# What gas is in a halogen light bulb
**URL:** https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872
**Category:** Factual Questions
**Created:** [December 31, 2005, 6:31pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872 "2005-12-31T18:31:07Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@ticker](https://boards.straightdope.com/u/ticker)
#### Post date: [December 31, 2005, 6:31pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/1 "2005-12-31T18:31:07Z")
I am currently watching the celebrity special edition of Who Wants To Be A Millionaire (the UK version). The last question was ‘What gas is commonly found in light bulbs’ and the possible answers were:
a) Argon
b) Chlorine
c) Methane
d) Hydrogen
The correct answer was Argon but I was wondering if Chlorine might not also be a valid answer. How Stuff Works only mentions that halogen bulbs use a gas from the halide group, not which one(s) are used in practice.
So, would Chlorine have been an alternate correct answer?
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### Author:  [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)
#### Post date: [December 31, 2005, 6:37pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/2 "2005-12-31T18:37:57Z")
Well, a halogen light generally has an inert gas like argon, and a halogen gas, like bromine or iodine. I couldn’t find any references to chlorine being used as the halogen gas in a halogen light, so absent some more knowledgeable doper coming in and showing where to get halogen lights which contain chlorine gas, I’d say that chlorine would not have been an alternate correct answer.
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### Author:  [@astro](https://boards.straightdope.com/u/astro)
#### Post date: [December 31, 2005, 6:38pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/3 "2005-12-31T18:38:42Z")
> [@ticker](#):
>
> I am currently watching the celebrity special edition of Who Wants To Be A Millionaire (the UK version). The last question was ‘What gas is commonly found in light bulbs’ and the possible answers were:
>
> a) Argon
> b) Chlorine
> c) Methane
> d) Hydrogen
>
> The correct answer was Argon but I was wondering if Chlorine might not also be a valid answer. How Stuff Works only mentions that halogen bulbs use a gas from the halide group, not which one(s) are used in practice.
>
> So, would Chlorine have been an alternate correct answer?
[IIRC chlorine is a fairly corrosive & noxious gas](http://en.wikipedia.org/wiki/Chlorine). Doesn’t seem like good bulb filler material.
> [@](#):
>
> Chlorine (from the Greek language Chloros, meaning “pale green”), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17. As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including the human body. As chlorine gas, it is greenish yellow, is two and one half times as heavy as air, has an intensely disagreeable suffocating odor, and is exceedingly poisonous. In its liquid and solid form it is a powerful oxidizing, bleaching, and disinfecting agent.
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### Author:  [@Tevildo](https://boards.straightdope.com/u/Tevildo)
#### Post date: [December 31, 2005, 6:41pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/4 "2005-12-31T18:41:16Z")
Even if some halogen lamps use chlorides rather than iodides, “Chlorine” would not be a correct answer - the bulbs might contain the element chlorine, but they don’t contain chlorine gas as such.
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### Author:  [@spingears](https://boards.straightdope.com/u/spingears)
#### Post date: [December 31, 2005, 7:28pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/5 "2005-12-31T19:28:28Z")
What gas is in a halogen light bulb?
> [@ticker](#):
>
> I am currently watching the celebrity special edition of Who Wants To Be A Millionaire (the UK version). The last question was ‘What gas is commonly found in light bulbs’ and the possible answers were: a) Argon b) Chlorine
> c) Methane and, d) Hydrogen
> The correct answer was Argon but I was wondering if Chlorine might not also be a valid answer. How Stuff Works only mentions that halogen bulbs use a gas from the halide group, not which one(s) are used in practice.So, would Chlorine have been an alternate correct answer?
NO! Not If you are considering ordinary incandescent bulbs as was asked on the TV program.
Lets not mix apples and oranges.
Halogen lamp bulbs use one and maybe more of the following Halogen gasses to prolong the life and increase the temperature and hence the light output.
- Fluorine \* Chlorine \* Bromine \* Iodine and \* Astatine
[\*\* How a Halogen Lamp Works to Live Longer \*\*](http://home.howstuffworks.com/question151.htm)
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### Author:  [@Tevildo](https://boards.straightdope.com/u/Tevildo)
#### Post date: [December 31, 2005, 7:36pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/6 "2005-12-31T19:36:33Z")
> [@spingears](#):
>
> Halogen lamp bulbs use one and maybe more of the following Halogen gasses to prolong the life and increase the temperature and hence the light output.
