Then what changed since then which made bare T8, T5HO and now LEDs acceptable ?

The F40 at full power has about the same intensity as the F20, but that is too much for some applications.
The lower intensity of the F40 operated at 25W was used not as a replacement for GLS, but more like equivalent of the linear incandescents used in Europe.
The refractors were used with incandescents, with classic bulb format there was no other choice, but they always had those disadvantages.
Using bare tubes operated at lower brightness meant just a cheaper, simpler and even more efficient way of getting similar result and getting rid of those disadvantages.

In Europe in a "progressive energy efficiency" legislation was formed (somewhere in the 60's till 70's) banning all gear that operated the lamps at lower than rated lumen output, effectively banning an equivalent approach in Europe. Not that European market wouldn't want similar solution, but it was the bureaucratic legislation written by activists what stopped that.
So way less efficient linear incandescents were used instead. It made money for the lamp makers (the lamps were very expensive; so the lamp makers were happy) and consumed way more power (nobody cared), but did not violate the standards (that is what the bureaucrats cared). The modern "energy efficiency standard" laws are nothing new in their stupidity...

I recently obtained this fixture that uses 4 13w PL bulbs, however the ballasts it came with are extremely loud and seem to heavily overdrive the lamps. The current ballasts are Robertson model SP2, which I will add an image of. Anyone know of quiet ballasts that would properly drive the lamps and that I could obtain at least 4 of? Thanks.


Here is a video of the fixture operating, showing the noise.

https://www.youtube.com/watch?v=MBGkSB_3NkI

Yep . 1974 Dodge Monaco .

Dodge Monaco?

The car the Blue Brothers drove.

Ford XC Flacon

May I ask what size tube they use?

Yep, I think they're M-2s. Those are UNMISTAKEABLE! They could probably be mounted back to back, with a small arm (like the ones that attach a shoebox) holding each. Or they could be custom? IDK how they're mounted, but correct me if I'm wrong, i think they are M-2.

F20T12 have about the same intensity as F40T12, a little less (1050lm 2ft F20T12 vs 2500lm 4ft F40T12 for 765)

I am trying to understand why then bare F40T12 was cosidered too high intensity in the US but ok in Europe

And why over the years bare incandescent lamps, T8, T5HO, and now even bare LED chips without any diffuser (in decorative luminaires for living room no less) became considered ok. What changed ?