Halogen Cycle Explained

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Halogen Cycle Explained

Be it automotive lamps, floodlighting or stage lighting, halogen lamps are used everywhere. The tungsten halogen cycle is the chemical process which boosts the longevity of these special lamps. To know all about how halogen cycle works to lengthen the life of a tungsten filament, read this article.

The incandescent lamp is the prime mascot of the industrial revolution, which has come to symbolize the proverbial ‘Eureka’ moment of invention and creativity. From the first incandescent lamp with a long-lasting filament, created by Edison, to the halogen lamps of today, we have witnessed rapid innovation in the technology that lights up our world. Every invention, which finds widespread application, evolves with time to provide better functionality, as changes are made in its earlier designs, to reduce imperfections. 

The halogen lamp is one such example of a device which has undergone continuous innovation, becoming the versatile choice for any lighting requirement. What lengthens the life of the tungsten filament, embedded in a halogen lamp, also enabling its high luminosity, is the mixing of a halogen element like bromine or iodine in the gas envelope surrounding the filament. In this ScienceStruck article, you will find the tungsten halogen cycle explained, demystifying the working of a halogen lamp in the process.

Tungsten Halogen Regenerative Cycle Demystified

Prior to exploring the phenomenon, that halogen cycle is, let me describe the backdrop where it occurs, which is the internal environment of the halogen lamp. Halogen is the generic name for a bunch of elements that include iodine, fluorine and chlorine. A binary compound, which includes a halogen, is known as halide. The central element in the working of halogen lamp is the tungsten filament, which emits photons or light in the visible range, when heated to high temperatures. 

To protect this filament from getting oxidized or undergoing chemical reactions, the filament is enclosed in a glass bulb, filled with a gaseous mixture, consisting of an inert gas (argon, krypton or xenon – which reduces tungsten’s chemical reactivity) and a trace amount of halogen compound (bromine or iodine).

Since the tungsten filament in a halogen lamp attains high temperatures, it’s generally made up of fused silica glass. The role of the halogen compound will be explained in the following lines. Here is a stepwise explanation of the tungsten halogen regenerative cycle.

Step 1: Tungsten Vaporizes at High Temperature

As the halogen lamp is switched on, current starts flowing through the tungsten filament, heating it up rapidly. The bulb surface, gaseous envelope and the filament are all heated up to different temperatures, creating a gradient throughout the bulb. This gives rise to convective currents inside the bulb’s innards. As the filament temperature crosses 2,500° C, tungsten atoms start vaporizing.

In non-halogen lamps, these vaporized atoms get deposited on the bulb’s inner surface, blackening it and thinning the filament, reducing the life of the lamp. In halogen lamps, there is a fail-safe mechanism, which prevents the atoms from being deposited on the bulb insides. This role is played by the trace amounts of halogen compounds mixed in the enclosed gas. Let us see how it swings into action.

Step 2: Creation of Tungsten Halide

The halogen atoms react with the vaporized tungsten to form tungsten oxyhalide or tungsten halide compounds. These halide compounds get caught up in the convection currents, created due to the temperature gradient and get directed towards the filament. Thus vaporized tungsten isn’t allowed to be deposited on the bulb surface. This is the beginning of the halogen regeneration cycle. The bulb surface temperature has to be in excess of 200° C, for the halogen cycle to trigger into action.

Step 3: Deposition of Tungsten Back to the Filament

When the tungsten halide compounds come in contact with the filament, the high temperature makes the halides dissociate again, depositing tungsten back to the filament. The vaporized halogen (bromine or iodine) is free again to catch more tungsten atoms, that are emitted by the filament. Of course, the tungsten atoms vaporized from a particular region of the filament are rarely deposited at the same place.

They are deposited unevenly and therefore, the filament is going to break at some point of time in the future, when it thins too much in some region. Still, the halogen regenerative cycle, slows down this process, quite substantially. Halogen lamp bulbs are compact and the gaseous mixture is filled at a high pressure inside them, to reduce the vaporization rate of tungsten. The cycle continues throughout the active life of the bulb.

In this way, the halogen cycle restores the evaporated tungsten back to the filament, lengthening the life of the bulb and preventing the blackening of the surface. This also enables the halogen light bulb to have a luminous efficacy of 10 to 30 lm/W, which is substantially higher than that of a non-halogen lamp. So next time you see a halogen lamp, you will be able to appreciate the technological marvel better, as you now have an insight into its working.

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