That's a big switch.!

Nice room heater..! Looks like it's specified for base-up operation, you might want to add that in the information fields..? I made sure to add that field, which was sadly lacking on LG.

Don't think it will get used too much but it is a good heater. It is really made for base-up fitting but seems ok for horizontal. I'll add that detail.

They really had faith in the mechanical stability of the split-frame construction of the lamp for them to use such a bimetal connection! Also, looking at your data, I had not realized how inefficient those MBIFs were for sodium-scandium lamps... this made me check Thorn's literature (note that the lamp power dissipation is 375 W, not 400). A 24-klm output is just 12 % higher than that of a 400 W fluorescent mercury lamp. The /H variant of the 400 W MBIF is even worse, its output at 2000 h is actually the same as that of the MBF 400W (21.5 klm). That's most peculiar given the very high lumen potential of the Na-Sc fill chemistry. The performances of the MBIFs in the long run are worse than those of what I considered the least efficacious 400 W MH lamp so far, the Osram HQI-E 400W/DH. The chemistry in those Na-Sc lamps must have been really unstable.

This makes you wonder why Thorn went for the challenging Na-Sc mix while other options such as the Na-Tl-In mix would have been much easier to implement... It looks like convenience (technical support from GTE-Sylvania in the 1960s and 70s), initial output (at 100 h), and light color quality (although not better than that of other MH fill options), were all that mattered back then. It's a bit ironical then that while the GTE-Thorn co-operation brought the first Na-Sc MH lamps to Europe, it's GE who eventually ended up acquiring Thorn's lighting operations in 1990-92. Thanks for your post, that was most enlightening! I had not realized that in Europe the HPI lamps from Philips actually performed better overall than any equivalents from the competition (except on the initial output level in some cases). It's then no wonder the Dutch stuck with the NTI fill chemistry for their main MH workhorses after all. This may not have delivered the greatest initial output but it permitted the most stable lamps, resulting in superior performances in the long run.

Funnily, my thoughts, after its first run up here was, it’s not as bright as expected. I then made allowances when saw it runs at 375W on Thorn gear. However, your right, it is still surprisingly low. I did think manufacturers struggled a little getting the chemistry and stability of early halide lamps good. As you mention, at the time, the quality of light was probably more the focus than efficiency. I don’t recall Thorn having issues with colour drifting with age, although that seems to be the reason for the more stable horizontal only burning lamp. I remember reading somewhere halide lighting was good enough for television studio lighting as being quite an accolade at the time. I also remember James Hooker mentioning that GEC basically gave up trying to make stable while halide lamps and stuck to the blue and green variants for flood lighting. Sounds like Philips had the overall early lead in the technology. James has some information on Thorn MBI lamps on his site.