🚀 go-pugleaf

Someone asked what the halogen gas did for these lamps.
Hope this helps . . .
(In this posting, "lamp" means light bulb, not a lighting
fixture).

A "halogen" lamp (also sometimes called a "quartz", "tunsten-
halogen", or a "Quartz-iodide" lamp) is basically the same 
as a regular incandescent lamp -- it produces light by heating 
a tungsten filament in a sealed envelope containing some gas.
It is still a form of incandescent lighting.

No ballast is needed for halogen lamps, although some small
fixtures use lower voltage lamps and need a step-down 
transformer.

The difference is in the envelope, which is made of quartz
to handle the high temperature operation needed; and in the
gas which fills the envelope.  The gas is a member of the
halogen family.  Regular incandescent lamps use an inert gas.

In a regular incandescent lamp, the tungsten "boils" off the
filament during operation.  It eventually contacts the cooler
glass envelope, where it deposits itself.  This results in
darkening of the envelope as the lamp ages, and in weakening
of the filament.

A chemical process (the halogen cycle) is used to improve this 
in these new fangled "halogen" lamps.  The tungsten chemically
bonds with the halogen gas in the envelope.  When the gas
contacts the hot filament, the chemical bond is broken, and 
the tungsten is re-deposited in the filament.  These lamps
won't last forever, because the tungsten isn't deposited
at the same spot it came off the filament.

This gives lamp designers 2 options:
- make the lamp produce brighter, higher color temperature
  light at the same life expectancy.
 -OR-
- make the lamp the same brightness with a longer life.

To maintain the halogen cycle, the lamp must run at fairly high
temperatures and pressures.  Dimming may reduce this somewhat,
and cause the envelope to darken, but operating a while
at full brightness will reverse the process.

Also, the quartz envelope shouldn't be touched by bare 
hands -- oils on the skin will weaken the structure and may
cause bubbles or lamp failure.

Tungsten-halogen (T-H) lamps are now used almost exclusively
for stage and television lighting applicatuions, where they
have been around for quite a while.  This is due to thier
longer life/higher color temp and the fact that light output
does not  decrease as the lamp ages, as it does with regular
incandescent lights.

Randy Brumbaugh
rando@skipper.dfrf.nasa.gov