Halogen Lamp Technology
By David Dayton
The incandescent lamp technology about 1920 was tungsten filaments mounted in a glass bulb with a vacuum atmosphere. The tungsten evaporated and condensed on the bulb. The rated lamp life was considered to be when the light output had dropped to 70% of initial, the bulb should be replaced.
The next technological improvement was to replace the vacuum with a gas fill, argon and nitrogen. The gas fill retarded the tungsten evaporation but the bulb temperature increased due to the thermal conductivity if the gas fill. For safe use, the higher the lamp wattage the larger the bulb. With gas fill, the tungsten still evaporated and condensed on the bulb wall but now there was convection and most of the blackening was at the top of the bulb.
In 1953, the halogen incandescent lamp was invented by General Electric. The initial lamps were double ended tubular bulbs. A halogen was added to the fill gas. The halogen group is Iodine, Bromine and Fluorine. The early halogen lamps were Iodine lamps. Iodine was used because it is a solid at room temperature and the amount added was small but not a critical amount. During lamp operation, tungsten would condense on the inside of the bulb and the Iodine would combine with the tungsten forming Tungsten Iodide and that molecule would migrated back to the filament and decompose to Tungsten + Iodine and the tungsten would deposit back on the filament and the cycle would begin again. These lamps were in production for a few years but the process was difficult to automate for high production and cost.
During the Iodine lamp era, experimental Bromine lamps were made mixing a small amount of pure bromine vapor with the argon/nitrogen fill gas. The process worked but the precise control over the amount of Bromine required made the lamp manufacturing process unworkable.
Later it was found that bromine could be added to the fill gas in the form of CH2Br2. When the lamp was turned on, the following breakdown took place
CH2BR2 -------> C + H + H + Br + Br
The C (Carbon) either condensed on the glass bulb or was adsorbed by the tungsten filament.
The Tungsten Bromine cycle is W + 3Br2------->WBr6
That is, Tungsten combines with Bromine at the bulb wall and the resulting molecule migrates back to the filament and when the molecule reached 1325°C, it decomposes to W + 3Br2. The W is deposited at the cool end of the filament.
Now, about the hydrogen that was added, the hydrogen readily combines with free bromine to from HBr. That is, the more hydrogen in the bulb, the more Bromine is removed from the cycle. Therefore, the amount of free bromine is a matter of the amount of hydrogen in the bulb. It was the addition of hydrogen that resulted in control of the amount of bromine required to keep the bulb clean. Free bromine will attack tungsten at about 200°C and is most aggressive at about 350°C and then is very slow attack at about 450°C.
The major advantage of the clean bulb wall is that a smaller bulb can be used and the smaller the bulb, the higher the permissible fill and operating pressure. The higher gas pressure in the bulb is the total advantage of the halogen cycle.
The higher the fill pressure, the lower the tungsten evaporation rate and the longer the life of the lamp. Double the pressure means double the lamp life. Higher lumens per watt can be traded for shorter life.