Recently, I begins to see more and more elliptical HPS lamps that have a phosphor coating of the type of deluxe white MV lamps, rather than the regular white diffusing coating.
All 70W internal ignitor HPS lamps inside the Schreder Alura we have in Zebulun st. have this phosphor, and there are two other ones in two globes at the playground at Qiryat Benjamin neighborhood.
Today, I exited to the area of Zebulun/Shivat Tsiyon, where the cycling mania occured in the past, in order to see the state there. I was interested to see the spectrum of the cycling elliptical diffused HPS lamp inside the AEG Koffer 150 during the hot restrike that I've seen millions of time. So I see its spectrum with my CD-R, only to find that this lamp have a phosphor.
During the glow, only the sodium D line appeared, and when the lamp hot restruck, the sodium D line remain unchanged, the mercury lines and a continum background (Which I don't know what it is) appeared. But I also seen a suspicious red emission in the spectrum, which looks exactly like an Eu: YPV phosphors emission.
This is the who know what time that I've see a phosphor inside a HPS lamp. But this is only recently.
Is this a recent trend to use a phosphors coating in HPS lamps?

That is strange, because the phosphor need the near UV to become excited, while the HPS discharge does not radiate virtually anything else than the broadened sodium D (so quite far from the area, where the phosphor could become excited)...

Moreover the Eu radiate in red, where the (broadened) sodium is quite rich on itself, so extra red would make the light even worse...

Only one possible reason came to my mind: If the lamp isn't in fact a "non-cycling" kind, where at the end of the life the sodium is gone, so only the buffer gas remain. And without the sodium, this could be quite rich in UV, so the phosphor could make sense to boost the efficacy (so light output, so it illuminate at least somehow) in this EOL mode. Otherwise I have no idea about the purpose.

The lamp is cycling: http://www.lighting-gallery.net/gallery/displayimage.php?pos=-87464
Probably this is a chinese generic lamp. But it is very redded out.

Considering their likely origin it's possible they have been made on the same production line as mercury lamps to save costs. A waste of valuable phosphors in any case.


BG

The common production line would not explain that, you may easily just not fill the phosphor reservoir, or use just a diffuser (silica,...) instead of the phosphor, the coating machine don't care.
So either it has some purpose (which is a mystery for now), or it is just an error on the manufacturing floor (someone filled the tank, while it should have been empty, or filled it by an MV phosphor instead of the diffuser).

I would rather guess for the error...