Incandescent lamps were concived in the early 1800's using various carbon based materials for the filament, but early lamps did not have a very long life. As time went on different filament materials and manufacturing methods enabled lamps to have a longer life and thus became a practical lighting source for many applications.
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Incandescent lamps have one major drawback, incandescent lamps convert only 15%–30% of electrical energy into light; the remaining 70%–85% of energy is dissipated as heat.
Even though incandescent lamps may have a low efficacy, they still have applications in decorative lighting.
Construction of a typical incandescent lamp, here a tungsten filliment lamp is shown
Construction of tungsten haolgen lamp is shown, there are different types available.
There is one problem with incandescent lamps and that is evaporation of the filament material with deposition on the inside of the lamp evelope, this increases with filament temperature and leads to loss of lamp brightness and early failure of the lamp. Filling the lamp with an inert gas (at very low pressure) slows this process.
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The halogen lamp addresses both the probem of lamp efficacy and and lamp life, this is acheived by different lamp construction and materials used and the introduction of a halide (Iodine or Bromine) into the lamp, which combines with tungsten evaporating from the filament and redepoisting the tungsten back onto the filament, thus enabling a higher filament temerature improving lamp efficacy and logevity.
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