Others have dealt with your three cases, but there is an additional one:

- Same supply, but half-wave rectified.

My understanding is that this (perhaps significantly) increases the 
longevity of at least household-grade bulbs, at the expense of at least 
some lumens.  There were products on the market which basically fit in 
the light socket and contained a diode internally.  Not sure how well 
this actually worked, but worth a shot.

I will say that running bulbs even slightly below their rating 
significantly improves their lifetime.  Most long-life bulbs are 
actually 130V bulbs being run at 120V.

-forrest

Apptech wrote:
> Any thoughts on the longevity of tungsten bulbs running on
>
> - AC supply
> - Same supply but full wave rectified.
> - Same supply but full wave rectified and filtered to DC.
>
> The bulb is in fact two 12V bulbs in series running from a 
> nominal 16 VAC supply.
>
> In the second case the filament has a constant polarity bias 
> but is exposed to cycles of heating and cooling which 
> intuitively (to me) feel the same as for AC.
>
> In the third case the DC voltage will tend to be higher than 
> the RMS AC voltage so the bulb will operate at a somewhat 
> higher wattage.
>
>
>
>         Russell
>
>
>
>   

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