The filaments of ordinary bulbs gradually evaporate,
cross the vacuum, and condense on the glass.

Halogen bulbs counter this failure mechanism by
the addition of a little halogen gas.

Tungsten vapour combines with the halogen to form
a tungsten halide, which diffuses toward both
the quartz glass and filament.

The hot filament tears them apart, returning
the tungsten to the filament, and releasing the
halogen. Clever huh?

The lamp is run hot enough to ensure the halide
doesn't condense on the quartz glass.
Quartz glass handles the temperature better.

Simply lowering the power is not a good thing.
They were designed to run hot for a reason.

Running cooler for long periods of time,
I suspect the halide will gradually accumulate
on the quartz glass. I _think it is transparent,
not opaque like tungsten film.

I _think_ you may be able to get away with
occasional dimming, if you run it a 100%
for a while every so often. This may boil
the halide off the glass and thus the
tungsten can return to the filament.