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.

The urban legend about halogen lamps is that reducing the voltage even slightly
causes the filament to burn out prematurely. The reasoning is that the slight
drop in temperature reduces the halogen self-healing effect much more than it
reduces the evaporation of the filament. I believe this is correct.

Thanks for the references.

Now... Does anyone know anything about helium reducing the life of incandescent
lamps? grin


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

They claim a 5% voltage increase will reduce life by 50%, which is
more like the 13.5th power.

The one time I checked the 12th power approximation against actual
testing of low wattage high voltage (mains) lamps it was off by more
than an order of magnitude, so take the whole thing with a grain of
salt, IMHO, unless your lamp type matches the type used for testing.
I'm sure a real lamp specialist could go on for hours about this sort
of thing.