Most high pressure lamps use a barium getter in the outer envelope (the silvery coloured part on the envelope near the base).
This can occasionally cause problems due to the fact that this barium film is elecrically conductive. This was a far, far greater problem with low pressure sodium lamps however, where the problem could be exacerbated by the fact that there's a semiconductor layer over the inside of the tube as well - which could be electrified as well as the getter itself, and cause sodium to be drawn from the arc tube, causing severe blackening of the semiconductor film - in addition to the basic problems of the getter being electrified and potentially causing seal failure through electrolysis of the glass...
Some newer lamps use a different getter type, a zirconium-cobalt pellet which eliminates the need for the old style getter being flashed onto the lamp envelope. Some companies which are more concerned than average about the quality of the vacuum even use both types, Iwasaki for example do this in their SDX range of White SON lamps
I've included a picture of both getter styles below. Images are of an Iwasaki EYE NHT-150SDX
Barium Getter. Silvery area right above the lamp cap.
Zirconium-cobalt getter. The small "tab" attatched to the leadframe with a circular raised area on it near the centre of the image.
