Why do MV lamps "green out"?
It has always been a mystery to me.

Why do MV lamps "green out"?
It has always been a mystery to me.

I think it has something to do with the metallic deposits on the arc tube blocking all light but green. I am probably wrong though.

They dont have to block all light, it is sufficient that they block both all 4 main Hg lines (2 near Blue + 2 near Green) but to different extents so the Green is less affected to get Green tint

It's odd that green would not be blocked as much.

Why ? Havent you seen stains on Glass (or in this case Quartz) that have Brownish tint ?

Not yet, I haven't used my lamps enough to see that.
But when I do see it, it's just plain black.

I mean in general, not specifically about darkening of HID arctubes. I dont know the reason for sure either and following this thread to get the real answer too, just saying that the Blackening of arc tube and filtering of Blue as result is plausible possibility, dont throw it out as wrong

What would really help is a spectrum of a new and greened out MV.

Generally, I'm much more interested in the evolution of M-H rather than MV lamp spectra, since these are much more common, at least for my cosmopolitan city.

I'll be comparing the spectra of identical M-H lamps shortly with 1 new, 1 @ EOL. And I'll try (to remember) to find a greened out MV lamp to do this as well, or use one until it gets greened out.

I should have added what I believe is happening with greened out MV lamps.

The arc-tube becomes blackened, and partly, much of the UV starts getting blocked. Less UV energy means that the phosphors won't get excited as much. Therefore, the reduced red wavelengths of the DX coatings leads to a greener appearance, closer to an uncoated lamp. I also believe the violet & blue wavelengths get reduced. When taking the spectra of the same lamps, one newer, one @ EOL, should confirm or refute any of this.

It's not just with DX MV lamps, clear MV also does it.

I have seen a old GE mercury lamp
So jet black that dont emit light
But when you touch it it still hot
Glass that burns your hand.

I have seen a old GE mercury lamp
So jet black that dont emit light
But when you touch it it still hot
Glass that burns your hand.

Maybe you could use it as a space heater.

Maybe you could use it as a space heater.

I wouldn't trust it, lamps that are in this bad of a state have a chance of exploding. The blackened arctube makes the pressure increase dramatically.

The pressure of MV lamps don't increase when the lamp blackens, as MV lamp is an unsaturated vapor lamp, so there is no additional mercury to vaporise, and the internal pressure is lower than that of MH lamps, and the mercury don't corrodes with the quaryz, so the likelyhood of explosion is minimal.
With MH lamps, the pressure is much higher to begin with, and this is a saturated vapour HPS, so there are always liquiid halides at the bottom part of the tube, that contribute to the internal pressure and corrosion as the lamp age, and so increases the risk of explosion at EOL.
The main reason for MH lamps to green out, is either leakage or halide deplation. Both can cause degradation of the halides lines in the spectrum. I've seen MH lamps that have very little sodium and scandium lines in the spectrum.