On Sun, 31 Aug 2003 10:38:58 GMT, Spehro Pefhany
wrote:

On Thu, 28 Aug 2003 12:44:54 -0700, the renowned "William Sommerwerck"
wrote:

Thanks for the confirmation of 12th power.

The 12th power approximation does originate at GE Lighting, AFAIK, but
is only valid for voltages rather close to the rated operating
voltage, and for typical high voltage incandescent lamps. Long-life
and halogen bulbs WON'T behave the same.

http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm


The problem with Halogens is LOW voltage reduces the life of the bulb
as well. The Halogen Cycle requires a minimum temperature in order to
re-deposit the tungsten on the filament.Low voltage boils the tungsten
off and deposits it on the glass envelope. Eventually the glass gets
dark and the filament gets too thin and burns out. Specified voltage
maintains proper temperature for thr reddepositing of tungsten on the
filament.
This operating heat is why quartz glass envelopes are generally used
for halogen bulbs.
This is copied from elsewhere on the web:

What is the difference between the internal conditions and mass
transports happening inside the noble gas fill, and the halogen
cycle incandescent bulbs?

Noble gases do not react with the tungsten vapor, leading to a layer
of semi-opaque condensed tungsten on the inner surface of the
bulb. Since the bulb is cooler than the boiling point of tungsten,
tungsten is gradually transfered from the filament to the bulb until
the filament burns out.

Halogens react with tungsten vapor, resulting in a layer of tungsten
halide on the inner surface of the bulb --- but since tungsten
halide is transparent, less light is absorbed. Also, tungsten halide
has a low enough boiling point that it can re-evaporate when the
bulb is hot enough. Finally, if tungsten halide molecules get close
enough to the fillament, they can disassociate back into tungsten
and halogen atoms, and the tungsten can be re-deposited onto the
filament, extending its lifetime, while the halogen goes back into
the fill gas.

There is an important elegance to the tungsten-halogen cycle. Because
the filament is a series resistance with a positive temperature
coefficient of resistance, any part that thins by sublimation runs
hotter than the rest. In a conventional bulb the heavy gas fill acts
to reduce sublimation and thermally insulate the filament so a given
power level gives more visible light. However, a localized thinning
gives positive feedback and failure.

In a tungsten-halogen bulb, sublimated tungsten reacts to give
volatile tungsten halides that thermally decompose and redeposit metal
at the hottest spots. The bulb does not darken from transported
tungsten. The hottest spots get rebuilt. Negative feedback allows a
filament to be run very hot indeed.

There is a price to be paid. The envelope must be made of fused
silica to take the high temperature. A mere trace of sodium (a
fingerprint) catalyzes crystallization of fused slica to cristobalite
at temp. When the envelope cools or heats it cracks from differential
coefficients of thermal expansion. When tungsten halogen bulbs fail
in use the results are often quite... attention-getting. Internal
pressure at operating temperature can be several atmospheres.
Tungsten halogen bulbs also have a pretty good UV component compared
to ordinary incandescents unless the envelope is doped with cerium or
such.