One of these beauties Citroën DS 21!

All seems so wild to me on the other side of the world....

Why would effort be made to produce dim, inefficient, big, and requiring expensive ballasts F25T12 for home use, and then put them in twin fittings anyway ?

when a single F40 does as good job ?

Or F20T12 at its normal power, half the size, with just a choke ballast, would put out the same light as one F25...

(And if it has a few Lumens less, then overdrive it by a couple Watts to get those Lumens up....)

The brightness of full power operation would be way too much for the domestic use, because how close these fixtures are.
Yes, it would be possible to use a diffuser with a single F40T12 tube, but that is either very expensive, prone to getting dirty or inefficient (wastes, absorbes, a lot of the light). Mainly in the past there were no suitable materials making a good, efficient long lasting diffuser for low enough price. When using more lamps and reducing the power density you get away with bare bulbs, so no cost for the diffuser, no light lisses in it, nothing to collect dust that is then blocking even more light, nothing to yellow down,...
Plus shorter tube costs more to make than the mainstream size made in huge quantities (because it needs retooling, not just replace the filament feed and the etch stamp and keep the rest of the machinery the same as with the mainstream F40T12), so also a cost benefit.
Plus two lamp ballasts cost about the same as a single lamp full power ballast, so no saving there.

Using two physically large lamps and feeding them by reduced power was just a cheaper way to get what was needed.

@Ash - Don’t worry, our residential single-lamp fixtures aren’t free from underpowered ballasts either. In fact, I think that’s where they started. The whole thing really is one of the biggest scams in lighting history. If the ballast underdrives the lamp by a few watts, fine. But under 60% power is ridiculous. And the companies still advertise the full lumen output on the lamps and not even mention that the fixtures are underpowered, so customers would buy these powerful-looking fluorescent fixtures expecting to get commercial brightness but would instead be met with regret and disappointment.

If anyone has a good Philips SRS201 MA60 or MA50 (Gear in lantern) MK2 bowl going spare and feels like doing a deal, please let me know. I saw some on ebay but this is for the mark 1 MA60 with the small shoe from the 1970s. Many thanks!    

Info taken from this page on Lamptech from @James of course. Here we go:

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1) Seriousness:
James writes "if measures are not taken to keep the sodium away from the seals, they will be attacked resulting in premature lamp failure". Alright, makes sense. But what kind of failure are we talking here? Will it just react and turn the glass black (I am assuming not), or will it crack and destroy the seal and make it leak? What actually happens when sodium attacks a seal?

2) Seal Design:
The seals are designed to remain hot while the lamp cools down so sodium doesn't condense on the seal. This was originally done (with some exceptions on question 3) with a magnesia bead fused to the end of the lead wires that were sheathed in inverted 2-ply glass tubing. That makes sense. But it also says that Philips later switched to a design that sheathed the wires in pure borate glass instead of 2-ply glass. Why wouldn't they do that from the start? That just seems like the simpler solution that is easy to think of. Also, is there any reason magnesia was chosen as opposed to alumina, or it it just convenience?

3) Glass types:
With the early SO dewar jacket lamps pictured on Lamptech, the ones made of soft glass had magnesia thermal mass beads and the ones using hard glass did not. Why does the type of glass used change whether or not it needs a thermal mass bead?

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Thanks!

All seems so wild to me on the other side of the world....

Why would effort be made to produce dim, inefficient, big, and requiring expensive ballasts F25T12 for home use, and then put them in twin fittings anyway ?

when a single F40 does as good job ?

Or F20T12 at its normal power, half the size, with just a choke ballast, would put out the same light as one F25...

(And if it has a few Lumens less, then overdrive it by a couple Watts to get those Lumens up....)

@LandryB — Check your PMs.

Yes that's correct. However, Xenon under atmospheric pressure does have most of it's spectrum in the infrared region, above 950 nm.

When under low pressure, such as LPS, HPS, some rare FL or even in industrial cold cathode lamps, Xenon is only used as a buffer gas,
that means like 95% the gas decelerates the ion movement and 5% is creation of light by electric discharge. Not very efficient.

When you raise the pressure, the ability for gas discharge becomes way more prominent, also the spectrum gets shoved into a more useful
range of wavelengths. You will need to have at least 20 bar or more for this effect to come into existence, more like 60 bar to really
create light efficiently from Xenon.

Xenon short arc lamps have a cold pressure of around 8 bar and a hot pressure of 60-80 bar, they are very efficient with no other gasses
present. Some ultra high pressure Xenon lamps (mostly text equipment in labs) run up to 200 bar and create a near sun-like light ... but
they are ungodly expensive.

Have you tried applying high voltage to a Xenon flash tube? You will see a faint glow, for the human eye it looks blue-ish since we
cannot see the 'red' of 950 nm and above. When you pass a very high current through the ionized Xenon gas, the pressure will spike by
around 40 bar in the plasma channel (of larger standard flash tubes like photo equipment or disco flashers). This is when Xenon becomes
efficient in creating visible light.

So the 'cold' pressure of an automotive MH lamp is already very high - the Xenon provides a substantial part of the overall light output.
Also, Xenon car lights are heavily boosted by their electronic ballasts.

@LightsAreBright27
Omg yes! The effect would be enhanced with some longer persistence phosphor (and a focusing setup with either a coil permanent magnets), but yes that would look very very cool

Very cool indeed! Imagine if someone put deflection coils, and did some slow scan vector images. That neon beam moving around would bw so cool.