The 18w Osram SOX lamp has now completed 3 years of dusk-dawn service on my patio.
All of the blackening aound the electrodes which was on the lamp from new has completey dissapeard and has now started to turn brown. I checked on this lamp several months ago and it had started turning white first.
There is also the very slightest hint of some orange staining around the bend at the bottom of the lamp.
There is no evidence of any sodium loss into the outer jacket at all.
The lamp is operated on a Parmar resonant start style ballast with no in-circuit ignitor.
Interesting to see the shadows of the electrodes from the bright sunlight.