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Author Topic: I'd like a good description of how a halogen bulb works.  (Read 12157 times)

Offline Lightbulb Collector

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I'd like a good description of how a halogen bulb works.
« on: September 21, 2008, 09:37:36 pm »
Whenever I see a description of the halogen bulb, it says about how the chemistry of the gas of the bulb using iodine gas keeps the filament in tact for a longer time. But what I've NEVER seen explained is why they use the gas at high pressure. On halogen bulb packages you see tons of warnings about bulb explosions due to bulb pressure. But WHY is such a high pressure required?

Offline Max

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Re: I'd like a good description of how a halogen bulb works.
« Reply #1 on: September 22, 2008, 03:34:50 pm »
The high pressure is not required for the halogen cycle to work, and in fact there are plenty of halogen lamps (most with bromine, not iodine filling) that do work a lower pressures and do not present any serious explosion hazard.
Higher buffer gas pressures can however improve lamp performances by reducing the evaporation rate of the filament. Why is that so critical since the halogen cycle brings tungsten back to the filament? Pressure can make a difference simply because the halogen cycle does not bring tungsten atoms back precisely to where they came from - meaning that there is still a transport phenomenon that will eventually lead to a part of the filament becoming thinner than the rest, and there will come the hot-spot failure. Lamps with higher rare-gas pressures, and/or filled with heavier gases (such as xenon) can be driven at higher temperatures (i.e. at higher lumen efficacies) at no or little cost to the lifetime.

Offline James

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Re: I'd like a good description of how a halogen bulb works.
« Reply #2 on: October 31, 2008, 07:55:59 am »
Also contrary to popular belief, many people believe that the halogen lamps are brighter and more efficient because of the halogen filling.  This is not true!  To explain how they really work, consider the following examples.

Starting with the ordinary incandescent lamp, we are all familiar that the filament evaporates during life causing the bulb to blacken.  When the filament has become sufficiently thinned at one point, it will fail.  Either because it can no longer support its own weight, or because the temperature at that point will increase and cause it to melt.

The blackening of the bulb is a problem, because it causes the light output to decrease during life.  In order to reduce the significance of the blackening, the filament has to be contained in a very large diameter glass bulb.  The larger the bulb diameter, the thinner will be the coating of tungsten that builds up over its surface, and the higher will be the light transmission.  Obviously however, it is commercially desirable to make lamps with smaller bulbs for numerous reasons.  The problem is that the bulb cannot be made too small, otherwise it will blacken too fast and the light output will drop too rapidly.

It is this problem, and this only, which is solved by the halogen gas filling.  The halogen cycle prevents the deposition of the evaporated tungsten on the bulb wall, instead bringing it back near to the colder ends of the filament.  So the advantage of a halogen-filled bulb is that the blackening is stopped and we can achieve nearly 100% lumen maintenance during life.

Since the blackening problem has been solved, there is no longer any requirement to maintain the traditional large diameter glass bulbs.  The volume of a halogen-filled lamp can be reduced well below 1% of the volume of a corresponding wattage of incandescent bulb.  When the bulb is made smaller it automatically becomes physically stronger.  This means that it is capable of withstanding much higher gas filling pressures.  Higher gas fill pressure is very important because it exerts a strong force on the the filament and reduces the rate at which tungsten atoms will boil away from its surface.  It is consequently the effect of high gas pressure which is responsible for increasing the lifetime of the halogen lamps. 

On top of this, because the bulb is so small, we can afford to fill it with much more expensive gases than are commercially viable for ordinary incandescent bulbs.  GLS lamps are filled with an argon:nitrogen mixture.  Volume for volume, you can liken the price of this gas with that of water, it's relatively cheap.  A better gas, however, is krypton.  Krypton has larger atoms which are successful in further reducing the rate of evaporation of tungsten from the filament.  More importantly its thermal conductivity is somewhat lower than argon, so it keeps the filament hotter.  The hotter filament is repsonsible for increasing the efficacy of these lamps. However you can liken the cost of a volume of krypton with that of milk.  Its rather expensive to fill 60mm diameter incandescent lamp bulbs with Krypton gas!  However for the small size halogen capsules, where less gas volume is required, it can be justified.

Taking things a stage further, in some special halogen lamps we can even consider the use of Xenon gas fillings.  Xenon has a still lower thermal conductivity than either Krypton or Argon, and also a higher atomic weight.  This is the best gas filling to fabricate high efficacy lamps of increased lifetime.  Xenon fillings are occasionally used in the current family of new energy-saving halogen lamps which are appearing on the market.  Unfortunately though, you can liken its cost per litre to that of a finest whisky!  And that cost is spiralling upwards out of control.  During the past 12 months we have seen the price of Xenon increase by around 400% due to massive increase in the rate of consumption (especially by the aerospace industry).  It is likely that it may soon become too expensive for use even in the smallest energy-efficient halogen lamps, and we will be limited to Krypton fillings with somewhat lower efficacy (but still better than argon of course!).

The best halogen lamps are those filled to high pressures.  Low pressure versions do exist on the market, but should only be used when absolutely essential (e.g. in open fixtures, where a lamp with no explosion risk is necessary).  Low pressure halogen lamps typically have half the life and 10-20% lower lumen output than their high pressure counterparts.

So to summarise:
1) The role of the halogen is to keep the bulb wall clean
2) The role of the high pressure filling is primarily to increase lamp life
3) The role of the high atomic weight gases is primarily to increase lamp efficacy