Increasing inert buffer gas pressure (within a reason) will just slightly increase arc (burning) voltage. What will happen at pressure decrease is more complicated, voltage will certainly increase again at some point. A tube with no buffer gas won't ignite at all at practical conditions.

Thanks James!

Interesting read.

Sci-hub rules, of course

Never thought of thorium migrating from the electrodes as a reason for changing lamp's color balance. Will try to catch some characteristic Th lines in some seasoned lamps!

It was to circumvent the Philips patent on high efficacy halide fills.  Sylvania has a much earlier patent on the use of the Lanthanum-Thallium halides as a mechanism of boosting efficacy, lumen maintenance and colour rendering.  I don't think that was ever commercialised, but with modern computer modelling techniques we were able to further refine the La-Tl system to deliver exception performance especially when combined with the benefits of cerium halide.

I like Aussie lights the most, they are my fav.

I had a thought experiment: Imagine 4 fluorescent tubes, all of them 1200 mm long and 38 mm diameter.
One of them is a standard F40T12 with about 3 torr of argon. Another tube is filled to 25 torr of argon. A third one is only filled to 0.1 torr of argon and a 4th one only has the mercury vapor pressure in it, no gas filling.

If we connect them to a current source supplying exactly 430 mA of current, am I right to  assume that the tube filled at 25 torr would have the highest arc voltage and the one containing only the mercury vapor pressure the lowest?

I assume this effect is not linear, so tripling the gas filling pressure wouldn't triple the arc voltage.

These experimental lamps wouldn't be practical, but it would proably offer the possibility of some interesting experiments.

I couldn't find any documentation about gas filling presures F30T12 and F30T8 (pure Ar in both). Since a reduced tube diameter increases the voltage, it's expected that the gas filling pressure in F30T8 is significantly lower to keep the arc voltage of the two tubes the same.

Another interesting topic would the the gas filling old Philips 16, 32 and 50 W tubes (pure argon despite being T8), which have significantly higher arc voltage than standard 600, 1200 and 1500 mm long tubes.

I've never seen CMH lamps with LaI. What Sylvania put it?

Because Scandium has an extremely high affinity for oxygen.  In the cheaper NaSc lamps the salts are dosed as NaI, HgI2 and the Sc as a solid metal chip.  During operation the HgI2 dissociates and releases free iodine which should then react with the scandium chip.  Hpwever, the scandium also melts and before it can be converted to iodide some of it will attack the quartz with which it is in contact.
4 Sc + 3 SiO2 ⇒ 2 Sc2O3 + 3 Si

Silicon is released.  Silicon is extremely soluble in tungsten and will dissolve in the hot electrode tip, reducing the electrode's melting temperature.  Additionally, some of the reacted ScI3 reacts with the ThO2 emitter which is used to dope the tungsten electrodes to give them good work function and high lamp efficacy.  This is actually very beneficial, the electrode temperature is reduced and tungsten vaporisation is also reduced.
3 ThO2 + 4 ScI3 ⇒ 3 ThI4 + 2 Sc2O3

However, some of the metallic scandium will exchange itself with the thorium atoms deep in the electrode structure.  The result is that some thorium metal is released.  That is excellent for boosting the red output and colour rendering of NaSc lamps, but as you may know from the old mercury lamps that used to have thorium emitter on their electrodes, that metal is quickly transported to the wall where it condenses as quite a heavy black layer.  However most of the blackening responsible for lumen depreciation is tungsten from the electrodes.

You now need to understand that NaSc lamps actually have their scandium dosed either as a metal chip, or as bi-component halide pellets of NaI+ScI3 as first invented by Thorn Lighting, and subsequently used by some other manufacturers especially for higher performing NaSc lamps.  One of the big benefits of dosing scandium in metallic form is that it functions as a getter to keep the arc tube purity high.  When changing to ScI3 dosing that beneficial effect is lost.  As a result many manufacturers did not make the change to ScI3 dosing.  It requires a substantial increase in chemical purity of the arc tube preparation, filling and pumping processes.  It is necessary to fill arc tubes in a glove box type environment which is slow and expensive.  Conversely, metal-dosed arc tubes can be processed on semi-conventional high speed automatic lampmaking machinery without need for a glovebox, so they are considerably cheaper.

However, for metal halide lamps containing Sc metal, due to its gettering action there is substantially no free oxygen in the arc tube.  That is a problem, a small amount of oxygen is beneficial because it reacts with free iodine and tungsten vaporised from the electrodes to form WO2I2.  That is a gas, and it prevents tungsten metal from being deposited on the wall and keeps the arc tube ends cleaner for longer.

Some of these challenges are discussed in the excellent paper that Wim van Erk of Philips presented at the Light Sources conference in Eindhoven in 2000.  See Transport processes in metal halide gas discharge lamps, https://www.pismin.com/10.1351/pac200072112159

Thorium iodide is very volatile

Yes

the end!!!

suite