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Re: (TFT) One-Hex Fire - how much light?

 >From: ErolB1@aol.com

 >Working over my rules for light sources (both magical and non-) I've come
with a question: How much light would a one-hex Fire spell (or
equivalent-sized non-magical fire) produce?

My initial w/i/l/d/a/s/s/e/d/ g/u/e/s/s/ estimate is that it should be
somewhere between 10x and 100x as much light as a torch, lantern, or Light
But I thought I'd ask the list: Does anyone have any better estimates?

First my guess, feel free to refute or improve on it.

For an "equivalent non-magical fire" I would scale (light produced) with (surface area of fire). So if the fire goes up in size from a torch (say a 15-cm blaze, measured horizontally through the widest part) to a whole hex sized bonfire (150 cm horizontally), surface area would go up by (150/15)^2, so the total light produced will be 100 times as great - or the equivalent of 100 torches all in one place. (However, 100 torches could be *much* better at lighting up a cavern, because they could be strategically placed to distribute the light better, eliminate shadows, etc.)

This assumes similar materials burning, visible light produced by same mechanism, etc (ie no special pyrotechnics or enhanced sodium content or whatever) between the torch and the bonfire.

Also, I could be conservative scaling by surface area rather than volume, as flames are basically transparent, so light produced in the interior of a bonfire is likely to escape. However, looking at a candle flame, the region which produces the most light is the surface of the flame, not the interior. This combined with the "inefficiency" of a single large light source vs. many small sources makes me want to stick with surface area (or even scale down some).

I would assume a "magic" bonfire is modelled on a "real" bonfire, and doesn't magically enhance heat output or filter out optical radiation, so I'd rate it just the same.

Date: Tue, 23 Sep 2003 02:58:51 -0500
From: "David Michael Grouchy II" <david_michael_grouchy_ii@hotmail.com>

DMG, much as I love your plot lines, I'd like to take issue with some of the scientific assumptions in the light reading :-), as below.

    1 lux = light needed to illuminate 1 square meter to 1 lux

Eh? This one seems circular. From the "Buyers Guide 2003" supplement to Physics Today,
	1 lux = 1 *lumen* / m^2

	where 1 lumen = 1 candela/steradian (oh great, *that* helps)

or, from http://whatis.techtarget.com/definition/0,,sid9_gci542011,00.html

	1 lux = 1.46 milliwatt / m^2

where the whole 1.46 milliwatt has to be visible light (555 nanometer wavelength, which (I think) is yellow).

Put another way, get a 1-watt (visible light output) bulb, focus the entire output onto a 1 m^2 panel, and you are hitting it with 683 lux average.

	The examples you gave were very clear.

SOURCE               LUX PER WATT (on 1m sq and 1m distance)

Units are funny again. I suspect it's just LUX, measured on a surface at 1 m distance from the source.

   Or put another way, we are talking about a fire that kills in fifteen

Gotta be careful. "Inflicts injuries which will prove to be fatal (and are immediately disabling)" may be pretty different from "turns to ashes" as a definition of "kill".

Spontanious combustion is defined as the tempurature were a
material burns without external oxygen and provides it's own fuel.

Maybe in magic terms. In physics, it means (loosely) the temperature at which a substance will ignite itself (even without a spark). It will still need oxygen (or some oxidizer - could be H2O2 or something) to burn, but because it could ignite pretty much all at once, it will burn *fast* once it starts.

celcius      material spontaniously combusts
  230            linseed oil
  200            leaves
  159            wood
  140            drying oil paints
  100            green hay
   95            cotton
   65 - 121      animal feed
   30 - 40       damp coal

Some of the numbers on this table are highly suspect. Damp coal will gang-fire at 30 C = 86 F !??!? On a hot summer day, Pennsylvania could erupt. Cotton in a double-boiler will explode??? I don't believe it.

Well, it turns out that things that burn hot tend to produce less light. The energy is converted to heat, not light.

Again, this is tricky. At high enough temperatures (like the filament in a light bulb, or above), most of the radiation is black-body radiation, which is produced just because the mass is hot. (Okay, hot -> atoms wiggling -> charges accelerating -> electromagnetic radiation generated. Hotter -> atoms wiggling faster -> higher-frequency radiation.)

For black-body radiation, the hotter it is the more radiation it produces. The majority of the radiation is emitted at a frequency proportional to the temperature, so very very hot things emit most of their radiation in the UV. *However* the total amount of radiation coming out is proportional to (Temperature)^4, and the amount emitted in the visible keeps going up even if the preponderance shifts to UV. The hotter it is, the brighter it looks to our eyes. See http://itl.chem.ufl.edu/4412_aa/origins.html

However, candles, oil lamps, etc. are not principally black-body emitters. The nice yellow light we see is mostly produced by sodium (and other) vapors. The atoms of sodium get electrons excited to a high-energy state either thermally (some atoms get more than their share of energy, just at random) or in reactions with Oxygen or other radicals in the flame. While returning to their base state, the excited electrons go through a transition that causes them to emit yellow light. This isn't very efficient - most of the emitted energy is in fact thermal - but it's enough to see by.

Hydrogen, methane, presumably linseed oil, etc. flames don't contain Sodium or other elements with good yellow-emitting transitions in their atoms' electron shell structure, so they don't make yellow light - unless they get really hot and start putting it out in black-body form. (You can, however, get light out of a methane flame by bubbling the methane through salt water before burning it so that it *does* contain some sodium.)

  linseed oil produces fewer LUX
than coal.  So if a magical fire burns exceedingly hot, one could reason
that almost no light is produced.

Doesn't necessarily follow. It's magic anyway, could produce light by design. Or if it gets above 2000 C or so, the *air* near it will get up into the range where its black-body emissions are visible, and start to glow. But I suspect it's not this hot.

   Now for the pseudo science.  Here are the various temps and times for
reducing a human body to ash.  ...  To do the same
work in a quarter of a minute would require most all the energy to go into
heat and not light.

Sure would. But I claim a fire kills by charring lung tissue or air passageways first - which would require *way way* less heat and time - and by skin trauma second - which would still require a lot less heat and time.

Medical experts?

  Magical fires may produce almost no light at all.  In
fact they may appear dark, even black, considering the supernatural temps
involved.  I would go with 0.00001 LUX.  Darker than a moonless overcast
night sky.

Even trying to assume the "ash case", the fire would have to be hotter than a crematorium (a *lot* hotter) to ash a body in 15 seconds, and the air around it would be up in the black-body glow region - so it'd still put out a lot of light, even though the "magic" part of the fire was somehow filtered to pure heat (or pure UV, whatever).

So I think a magic fire will still produce a lot of light, no matter how you cut it. And my preference is to have a magic fire appx. = a natural bonfire. But to satisfy my preferences, a magic (or natural) fire could *not* turn a body to ash in any short length of time - it'll just kill by surface damage (and lung and airway damage).

Apologies all if this is a long-winded rehash - I'm on digest mode.
					- Mark
			210-522-6025, page 888-733-0967
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