While doing some research into the history of LPS lamps, I have come up with a few questions:

1) Does anyone know why the 45W SO/SOI never made it past SOI? 60W, 85W, 140W, and 200W SOIs evolved to 35W, 55W, 90W, and 135W SOXs respectively, but 45W SO/SOIs never got a SOX lumen and size equivalent. Only later was an 18W SOX-E made, but that's physically much smaller than 45W SO/SOI and as such outputs fewer lumens, not an equivalent.

2) Another observation I have is that the predicted ballast impedance of 45W SO/SOI and 60W SO/SOI are very similar (770@480V and 760@480V respectively), were these lamps designed to be able share a ballast in the same manner as 35W and 55W SOX are?

3) Interestingly, the predicted impedance for 45/60W SO/SOI is very similar to the standard 35/55W SOX ballasts which are both 775@480. Has anyone tried running 45W or 60W SO/SOIs on 35W/55W SOX ballasts? If that does indeed work then I think ~12" LPS lamps are the only ones that all can run on the same ballast (including 60W SO, 60W SOI, 35W SOX, 26W SOX-E, all of which are ~12"), meaning if a lamp physically fits then it is electrically compatible (a rare property in discharge lighting).

Thanks!

1) During the mid 1980s sodium lamps had almost only one application  : streetlighting.  They were developed with luminous fluxes to suit the requirements for lighting levels and uniformity of Group A and Group B roads.  There was no need for a lamp with lower lumens than the 45W SO/SOI, and in parallel public lighting regulations called for higher outputs.  The modern 40/35W SOX delivered slightly more light than the 45W SO/SOI, so there was no purpose to develop a new lower wattage SOX because it would have had no commercial application.

There was no real interest in smaller SOX to use less power than other small light sources because back then electricity was so cheap compared to these days. 

Some years later around the time of the first middle east oil crisis, electricity prices suddenly increased (although still exceptionally cheap compared to today’s horrific increases). Philips developed a new application for SOX lamps to replace 100W incandescents, then the workhorse of outdoor security lighting, to achieve an energy saving.  That opened up a new market and resulted in the development of the 18W SOX, quickly followed by the 10W of GEC and 6W of Thorn.


2. Yes, in fact many companies offered a single ballast to run 45/60/85W SOI.  These are all 0.6A lamps.


3. The 40/60W SOX and 35/55W re-rated SOX were also all 0.6A lamps and designed to run on 45/60/85W SO/SOI ballasts.  In some cases you can also run 45/60/85W SO/SOI lamps on older 35/55W SOX ballasts, but after some years the OCV of the 35/55W ballasts was reduced, and they could not always keep an 85W lamp running as it warmed up.

In a similar way, the 90/100W SOX was a 0.9A lamp and designed for use on a 140W SO/SOI/SLI ballast.  The 135/150W SOX was also 0.9A and could run on the 140W or 200W SO/SOI ballast.

@James
1) That makes plenty of sense, I guess street lighting needs a lot more light than security lighting.

2) How is that so? I totally get that the current between a 110V lamp (60W SO) and a 70V lamp (45W SO) can be similar, but once we get to the 165V lamp (85W SO) that seems like a big jump. How is the current kept so constant throughout all of these arc voltages? I assume these ballasts were the equivalent to choke/HX ballasts, but maybe they were in a CWA-type arrangement that has a more constant-current-ish output. Or maybe a ridiculously high OCV

3) That makes plenty of sense, I really had no idea that there were any ballast compatibilities at all across the SOI-SOX dividing line, but I am happy to be incorrect on that one. I also had suspicions that 140W SLI was meant for 140W SO ballast due to their similar specs and very similar predicted impedance.

Thanks so much for clarifying all of this!

@James
1) That makes plenty of sense, I guess street lighting needs a lot more light than security lighting.

2) How is that so? I totally get that the current between a 110V lamp (60W SO) and a 70V lamp (45W SO) can be similar, but once we get to the 165V lamp (85W SO) that seems like a big jump. How is the current kept so constant throughout all of these arc voltages? I assume these ballasts were the equivalent to choke/HX ballasts, but maybe they were in a CWA-type arrangement that has a more constant-current-ish output. Or maybe a ridiculously high OCV

3) That makes plenty of sense, I really had no idea that there were any ballast compatibilities at all across the SOI-SOX dividing line, but I am happy to be incorrect on that one. I also had suspicions that 140W SLI was meant for 140W SO ballast due to their similar specs and very similar predicted impedance.

Thanks so much for clarifying all of this!

i'd highly recommend you check out the literature section on James's website, lots of juicy technical details in the various catalogs, technical data sheets and technical manuals that will answer many of your questions and I am sure you will find most fascinating (I have certainly lost many an hour to it in the past)

https://www.lamptech.co.uk/Literature.htm


with regards to 45W-85W 0.6A lamp on the same ballast, a constant current is kept simply by having a massively high OCV to lamp voltage ratio such that lamp the total change of voltage drop across the "Choke" is relatively small so current changes little, the higher arc voltage of a Sodium lamp when striking is why the start up current for those lamps is actually less then their running current, because the high arc voltage will start to push over the curve of the ballast finally


and this is where some lighting enthusiasts run into problems, as in later years they came out with low-loss simple choke+ignitor ballasts for 35W/55W SOX, which can run a 45W or 60W SO fine, but will not be able to run an 85W SO properly at all


Ditto in later years 240V Choke+ignitor setups for 90W SOX lamps came out and lighting enthusiasts of the time quickly learned of their unsuitability for 140W SLI/H lamps!


(Philips later in the SOX era did come out with a series of CWA ballasts for SOX lamps, especially the SOX-E range and general larger SOX lamp range, but thats a whole separate thing )