They're not gone yet here, but i do miss them a bit on the highways.

The golden yellow light of HPS always separated 'Away from home' from 'Home' for me. As soon as the lights change from HPS to fluorescent, you know you're in a residential area, which i find a pleasant switch from a travel mindset to a being home/relaxation mindset.

I have few memories about LPS lit streets. In 2021 i thought for the first time 'Let's make a picture of the LPS streets' because in 2020 we still had one big inner city artery lit with LPS, but literally just months before i wanted to capture those scenes, they turned out to have been removed.

The good thing is that i now have all of those lamps myself and could get them for cheap because for the professional user, the lamps are now practically worthless.

Looks like most CMH lamps have smaller electrodes than quartz MH lamps, as most of them are rated for electronic ballasts.

My 50w and 70w SON/CDM/CDO drivers are both electronic.
My LPS ballasts are inductive, as are my 150w HPS, 100w SDW-T and 50/80w MV ballasts. I tend to call those ballast, and an actual switching/oscillator circuit a driver because it's an active thing rather than something just being a impedance in series with sometimes an ignitor that only serves to start the discharge.

But interesting to know how those things work and what makes them different.

Aside from the difference in power factor, extremely important is also the current crest factor which determines the required size of the electrodes.

Magnetically ballasted lamps suffer a high CCF which causes considerable electrode heating cycling variation during the 50/60Hz sinus, and the electrodes must be made large enough to accommodate that.  These significant heat losses reduce lamp efficacy and shorten the life.

At higher frequencies above about 400Hz, or on squarewave operation, the current peaks are drastically reduced.  The electrode size can be reduced drastically - just look how small are the CPO electrodes vs the equivalent SON types.  Even if you can set up a magnetic ballast that operates the lamp at the same power as electronic (ignoring voltage and current differences which will always be significant due to the different waveform), it will quickly destroy the electrodes by severely overheating them at the peak of each current cycle and causing higher continuous temperature operation.  For this reason, electronically ballasted lamps of almost any type should not be run on magnetic gear if there is a desire to achieve anywhere close to the rated life.  In some extreme cases life may be reduced to only a few hundreds of hours.

@James: Here is the lamp: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-246513

Because the particular halides do not have high enough vapour pressure to attain the desired spectral performance at the available arc tube wall temperature.  So more halide is added to raise the total vapour pressure, and improve performance.

It is maybe a rather old lamp, over time all manufacturers optimised their thermal designs to be able to reduce the salt dose.  And in Sylvania’s case changed from dosing with pure mercury to 3% cadmium in mercury to improve colour rendering without need for halides.

The bromides are more chemically active than iodides and help to protect the quartz from attack by recombining with corrosive metal vapours further away from the wall.  They are also more active in accelerating the halogen cycle to keep the wall clean from sputtered tungsten in lamps that are sufficiently highly loaded - but also have the disadvantage of being more active in causing beavering of the electrode shank at the cold spot where it passes through the arc tube wall, which can reduce life.

The bromides also have different vapour pressures than iodides, and depending on the metal may be more or less effective in bringing their vapour into the plasma.

Many metal halide lamps contain a finely balanced mix of bromides, iodides, and in highly loaded lamps also the chlorides, to balance each of these processes for optimum initial performance, lumen and colour maintenance, and life.  With some elements it is not easy to obtain the bromides, or it is desired to have a bromine-iodine mixture.  In this case they are added as iodides, and part of the metallic mercury mix is eliminated and dosed as mercuric bromide to get that halogen into the fill via an easier mechanism.  The same is done for lamps where iodine is consumed too rapidly during life, by replacing part of the mercury fill with mercury iodide.  Those are often easy to spot thanks to the intense reddish-orange colour of that salt when cold.

I do not miss HPS, what I do miss is mercury vapor. But it is clear to me the MV just were not able to hold.

Well, I certainly hope so! As soon as you can get a photo of one, please let me know!

Remembered them as a kid (mainly HPS) when they were all over. Out on late night drives to all the various places we went to back then. Loved falling asleep under them. Don’t miss them too much these days, as I do enjoy the better light and color rendering of LEDs but I still feel nostalgic whenever I go into an area still lit with HPS.