Introduction
Tungsten Halogen lamps were invented in 1958 by the engineers of General Electric. Producing a very strong, white light they are now used in many applications from automotive to retail display lighting. To understand their many advantages and consequent popularity we will compare the way in which ‘normal’ incandescent lamps work.
Incandescent Lamps
A normal incandescent lamp comprises a tungsten filament inside a glass envelope filled with inert gas such as argon or nitrogen. An electric current heats up the filament to about 3000 degrees making it a thermal radiator. At this temperature about 5-10% of the electrical input is converted to visible light and about 90% to heat, which is not particularly efficient!
During this process the filament vaporises and tungsten atoms stream towards and deposit on the cooler glass walls of the lamp envelope, which, over time becomes black. You will probably have noticed this in some of your older household lamps.
This is bad news on two fronts: The filament is constantly degrading, and thus limits the life of the bulb. In normal use incandescent lamps typically last only 750 to 1,000 hours. The blackening of the glass also impairs the performance of the lamp for the deposited Tungsten material absorbs light. To decelerate this blackening conventional incandescent lamps have a ‘big’ envelope with a wide surface area to spread the deposited tungsten.
The Halogen Breakthrough
The engineers of GE discovered that when Halogen gas was added to the envelope tungsten atoms did not collect on the walls. The Halogen atoms attach themselves to the vaporised tungsten atoms and transport them back to the filament where they are reused. The Halogen atoms then go back out into the lamp atmosphere and search for more vaporised Tungsten atoms. This continual process is known as the ‘Halogen Cycle’, and it signalled a breakthrough in lamp technology and design.
- Since the tungsten filament is continually ‘re-fed’ with atoms it may be operated at much higher temperatures. This results in a much brighter, whiter light. Halogen lamps therefor have a better ‘light efficacy’. They produce about 20% more light per watt than an incandescent lamp.
- The whiter light that halogen lamps produce give a ‘better colour rendering’, as they identify more colours of the visible spectrum. Their use initially became widespread in retail display lighting for this reason.
- Because the filament is continually re-fed with tungsten it lasts much longer. Halogen lamps are now being produced with an expected life of over 4000 hours.
- They do not suffer from blackening meaning the lamp performs consistently throughout its life.
- Since blackening does not occur, halogen lamps have no need for a big envelope, and are thus much more compact. The envelope of a G4 12v 20w capsule, for example, is just over 1cm long
Conclusion
These advantages have made halogen lamps the number one choice of architects, lighting designers, retailers and home owners. Continual research and development by the major lamp manufacturers is leading to improvements in light efficacy, lamp life and performance. The future for halogen lighting looks bright indeed!
