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12V versus 120V bulbs

>Date: Thu, 10 Apr 1997 19:33:12 -0700
>From: Paul Nicholson <paul at eisusa_com>
>Subject: Re: Lighting question. . . 
>At 15:39 -0400 4/10/97, Edziu Iskra  wrote <edziu at cybernex_net>:
>>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
>>                        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

Exactly, the problem is TEMPERATURE. Given wattage will make a longer
filament on a 120V (or even worse - 220V) systems, and shorter ones on 
12V systems (providing that a material with the same conductivity is used).
This means, the same power will be concentrated in a larger area (in 120V
system), thus a smaller temperature of the filament - 
both bulbs will be using same power, but spectrum will be more shifted to
infrared region (heat) at lower temperatures, and more towards the
ultraviolet region with higher temperature. 

It is easy to check this: a 12 V 20W bulb shines much brighter (in the
visible spectrum) compared with a 220V 25W bulb (which uses 20 percent more

So, indeed, using 12V bulbs will be more efficient (from the point of view
of growing aquatic plants versus simple heat dissipation) compared with
higher-voltage designs. 

But, mind you, halogens get VERY HOT, so you will have to make a special
design on your hood... 

Hope this helps,