>
> - Fluorine \* Chlorine \* Bromine \* Iodine and \* Astatine
**Astatine**? The rarest of all elements, one of the most radioactive? I doubt it, somehow. 🙂 Indeed, have more than a few ug of At-211 ever been present in the same place at the same time?
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### Author:  [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)
#### Post date: [December 31, 2005, 8:11pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/7 "2005-12-31T20:11:30Z")
> [@spingears](#):
>
> Halogen lamp bulbs use one and maybe more of the following Halogen gasses to prolong the life and increase the temperature and hence the light output.
>
> - Fluorine \* Chlorine \* Bromine \* Iodine and \* Astatine
_That_ has already been established. We teeming millions now wish to know _which_ halogen gases are used.
And as **he Prince of Cats** (great to have you here as a member, **Tevildo**!) has noted, I hope you are not claiming that Astatine is used for this purpose.
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### Author:  [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)
#### Post date: [December 31, 2005, 8:14pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/8 "2005-12-31T20:14:17Z")
Incandescent lamps contain argon.
Halogen lamps contain either iodine or bromine.
Therefore, chlorine is an incorrect answer in any case.
What about the other halogens?
Chlorine and fluorine, in addition to being toxic and reactive, would be unlikely to work in any known design of halogen lamp. Being the most electronegative elements, fluorine and chlorine would bind too strongly to the tungsten vapor and and therefore disrupt the re-deposition process, causing the bulb to fail quickly. (also they’re highly toxic and reactive, which makes them less attractive for consumer applications).
The only remaining halogen candidate would be astatine. This wouldn’t work because it is not only one of the rarest elements on earth, but also radioactive. Its longest-live isotope has a half life of just a few hours.
So, that’s why halogen lamps only use iodine or bromine.
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### Author:  [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)
#### Post date: [December 31, 2005, 8:17pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/9 "2005-12-31T20:17:51Z")
Thanks, **Brain Wreck**!
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### Author:  [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)
#### Post date: [December 31, 2005, 8:23pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/10 "2005-12-31T20:23:24Z")
> [@astro](#):
>
> [IIRC chlorine is a fairly corrosive & noxious gas](http://en.wikipedia.org/wiki/Chlorine). Doesn’t seem like good bulb filler material.
N.B. - so are bromine and iodine, just slightly less so. Trust me, you get a snoutful of any halogen gas, you’re going to be regretting it for days at least. (Can’t vouch for astatine for reasons already discussed). In lamps I’m guessing they consider it safe enough because it’s in a very lean mix with an inert gas.
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### Author:  [@Tevildo](https://boards.straightdope.com/u/Tevildo)
#### Post date: [December 31, 2005, 9:18pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/11 "2005-12-31T21:18:59Z")
Can I just clarify one point? A normal halogen lamp actually contains iodine or bromine gas, rather than just tungsten iodide/bromide?
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### Author:  [@Tevildo](https://boards.straightdope.com/u/Tevildo)
#### Post date: [December 31, 2005, 9:22pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/12 "2005-12-31T21:22:12Z")
And, while we’re on the subject - why can’t you touch them with your bare hands? What happens if you do? I appreciate it’s because the bulb is made of quartz rather than glass, but what are the consequences of getting fingerprints on it?
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### Author:  [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)
#### Post date: [December 31, 2005, 9:25pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/13 "2005-12-31T21:25:16Z")
The salts and acids present in sweat would etch the glass and cause premature failure.
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### Author:  [@ticker](https://boards.straightdope.com/u/ticker)
#### Post date: [December 31, 2005, 9:37pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/14 "2005-12-31T21:37:17Z")
Group I metals, including sodium ions in sweat from fingers, act as network modifiers in glass which makes the molten glass less viscous and melt at a lower temperature, reducing manufacturing costs. This is why why most glass items are made of soda glass. Soda glass also undergoes much greater thermal expansion so quartz glass is used in halogen lamps which operate at higher temperatures. If you get salt from your fingers on the glass you create a region where thermal expansion is greater than the rest which leads to cracking.
