| Filament Lamps
In conventional filament lamps or light bulbs, electric current is passed through a coiled tungsten filament contained in a glass envelope that is filled with an inert gas. The filament is heated by the electric current and consequently emits electro magnetic radiation. At lower temperature, radiation is emitted mainly in the infrared part of the spectrum as heat. At higher temperatures the proportion of radiation at wavelengths in the range between 380 and 780 nm increases and visible light is produced. In a conventional lamp the filament temperatures are limited to about 2700 K as a result of the tungsten filament starting to evaporate and blacken the inside of the lamp envelope. |
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| Halogen Lamps
Halogen lamps are also filament lamps. However halogen is added to the fill gas to prevent evaporated tungsten from the filament condensing on the inside of the lamp envelope. This feature is used to exploit higher filament temperatures of 3000 K and beyond and allow the size of the lamp envelope to be significantly reduced. |
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Gas Discharge Lamps
In a gas discharge lamp, electrons are emitted from a heated electrode once a sufficient voltage is applied creating a plasma or a gas capable of conducting electricity. In the plasma the mobile electrons collide with atoms (predominantly mercury). In this process the electrons transfer energy to the atoms which elevate to an "excited" state. When these atoms fall back to their original status they emit photons (packages of energy). |
| In many low pressure gas discharge lamps the wavelength of the emitted photon is not in the range of visible light. Mercury, for example, has its major emission in the ultraviolet at 254 nm. |
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Principle of low-pressure mercury gas discharge fluorescent lamp
Ultraviolet photons have the capability to excite the fluorescent powders, which are coated on the inside of the tube, with a high degree of efficiency. As a result these emit visible radiation in a range of colours. Lamps based on these principles and operating at low internal gas pressure are called "fluorescent lamps".
High pressure gas discharge lamps emit radiation directly as visible light. In this type of lamp the mix of different element atoms in the hot gas plasma, each emitting at specific wavelengths, determines the colour characteristics of the lamp as a whole as well as the quality of colour rendition properties. |
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| Most gas discharge lamps need at least one free electron combined with a high pulse start lamp operation. Usually minute quantities of materials like tritium or krypton-85 are applied either in the lamps themselves or in starter devices as a source for electrons. |
