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Re: Lighting question. . .

At 15:39 -0400 4/10/97, Edziu Iskra  wrote <edziu at cybernex_net>:

>My understanding, albeit from a non-technical source (a non-aquarist
>lighting design manual which did not go into technical specs {meant for
>contractors and designers}), was that in terms of lumens per watt, a 12v
>halogen system was markedly more efficient than a 120v system.  From my
>understanding of lighting systems, I believe this efficiency may have to do
>with the fact that 5 20 watt incandescent bulbs will produce more lumens
>than 1 100 watt bulb.  Apparently, this effect overcomes the necessary loss
>of running the transformer, so that, in sum, a 12v system is significantly
>more efficient than a 120v.  If anyone can explain where my reason is wrong,
>please do so, and if anyone can find lumens/watt or similar ratings for 12v
>vs. 120v systems, please do.
>
>                        Edziu

Does not make sense to me. An incandescent light emits light from the
filament simply because the filament is hot. You can make a long thin wire
just as hot using 120 volts as a short fat wire using 12 volts. Energy is
lost from the wire by thermal conduction and electromagnetic radiation. The
shortwave radiation is called light and the longer wave stuff is heat. We
want the light, not the heat. Heat is conducted away from the filament by
the filament posts, filament supports if used, and the halogen gas in the
envelope.

As the temperature of the filament goes up, the ratio of light to heat
increases, until such time as the filament destructs. Running the filament
hotter results in greater effiency but shorter life. So if your supposition
is true then there are two possible explanations, or a combination of the
two:

1) 120 volt filaments possibly run at a lower temperature.
      and/or
2) 120 volt filaments possibly lose more heat through conduction, to the
envelope and/or to the halogen gas.

I can't see any inherent reasons why, except possibly that a long thin
filament might have to run cooler to maintain it's mechanical strength,
that either of the factors would be significant.

Incidently, there's a reason low voltage lights are used in vehicles. Short
fat filaments are mechanically stronger than long thin ones. Even on
aircraft with 400 cycle 120 volt power, 28 volt landing lights are used.

Why even bother with halogens, if you care about effiency get flourescent
or metal halide.

Paul