Why was low pressure Thallium not ever considered? I get it, thallium does produce a heck of a lot of UV, but a simple phosphor can convert the useless UV into white light to help improve the almost monochromatic green into higher CRI light. Thallium in the low pressure phase will also not generate much IR, but with a IR reflective coating like in SOX lamps, that tiny amount of IR can help keep the arc tube warm, although a neon argon penning mix would start and warm up the lamp from the intense IR from the Neon still. Theoretically this would be more efficient than LPS because thallium produces a main line at very close to 555nm, and under scotopic, night, vision, is way more efficient than SOX. For the sake of even better efficiency the UV from the thallium could be converted into green light close to 555nm using a phosphor, this would also make the light source less intense than SOX.

I would imagine the spectrum of such a lamp would look like this, of course the UV in the spectrum would be converted into usable visible light to either Improve CRI by adding some red to the mix or improving efficiency by adding more green:

Thallium is dangrously toxic, much more than Mercury. A broken lamp would very likely be a situation on par with a hazardous chemical spill, requiring evacuation, professional cleanup etc

(I don't know what quantity of Thallium would be needed to get the correct vapor pressure in such lamp, but it is way more than the tiny quantity used in some MH lamps. If it is anywhere near the quantity of Sodium in a SOX, it is immediately dangerous to life)

So do we need a lamp that is basically FL, but more complex to make, needs warm up, and is dangerous ?

Also: Metallic thallium have very low vapor pressure compared to thallium iodide.

Thallium is dangrously toxic, much more than Mercury. A broken lamp would very likely be a situation on par with a hazardous chemical spill, requiring evacuation, professional cleanup etc

(I don't know what quantity of Thallium would be needed to get the correct vapor pressure in such lamp, but it is way more than the tiny quantity used in some MH lamps. If it is anywhere near the quantity of Sodium in a SOX, it is immediately dangerous to life)

So do we need a lamp that is basically FL, but more complex to make, needs warm up, and is dangerous ?

Well I would imagine that would be a problem. And that also probably explains why such a lamp was never created.

Setting safety aside (assuming it's the 1940s), would it be more efficient?

Given how the LPS were fighting with retaining heat in the arctube in order to get some reasonable efficiency, something requiring way higher temperatures would be impossible at such low power densities. So I very strongly doubt.

Plus a iodine lamp needs the arc to have hot enough core to break the salt bonds into atomic metal, as only that is what generates the light (it breaks once the halogenide gas reaches the hot part of the arc, the metal atoms do the light emitting work there and it recombines back to the iodine salt gas once it leaves the hot part of the arc into the colder region).
In high pressure lamps the concentrated arc temperatures are by far sufficient, but with a low pressure the temperatures won't be enough, I'm afraid.

The problem with metallic thallium is a *significantly* higher boiling point than sodium (1473C vs 883C at 1bar). So while with sodium one ends up with relatively benign conditions in LPS lamp, with thallium fill the arctube temperature needs to be much higher. If you don't trust me, look at somewhat rare thallium spectral lamp construction - it has a quartz burner, shrouded by a metal cylinder to conserve heat, and overall just struggles to work having a very short life. Today, it is probably possible to overcome this by repurposing something like white SON arctube, but why, when existing green metal halide lamps work *so well*!

And I won't always trust atomic-spectra.net for spectral output content. Author uses NIST tables to generate his pictures, but excitation conditions for NIST tables do not necessarily match the ones in the arclamp. So while the wavelengths are of course correct, spectral balance may be way off.

Ah, I see. Would a metal, halide lamp, that only uses Thallium work? Would it still dangerous too?

Sure, MH lamps containing mercury as a buffer and thallium halide are very successful green light sources. See here:

Of course, not dangerous in normal use as thallium salt is behind a couple of glass bulbs. No problems until you deliberately crush them and consume the contents. I suspect the amount of Tl is still significantly below LD50 for a human, but am not 100% sure here.

 

@RRK According to the National Institute of Health, a lethal dose of Thallium is 10-15mg/kg, although lower doses per kg have been reported fatal. So you would have to keep Thallium levels at 200mg at most to be safe.

Visually, the amount of Tl is significantly lower even in large 400W lamps.

https://www.lighting-gallery.net/gallery/displayimage.php?pos=-229051

I would love to have one of those lamps. I have a Cooper Steeler in 400W probe start MH coming in that I could run it on. Do you remember where you bought that Chinese 400W probe-start green MH lamp?

Sure, these were some remaining stocks from a large lighting distributor's shop, TekLED in Astana, Kazakhstan. I bet this is quite far from you

But, green MH lamps are still easy to find, and I believe in US too...

Thanks. I'm thinking about getting a Plusrite 1053 green MH bulb; it is around $25 after tax, though.