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lamp types
Incandescent 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. When heated by an
electrical current, the filament emits electromagnetic radiation. At lower temperatures, radiation is mainly emitted in the infrared
part of the spectrum as heat. At higher temperatures, the proportion
of radiation at wavelengths ranging from 380 to 780 nm increases
and visible light is produced. In a conventional lamp, the filament temperatures are limited to about 2700 Kelvin. The tungsten filament starts to evaporate and as a result it leads to the blackening of the inside of the lamp envelope.

Halogen Lamps

Halogen lamps are also filament lamps. However, halogen is added
to the fill gas to prevent evaporated tungsten from condensing on the inside of the lamp envelope. This feature is used to exploit higher filament temperatures of 3000K and beyond and allows the size of the lamp envelope to be significantly reduced.

Halogen Lamp

Gas Discharge Lamps

In a gas discharge lamp, once a sufficient voltage is applied, electrons are emitted from a heated electrode, creating a plasma or a gas capable of conducting electricity. In the plasma mobile electrons collide with atoms (predominantly mercury), transfer energy to the atoms and elevate them 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.

Principle of low-pressure mercury gas discharge fluorescent lamp

Ultraviolet photons have
the capability to excite fluorescent powders, which are coated on the inside of the tube, with a high degree of efficiency. As a result, these powders emit visible radiation in a range of colours.

Lamps based on these principles and operating at low internal gas pressure are called “fluorescent lamps”.

Principle of a high intensity discharge lamp (e.g. metal halide lamp)

High pressure gas discharge lamps emit radiation directly as visible light. In this type of lamp the combination 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.

Most gas discharge lamps need at least one free electron combined with a high pulse to start the lamp operation and to produce light. 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.