So, I get the concept of a fluorescent tube or an HID lamp where the arc starts at an electrode at one end of a sealed tube and goes to the electrode at the other end.
How can this work if the tube is open to allow the gas to escape?
I assumed the heavy glass covers of early CFLs was purely decorative and to protect the inner tube, at least I have a Panasonic one that can even be unscrewed and removed.
In these the "heavy glass" is in fact the main envelope sealing the air out and the desired working mix (Ar, Kr, Hg,...) in.
The inner tubes are then just a kind of arc guide - form a designated path for the arc.
This concept has few advantages:
- The outer envelope is rather cool, so it is easier to maintain the desired Hg pressure in the arc (dictated by the temperature of either the coldest spot where the liquid mercury reside, or the mercury dosing amalgam pellet; here the low temperature of the outer may have been enough to suffice with at the time simpler and cheaper liquid mercury system).
- The phosphor is on the outer, so quite away from the otherwise aggressive arc chemistry (the ionized components tend to react with it), prolonging its life (so way slower lumen loss)
- The electrodes are encapsulated by the inner tubes without phosphor, which then could be narrower, so limitting the area affected by sputtering, so other factor improving the lumen maintenance.
But has quite a few major drawbacks:
- The volume containing Hg gas between the discharge and the phosphor is rather large, so there is quite high selfabsorbtion
- The glasswork is rather complex (two tubes sealed within a jar and seal that), so expensive. The open ends (instead of e.g. just a breather hole) are in fact motivated by having only one spot where these are fixed to other structures, so prevent stress buildup in the glass.