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### Author:  [@ticker](https://boards.straightdope.com/u/ticker)
#### Post date: [December 31, 2005, 9:44pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/15 "2005-12-31T21:44:09Z")
> [@spingears](#):
>
> Not If you are considering ordinary incandescent bulbs as was asked on the TV program.
The TV show did not specify ‘ordinary’ bulbs, just light bulbs. I was wondering if this could have been ambiguous given the four possible answers. It appears not (thanks everybody), but not because halogen bulbs were excluded by the question as posed.
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### Author:  [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)
#### Post date: [December 31, 2005, 11:51pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/16 "2005-12-31T23:51:41Z")
> [@Tevildo](#):
>
> Can I just clarify one point? A normal halogen lamp actually contains iodine or bromine gas, rather than just tungsten iodide/bromide?
It appears that some do, some don’t, and some have a combination. I had a heck of a time digging up a reference. Here, apparently, is the one site on the entire internet that isn’t content to hand-wave over the magic “halogen gas” ([link](http://www.sylvania.com/ConsumerProducts/AutomotiveLighting/Products/Halogen/HowHalogenWorks.htm) )
> [@](#):
>
> Atmospheres of tungsten-halogen lamps comprise an inert gas with about 0.1% to 1.0% of a halogen vapor added. The inert gas may be xenon, krypton, argon, or nitrogen, or a mixture (e.g., krypton/argon or argon/nitrogen) having the highest atomic weight consistent with cost as well as arc-resistance suitable to the lamp design and the operating voltage. The halogen vapor may be pure iodine (I2) or a compound of iodine (e.g., CH3I) or of bromine (e.g., HBr, CH3Br, or CH2Br2). Iodine is still used in long-life lamps for general illumination but bromine is now used in most tungsten-halogen lamps, especially those for photographic and reprographic applications. The minimum bulb wall temperature for operation of the halogen-cycle is about 200°C for bromine which is significantly lower than the 250°C for iodine. Bromine is also colorless while iodine has a very slight absorption in the yellow-green.
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### Author:  [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)
#### Post date: [January 1, 2006, 7:58am UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/17 "2006-01-01T07:58:05Z")
> [@Tevildo](#):
>
> And, while we’re on the subject - why can’t you touch them with your bare hands? What happens if you do? I appreciate it’s because the bulb is made of quartz rather than glass, but what are the consequences of getting fingerprints on it?
This is also true of the incandescent bulbs used in stage lighting – you don’t touch the glass of the bulb with bare hands, because the oil from your hands left on the glass can cause the bulb to burn out much faster than normal.
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### Author:  [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)
#### Post date: [January 1, 2006, 3:59pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/18 "2006-01-01T15:59:27Z")
Regarding the handling of the lamps, I don’t know exactly who says that or why. I do know that glass is transparent, fingerprints are not. So compared to the clean glass adjacent to it, the fingerprinted glass would get hotter and undergo different rates of expansion. Over time this could cause accumulation of micro-cracks and eventually speed up the failure of the bulb. I know this is a concern for high-energy laser optics, I’m just guessing it applies to halogen bulbs as well.
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### Author:  [@Padeye](https://boards.straightdope.com/u/Padeye)
#### Post date: [January 1, 2006, 6:04pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/19 "2006-01-01T18:04:18Z")
> [@Brain Wreck](#):
>
> Incandescent lamps contain argon.
Ordinary incandescent lamps are typically filled with vacuum. That’s why household bulbs pop when broken.
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### Author:  [@psychonaut](https://boards.straightdope.com/u/psychonaut)
#### Post date: [January 1, 2006, 8:14pm UTC](https://boards.straightdope.com/t/what-gas-is-in-a-halogen-light-bulb/337872/20 "2006-01-01T20:14:39Z")
> [@Tevildo](#):
>
> And, while we’re on the subject - why can’t you touch them with your bare hands?
Because they get very, very hot.
> [@Tevildo](#):
>
> What happens if you do?
You can get severely burned.
😃
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