Which reminds me, when did SON arrive? I've heared conflicting dates. Some have claimed it came in as long back as the seventies and I think I've heard people on here say it's from the late 80s, but all of Liverpool's Alpha 3 stock was already wholesale converted SON-T from my earliers memories (mid/late-80's). Perhaps the SON-E bulbs for mercury lanterns weren't as old. Certainly, Liverpool still replaced mercury bulbs with mercury before the early nineties.
I'm not an expert on lighting terminology so be gentle with me. Many years before I encountered the terminology SOX or SON, I heard the terminology low or high pressure sodium.
I'm assuming that
SON = high-pressure sodium = pinkish-white light
SOX = low-pressure sodium = orangey-yellow, almost monochromatic light
(actually I think it's mainly two nearby spectral lines).
There was a long interval from the time when SON first appeared to the time when it began to be fairly commonplace, and a further long interval before it became almost universal on new installations.
As far as I can recal, SON first appeared in the 1970s (maybe the mid-70s?), but SOX continued to be the usual type of lighting on new trunk road installations.
SON began to be fairly commonplace in the early-to-mid 80s (e.g. used on the A55 around Bangor/Llanfairpwll) but there were still a lot of new SOX installations even then.
And SON became universal on new trunk road installations around 1999.
When SON first appeared, I think I read something about high-pressure sodium being more efficient than low-pressure and giving better colour rendering, but being dependent on new types of glass to resist attack from the sodium.
In Edinburgh the first SON lanterns - GEC Z8526 "Turtles" - appeared in 1972, on Leith Walk. The survivors of this installation are still in-situ today.