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If a bulb is cooled by air flowing over it, would that change the way light is emitted?

I see contradicting evidence that air cooling bulbs changes how much light is emitted because the cooler air outside the bulb changes the chemical reaction temperature inside the bulb.

But then there are other sources stating that inside the bulb there is a vacuum that maintains a certain temperature, so it has nothing to do with its surroundings (obviously to an extent, not like we're in the Arctic).

LorenzoDonati4Ukraine-OnStrike
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evu
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  • It's great that you have done the preliminary research and found evidence (albeit contradicting evidence). Could you edit your question and add links to what you have found? That would provide more context to your question, which leads to better answers. – Nick Alexeev Aug 17 '15 at 22:37
  • A vacuum cannot have a temperature (to first approximation). Otherwise please clarify if you intend the air to flow over the filament or the surface of the glass. The outside glass surface is not realy involved with light generation, but the temperature can affect the diffusive coating (pale surface that makes the bulb into a soft light) and absorption profile of the glass. The vacuum isolates the filament from the glass so the light emission will not be affected. – crasic Aug 17 '15 at 22:41
  • OTOH For fluorescent lights, the light is generated on the inside surface of the bulb so glass temperature is very relevant. – crasic Aug 17 '15 at 22:47
  • Assuming that the air flow can lower the temperature faster than the heat from the light source can raise it. – Passerby Aug 17 '15 at 22:57
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    @crasic In [halogen light bulbs](https://en.wikipedia.org/wiki/Halogen_lamp) the space around the filament contains a small amount of halogen gas. The gas is one of the reagents in the chemical reaction (so called *transport reaction*), which preserves the filament. [Having said that, this question needs more context, which the O.P. should post.] – Nick Alexeev Aug 17 '15 at 23:10

2 Answers2

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The answer to this question is heavily context-dependent.

Short answer: it depends on which kind of bulb you are talking about.

As someone already pointed out some bulbs are filled with some gas, therefore ambient temperature variation can be transmitted to the internal environment of the bulb, thus the vacuum problem is essentially moot (probably the only bulb with no gas filling is the old edison-type incandescent bulb, which is going the way of the dodo due to legislation in many countries).

Anyway, incandescent bulbs most probably are unaffected by the external temperature because the light is produced by heating a filament to extreme temperatures (thousands of grades) hence any reasonable variation in external temperature is negligible compared with the filament temperature and the temperature of the gas the filament could be in contact with. Since the luminous flux in such bulbs essentially depends on the temperature of the filament and its geometry, it is unlikely that ambient temperature will affect their efficacy (lumens emitted per watt absorbed from the supply):

enter image description here

Here the black-body is the filament.

Bulbs working on other physical principles are another story.

In fluorescent tubes the temperature can effectively affect the luminous output, as you can see here. And also here:

enter image description here

LED bulbs light output is adversely affected by temperature too, a you can see here:

enter image description here

note that the graph shows the effect of junction temperature, not ambient. But ambient temperature will affect junction temperature, especially if proper heatsinking is not provided or is insufficient (for example you installed the LED bulb in a very recessed cabinet with no proper air flow).

LorenzoDonati4Ukraine-OnStrike
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Halogen bulbs depend on the envelope running very hot (600-900C according to this forensics report) to recover tungsten, otherwise it can end up deposited on the inside of the quartz or high-temperature glass and both light output and filament life will suffer. You can see this effect with halogen bulbs that have been operated with a dimmer for long periods of time.

Halogen bulbs with a double envelope and most other light sources including normal incandescent bulbs will be probably unaffected for practical purposes. See Lorenzo's answer for some well researched curves of light output vs temprrature.

I guess candle light or even carbon arc light could be blown out, but most would not call those sources 'bulbs'. Cooling an LED die will usually increase the light output somewhat.

Spehro Pefhany
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