Regarding the Hg lamp explosions, a couple of further points may be of
interest in this vein. First, in the 100 Watt Hg lamps the electrode
spacing is very close and if the reflected arc image should fall onto one
of the electrodes there will certainly be overheating of the electrode and
chance of explosion.
 
A second point is that the power supplies used can be a large influence on
the lamp heating. The arc voltage varies with operating temperature and
always increases (at a given current) as the electrodes burn back with
age. In power supplies that regulate to a fixed lamp current there will be
a continuous increase in the power dissipation of a lamp over time with
the possibility of overheating. Osram specifies a maximum temperature of
the lamp bases as 230 C (200 C is better).
 
It sounds like most of these failures that have appeared on the list have
occurred in equipment with a track record where only the lamp had been
changed recently: is this correct? This would indicate a difference in
the lamps that are being supplied.
 
 
Sincerely,
 
Dale A. Callaham
Central Microscopy Facility
University of Massachusetts
Amherst, MA 01003 USA
 
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