I have a 100W GE mercury vapor lamp that has a strange feature. The arc tube has a silver-colored coating on the outside of either end of the arc tube. I have searched the gallery for other clear mercury vapor lamps and haven't seen other lamps that look like this. I only see the white coating on metal halide lamps and no coating on mercury vapor lamps.

I have attached images of the etch, the lamp innards, and a close up of just the one end of the arc tube.



What is the purpose of this? Has anyone seen this before? My guess is that it just keeps the electrodes warmer so they last longer, but IDK.

Thanks!

 

I have a GE lamp made in Mexico that has a similar silver coating. I haven't seen this on Sylvania lamps so I wonder if this is exclusive to GE lamps.

 Apparantly it has something to do with thermal losses . See here : https://www.lighting-gallery.net/gallery/displayimage.php?album=5200&pos=150&pid=158914

The big seal flanges are sucking a lot of heat away from the region behind the electrodes, forming there a cold spot. The reflective coating is supposed to prevent that (it reduces the emission coefficient of the arctube in that area, so reduces thermal radiation from there).
It is practically on all 50W MV's, but maybe GE decided it is worth here as well...

The thing is, the MV arc is the most efficient when the arc is fed by 50W/cm of its length or more, lower loading yields drop in efficacy. That means the lower rated power the lamp is, the shorter the arc needs to be.
But at the same time the arc needs to have sufficient voltage drop, so the 10..15V cathode losses do not form that much percentage, practically all MV's ended up with about 100V arc.
So lower power lamps need to have the same voltage over shorter arc, so the voltage drop per unit length must be higher.
And higher voltage drop per unit length means you need higher operating pressure.
And to get higher pressure means the gas vs liquid transition happens at higher temperature, so you need to make sure the coldest spot on the arctube (the area behind the electrodes) runs hotter on lower power lamps. That may lead to use vacuum outer and on mainly the lowest power lamps it is done that way.
But at he same time you have to make sure the hottest part (which uses to be the center section of the arctube, mainly the part above the arc when burned horizontally) does not exceed the limit the quartz can handle, so you need to ensure sufficient cooling there. And the only cooling available without also blocking the light is gas flow around the arctube, so gas fill in the outer.
But the last two aspects are exactly in their opposite, so you have to find some measures to squeeze between. On top of that you are pretty limited in outer pressure selection: When gas filled, you need sufficient pressure to prevent discharges there. Or you need to use really vacuum. You can not go "somewhere between", as that would create tendencies for arcovers in the outer.
All these aspects you may to some extend control by the way how the lamp assembly is designed, but evrything has its downsides (less material left in the seal pinches mean better heat retention but structurally weaker assembly, careful inner wiring and armature arrangement means you may afford lower outer fill pressure and still do not suffer from arcing, larger arctube diameter leads to colder running arctube walls but more mercury amount so longer warmup snd even more complex tapered shape to prevent the end sections from running too cold), the reflective paint is one of them (allows the seals to be bulkier so stronger).

On really the lowest power lamps all such measures, include the coating are practically a must have, along with vacuum outer and so on. The higher you go with the power, the more freedom you have, even up to the (oldest MV) 400W sufficing with hard glass instead of quartz and yet losing only about 20% of light output compare to the modern high pressure designs.

I thought only 40-50W MV lamps have heat reflective coatings at the ends of the arctube.

Interesting why a somewhat more expensive noble metal coating is preferred in mercury lamps instead of that ubiquitous white flame sprayed zirconia we are all used to see in MH lamps?

Just came to my mind: Is it really more expensive? Yes, gold is more expensive material per mass, but isn't there so little amount of it that the process of applying it becomes actually more expensive than the used material itself. So isn't it possible that in the scope of MV manufacturing, the metal coating application may become actually cheaper? The MH may need to go for the Zirconia, as an easy to deposit metals won't srvive the higher temperatures of MH.

But this above is just entertaining a hypothesis which at the end could be wrong, I'm not expert in these lamp making steps...


What I know, for decades gold has been and used to for connection wires for semiconductor devices because it for long term was the cheapest way, even when using one of the most expensive metal for this. So I just tried to apply the same reasoning...