I'm not certain of all the details, but here's a quote from
Ken Archibald, National Safety Directory for the American Gold Wing
Association.
http://www.christmasinjuly.org/safety16.html
>>>
As tungsten evaporates from the filament, it usually condenses on the inner
surface of the bulb. The envelope gas reacts chemically with the tungsten
deposit of the glass to produce tungsten halides, which evaporate more
easily. The tungsten halide reaches the filament while it is very hot
causing the halide to break down and release tungsten back to the filament.
This cycle tends to be self-cleaning for the glass, and allow the bulbs to
stay fully bright throughout the age of the bulb. Two operating parameters
of this bulb are increased to give off more light, a whiter light. The
operating temperature is increased and the pressure of the gas inside the
bulb is increased. In order for the replenishment cycle to work the
operating temperature must be high.
<<<
My understanding is that, if the voltage is too low, the filament does not
reach the temperature necessary for this cycle to work, and the evaporated
tungsten does not get recycled back to the filament.
All I know for certain is that the very first set of H4 bulbs I put in Dad's
TR3A only lasted about 6 months. I no longer recall who told me to add
relays, but I did, and the next set of bulbs lasted much longer (may still
be in the car <g>).
Randall
Phil Ethier wrote :
>
> >The other reason they're a good idea is that, unlike ordinary
> >incandescent bulbs, halogen bulbs do not 'like' low voltage, it will
> >shorten their life.
>
> I did not know that. I'd be very interested in the mechanism by
> which this
> happens.
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