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I'll take a hack at this.  I thought it would be easy to just look up the 
luminous efficacy (lumens per watt, lm/W) of a candle, but I couldn't find 
it in my usual references.  The closest thing I found, in the IES Lighting 
Handbook, was that an acetylene torch flame produces 0.2 lm/W.

Just for an estimate, a candle puts out ~ 10 lumens, and probably burns at 
50-100 Watts, giving ~ 0.1-0.2 lm/W.  But a candle flame is optimized for 
producing light, so I'd guess even a roaring campfire produces somewhat 
less than 0.1 lm/W, and much less as it cools.

For comparison, incandescent bulbs produce 10-20 lm/W, and fluorescent 
tubes and the latest LEDs achieve 50-100 lm/W. Low pressure sodium lights 
are the efficiency winners,  producing 200 lm/W.

The theoretical maximum luminous efficacy obtainable would be 683 lm/W, 
with a monochromatic light of 555 nm wavelength (a greenish yellow).  For 
broad spectrum white light, the theoretical limit is about 220 lm/W 
(because wavelengths other than 555 nm are less efficient for human vision 
- note that SI photometric units are the only SI units that incorporate 
human biology).

-Richard

At 07:42 AM 10/18/2005, you wrote:

>ISEN-ASTC-L is a service of the Association of Science-Technology Centers
>Incorporated, a worldwide network of science museums and related institutions.
>*****************************************************************************
>
>duck!, here's a wild pitch:
>
>How many lumens per watt does a campfire produce?
>
>It was suggested that I start by figuring out the lumens per watt for a 
>candle, because conversion from (Thee Standard) candle to lumens is fairly 
>straight forward.
>
>Of course, I can't be sure a candle consumes watts of energy the way a 
>burning log does, but, you know, I just want a number--an interesting--if 
>fuzzy--statistic to help compare and contrast various light sources.
>
>Hints and suggestions?  Words of encouragment/discouragment?  Thank you 
>very much,
>
>Jason
>
>
>__
>JasonJayStevens
>Exhibits Design + Fabrication at the Ann Arbor Hands-On Museum
>[log in to unmask]
>www.aahom.org
>
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