochromatic, and also that it has a negative color rendering makes it less desirable. If I were to install a yellow light source, I'd be HPS...but I'd much rather use CMH.

There are two main reasons why LPS originally won the battle vs HPS in many parts of the world : cost and safety.  One of those has since disappeared.
A third reason also exists in areas of astronomical observation e.g. South California, because the light pollution from LPS can be more easily filtered out.

The initial investment cost of both the lamps as well as the lanterns used to be far cheaper for LPS than HPS.  It was only after the 1990s that HPS lamp production technology had matured enough to be able to undercut LPS lamp prices.  Prior to that date, and since streetlighting is state-funded and they will do anything to lower their investment costs, LPS was always the clear winner for new installations.  LPS was however slightly more expensive than mercury in terms of initial investment cost.

The most important cost though is not the price of the lamps and the equipment, but the cost of the elecricity they consume.  In countries like the USA where energy used to be among the cheapest in the world, lamp efficacy never really mattered.  Hence that country mainly developed lamps having e.g. longer lifetimes, better colour rendering etc, but never really focussed on creating energy-efficient sources like LPS, CFL, CMH etc.  However in EU where energy is among the most expensive in the world, lighting is extremely expensive to operate.  Since LPS has by far the lowest energy consumption for a given light level, this was again the clear winner over HPS and Mercury in terms of cost saving.

Secondly as Ria already indicated, contrast and visibility is greatly improved under LPS lighting.  Countless studies over the decades have shown that the accident and death rate on roads can be reduced by providing lighting, and reduced to the absolute minimum when that lighting is monochromatic yellow from LPS.  It therefore follows that the countries having the highest population and traffic densities tended to favour LPS for their streetlighting so as to keep their road death rate figures not so much higher than in the countries with lower population densities.  This is why you find LPS above all in Holland, Belgium, UK, Taiwan, Japan and Korea.  Many other countries also use LPS but only on pedestrian crossings.  Even in Germany and Italy, the big mercury countries, you can find LPS in such places.  It is well proven that motorists can see pedestrians from further away when they are lighted with LPS light.

In recent years, the economic advantage of LPS has been lost.  That is because there is a third significant cost : lamp replacements and maintenance.  Going back a few decades, LPS lamps always used to be replaced on a two-year cycle and the early HPS required annual re-lamping.  By the 1970/80s HPS was able to match the life of LPS.  Then in the 1990s when the HPS lamps with high xenon pressure became lower in cost, they suddenly extended the maintenance cycle to 3 and then 4 years.  Today's best HPS lamps can even stretch to 6 years service.  Moreover especially from the 1990s onwards, advanced new reflector optics were developed for streetlighting lanterns and these only worked with compact light sources, i.e. an HPS or CMH arc tube.  With these new lanterns it became possible to direct a greater percentage of the lamp's output onto the road, with less stray light flooding the surrounding area.  The gain in optical efficiency was such that an HPS lamp could match the light levels on the road from LPS, for the same or recently even lower power consumption.

Thus the key reason for using LPS, of lower total operating cost, was gone.  Starting in the mid-1990s the SOX-Plus lamps were introduced with the goal of extending the re-lamping cycle from 2 to 3 years and for a while that worked, but when the 4-year HPS lamp was introduced it was clear from that moment that LPS would die.  The lampmakers all made their plans at that time to gradually exit the LPS business over the following decades, and all further research in LPS technologies was stopped.  From the later 1990s authorities began changing their old LPS lanters to HPS and CMH at increasing rates, which resulted for the first time ever in the annual production volume of LPS decreasing.  The lower volumes naturally led to increased production costs, which drove customers to accelerate their change to other light sources

Unfortunately it is true that the road accident and death rates in the countries that used LPS have once again begun to increase.  However that issue is swept under the carpet by the authorities : cost savings are more important.  And when the death rates really started becoming problematic as in recent years for the countries with high population densities, the universal solution is to decrease the speed limits in an effort to solve that thorny topic.  Sometimes they try adding higher levels of light which is now easier to do with the precision optics of LED lanterns, and that also has some benefit.  In a curious twist, at least one company has now started marketing LPS-colour LED lanters with a deep yellow light and has dug up the old arguments that this colour of light improves visiblity and lowers the accident rate.  For sure it does help in humid weather conditions by eliminating the blue light scattering, but they are using broad-band rather than monochromatic LEDs, which do not offer the same advantages in contrast enhancement.

Thank you as always for that detailed explanation, James. One thing I have to mention from a pedestrian's point of view, you mention that "With these new lanterns it became possible to direct a greater percentage of the lamp's output onto the road, with less stray light flooding the surrounding area." That's all very well, but as a pedestrian, you need light on the pavement. With the abysmal quality LED's they've fitted around here (which certainly don't have "precision optics") there are simply small pools of light under the lanterns and huge black patches in between. As I've said, it's so dark in our street that we don't feel safe at night any more.

We used to have HPS post tops outside the back of our building, where the kitchen window looks onto. Now, their abysmal replacements merely illuminate a fraction of the area. There is a significant step on the footpath leading to one of the entrances to our block. At night, this is now completely invisible, and even when you know it's there, it's difficult to make out unless you have a torch. This to me is not progress.

I hope I'm not descending into an anti-LED rant, I don't mean to. But it just seems to me that everything is being driven in that direction with little or no thought, and that saddens me 

Massive thanks to James for the incredibly detailed explanation! That's exactly the kind of information I was looking for when I created this topic.
Where would LG be without the talent of this man.
My hat is off to you sir.

Thanks for the detailed & interesting info James (as always)!


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  the abysmal quality LED's they've fitted around here

I think the key detail there is 'abysmal quality'! Some of the LEDs I've seen around here actually seem pretty decent. I think its the low-quality junk is in part what's given LED such a bad name here. An honestly I see nothing wrong with an anti-LED rant...when it'd directed at crap that doesn't even do the job it was put in place to do.

Agreed !

LPS is an interesting technology...kinda similar to fluorescent. While LPS never really caught on here in the US, except for Southern California, I find it just to monochromatic, and also that it has a negative color rendering makes it less desirable. If I were to install a yellow light source, I'd be HPS...but I'd much rather use CMH.

Yeah I remember seeing some when I visited family, there aren’t really any in Canada anymore, most were in either rural areas (in Alberta) or more common in the east, like in Manitoba I think.

Thank you as always for that detailed explanation, James. One thing I have to mention from a pedestrian's point of view, you mention that "With these new lanterns it became possible to direct a greater percentage of the lamp's output onto the road, with less stray light flooding the surrounding area." That's all very well, but as a pedestrian, you need light on the pavement. With the abysmal quality LED's they've fitted around here (which certainly don't have "precision optics") there are simply small pools of light under the lanterns and huge black patches in between. As I've said, it's so dark in our street that we don't feel safe at night any more.

We used to have HPS post tops outside the back of our building, where the kitchen window looks onto. Now, their abysmal replacements merely illuminate a fraction of the area. There is a significant step on the footpath leading to one of the entrances to our block. At night, this is now completely invisible, and even when you know it's there, it's difficult to make out unless you have a torch. This to me is not progress.

I hope I'm not descending into an anti-LED rant, I don't mean to. But it just seems to me that everything is being driven in that direction with little or no thought, and that saddens me 

OMG YES! You explained perfectly.

A busy road that has all LED now seems to be a lot darker now, they are  just small patches of light, surrounded by dark. I can’t see pedestrians well either, and this is on a main road, the streets are almost worse.

Also if your windshield is dirty, you can’t see a thing.



But to be fair the LED can illuminate well, but only if the posts are very tall in my observation.