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Halogen Light Bulbs

What is the difference between a halogen and an incandescent light bulb?

Halogen bulbs are technically incandescent light bulbs - illumination is produced in both when a tungsten filament is heated sufficiently to emit light or "incandescence." The difference between the two is in the composition of the glass envelope and the gas inside the envelope.

A standard incandescent bulb has a heat sensitive glass envelope that contains an inert gas mixture, usually nitrogen-argon. When the tungsten filament is heated it evaporates and deposits metal on the cooler glass envelope (this is why incandescent bulbs appear black at the end of life). Eventually enough tungsten evaporates causing the filament to break.

Halogen light bulbs utilize a fused quartz envelope ("capsule") allowing for higher temperatures  -  at least 500°F. Inside the quartz envelope is a gas, usually bromine. The tungsten filament evaporates as usual but the higher temperatures are sufficient to cause the tungsten to mix with the gas vapor instead of depositing on the bulb as with incandescent. Some of the evaporated tungsten is re-deposited on the filament. The combination of this "regenerative cycle" and higher filament temperature results in a bulb that has a longer life and slightly higher efficiency than standard incandescent bulbs. The higher temperature filament also produces the "white" light often associated with halogen bulbs. The color temperature of halogen ranges from 2850K to 3200K compared to incandescent at 2700K.

Where does the term halogen come from?

Halogen is the name given to a family of electronegative elements, including bromine, chlorine, fluorine and iodine. Halogen bulbs are referred to variously as "tungsten halogen," "quartz halogen," or simply "halogen." The term "J" bulb (some manufacturers use the letter "J" in their halogen bulb designations) probably comes from the German word "Jod" which in English is "iodine," one of the elements in the halogen family.

Does dimming affect halogen light bulb performance?

The answer, perhaps unexpectedly, is maybe. One of the assumed benefits of halogen technology is that it does everything incandescent does with the added benefit of longer bulb life and consistent bright appearing light. Halogen bulbs generally last longer because halogen gases like bromine and iodine cause the evaporating tungsten filament to deposit back on the filament rather than on the bulb glass as happens with incandescent. By regenerating the filament, halogen bulbs have longer rated life and better lumen maintenance than incandescent. The halogen cycle requires high temperatures, above 500°F. That is why the light appears bright and why quartz capsules are used to contain the tungsten filament and gas. When halogen bulbs are dimmed, the lower voltage results in lower temperatures in the quartz capsule. If a halogen bulb is dimmed below 60% of full light output, the benefits of the halogen cycle are compromised. Evaporated tungsten now may blacken the capsule wall instead of regenerating the filament. Brightness will be reduced along with expected bulb life.  Here is a tip: after dimming your halogen bulbs to a low light level for an extended period, before you turn them off, burn them at full light for a few minutes. This should help clear any dark deposits on the capsule.