What is HALOGEN BULB?
Tungsten halogen cycle bulbs have tungsten filaments and are halogen gas filled, and require high temperature quartz glass.
These specially designed bulbs exhibit unique traits over standard Incandescent bulbs.
Directly summed up; glass remains clear (doesn't darken) and filament life is doubled.
What this really means, is lumen output over the life of the bulb remains high (brighter) so that the Lumens Per Watt (efficiency) remains high and the color of light remains and stays whiter.
Also, bulb lasting twice the life is an obvious benefit.
Halogen is the main family element name which include iodine, bromine, chlorine, fluorine and astatine.
These days, bromine is now the most commonly used halogen element in Tungsten halogen cycle bulbs.
The Tungsten halogen cycle is as follows...
Tungsten metal filament is heated by electricity to point of incandescence (emitting photons).
A few tungsten particles evaporate (boil off) from the filament and are carried away by convection current where they combine with bromide vapor forming tungsten bromide.
If the glass (quartz) wall is above 250 degrees centigrade, then tungsten bromide particles will not adhere to glass.
They continue to circulate in the hot gas envelope by convection current and when they come back close to the hot filament the particles reduce back to tungsten metal which is randomly re-deposited back onto the filament thereby releasing the bromide vapor, and the whole process then repeats itself.
If every tungsten particles happened by chance to land back exactly where it came off, the filament could last forever.
But, they don't and eventually some parts of the filament get thinner and eventually burn out, so good news is filament life usually is double over common incandescent bulb.
For the Tungsten halogen cycle to work, the bulb has to be on long enough to heat up completely and keep the glass at over 250 degree centigrade. To make this happen, manufactures design their bulbs positioning high temperature quartz glass as close to the filament as possible (but far enough away so that the glass doesn't melt) and designing the entire gas envelope so that the coolest area's are at least 250 degrees centigrade. Any area inside the glass envelope which is lower than 250 degrees will attract tungsten bromide particles and the tungsten molecule will revert and cling tightly to the cool areas on the glass and cool internal supports thereby causing the Tungsten halogen cycle to fail.
Obviously short cycling the bulb or using a dimmer switch on the bulb (running the bulb at lower than design voltage) will keep the bulb from fully heating up causing the cycle to fail; where then bulb will then give only the same service as a common inexpensive incandescent bulb. Even drafts or too much forced cooling can prevent the cycle.
An interesting fact is that if the bulb's glass at a later time gets heated to the point of re-vaporizing the tungsten soot build-up, then the tungsten vapor will re-combine with the bromine molecules and the tungsten bromine particle will be carried off again by the convection current to be re-deposited back on the tungsten filament, thereby eventually completing the cycle.
Tungsten halogen cycle bulbs make no sense for use as automobile brake light or turn signals as they would never be on long enough to heat up to cycle temperatures.
Not all halogen bulbs are designed to take advantage of the Tungsten halogen cycle. Some deceitful manufactures advertise halogen bulbs by putting halogen gas in otherwise standard Incandescent bulbs, which serves no benefit other than confusing you into buying a common incandescent bulb at premium price.
references for this page: HALOGEN BULB, HALOGEN, HALOGEN BULBS, QUARTZ HALOGEN, TUNGSTEN HALOGEN, TUNGSTEN HALOGEN CYCLE
references to this page: /U, GLASS, INCANDESCENT BULB, LUMEN, PAR LAMP, RE-RATE