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The original electric light bulb has been in practical existence for around a century. It produces light through the passing of an electric current through a thin and fragile tungsten wire or filament, causing it to glow and emit light. This type of bulb is now generally categorized as an incandescent light after its primary cause of light emission. Most people refer to this as the electric light bulb and its creator or inventor as Thomas Edison.

What is Incandescence?

Incandescence occurs when a substance is heated to the level whereby it emits light. Most substances start with a low level of red glow, which increases to a stronger white light as the temperature is increased further. The temperature level required to produce this initial visible red glow is a little under 500°C and increases in temperature are roughly proportional to the increase of irradiated or emitted light. So to summarise, one can say that incandescent light is created through the production of heat.

How Do Incandescent Bulbs Create Heat?

The light of the incandescent bulb is generated through passing an electric current through a thin filament of wire, normally tungsten wire. The electric current agitates the molecular structure of the thin wire causing it to increase in temperature. By increasing the electric current so the temperature is increased until light is emitted. This process happens almost instantaneously. Were this process to occur in normal air, the filament would quickly oxidize and burn out. To prevent this from occurring, the filament is encased in the familiar outer glass covering, often a soda lime glass envelope. This envelope is filled with an inert gas. An inert gas is one that does not interact chemically with its immediate surroundings, and so is harmless or non-toxic.

Why Are Incandescent Bulbs Being Phased Out Of Production?

The tungsten filament however does slowly but steadily evaporate and become thinner until it finally breaks, and the bulb becomes useless. With the vacuum glass container, this occurred more rapidly and resulted in a blackening of the outer glass cover which in turn reduced the light. The use of an inert gas slows down this process thus prolonging the life of the bulb to up to 1000 hours. This process of heat generation to create light emission is very energy inefficient compared to later and more modern light technologies such as fluorescent lighting and LEDs. With the world under a growing environmental pressure, most governments are implementing legislation to ban incandescent lights since there are far more energy efficient lighting technologies to replace them.

What About Halogen Lamps?

Halogen lamps also use incandescence as its method for light emission and is slightly more energy efficient than the traditional electric light bulb, or what I shall refer to asthe “Edison” bulb. They generally operate at a higher temperature for optimal functionality and also produce a brighter light - one that covers a broader lighting spectrum - and so is considered a more effective form of lighting. They have also been considered slightly more environmentally friendly than the original electric light bulb since halogen lights use approximately 25% less energy per unit of light emitted. They have been available in the public domain for the last 50 years. Since their filament is smaller, the source of light can be more directed. So they have proved to be valuable for task lighting and have found specific lighting niches such as spotlights and particularly headlights for automobiles.

How Do Halogen Lamps Work?

Halogen is the name for a collective group of five chemicals or non-metallic elements, namely astatine, bromine, chlorine, fluorine and iodine. The halogen lamp is so called because it uses one of these halogen gases as an additive to the traditional inert gas within the envelope that surrounds the tungsten filament. The inner surrounding envelope is made from fused quartz. The tungsten from the heated filament also evaporates, but the halogen additive allows the tungsten to reform back onto the filament in a cyclic chemical reaction.

The Halogen Cycle Chemistry

More specifically, the tungsten evaporates from the filament and condenses onto the surrounding surfaces in a similar way to the blackening process of the Edison bulb. Now, by increasing the electric current and so heating the bulb to around 250°C, the halogen additive reacts with the “blackening” tungsten deposits that have re-condensed on the bulb surfaces to create another gas, tungsten halide. This chemical reaction effectively removes the blackening particles and so cleans the bulb. In addition, as the tungsten halide comes in contact with the heated tungsten filament, the gas is broken down to re-create the halide gas, whilst the tungsten is precipitated and recycled back onto the filament.

The Physics of the Halogen Bulb

For the halogen chemical reaction cycle to happen, the bulb size needs to be reduced so that the (inner) envelope surfaces are able to reach this high temperature (for the formation of the tungsten halide) through their proximity to the heated filament. The envelope also needs to withstand higher temperatures and is generally constructed from quartz or similar more resilient glass compound. The smaller volume of the envelope allows for the use of a higher grade inert gas, which in turn extends the life of the bulb by slowing down the evaporation rate of the tungsten. By this juggling of envelope size, the gas constituents, their proportions and pressurizing the envelope, an energy saving of up to 25% is achieved, over the Edison bulb. A further necessity is the added outer envelope or casing.

Halogen Lighting Foresees a Blackening Future

The future for incandescent lights is dark. Lighting technological advancements have produced far more energy efficient and eco-friendly lights, CFLs (Compact Fluorescent Bulbs), LEDs (Light Emitting Diodes) and several other innovations besides. There are presently legislations in many countries for the phasing out of the Edison bulbs by 2015. Earlier forms of halogen bulbs that do not reach certain standards will also be included. Automotive lighting has relied heavily on the halogen technology. Likewise, household lighting and the more elaborate or decorative lighting systems like crystal chandeliers have relied on the variety of shapes of the Edison bulb. LED lights and similar lighting will almost certainly become the preferred system. The future of lighting is presently unfolding rapidly in front of our eyes.