In discharge lamps, light is generated when an electrical current is passed through gas or metal vapour plasma in sealed discharge tubes. There are two types of discharge lamp depending on the pressure of the filler material: high-pressure discharge lamps and low-pressure discharge lamps.

High-pressure discharge lamps
 
The most common representatives of this group of lamps are the metal halide lamps. Under high pressure (~10 bar) the filler substances in the gas atmosphere emit directly visible light through electron excitation. The light colours and the colour rendering properties can be influenced by adding carefully controlled doses of what are known as "rare earths".

Low-pressure discharge lamps
 
A distinction is made here between low-pressure mercury and sodium lamps. The fill pressure is only a few millibars. Like the high-pressure discharge lamps, low-pressure sodium lamps emit directly visible light, but exclusively with a wavelength of 585 nm.

In low-pressure mercury lamps (fluorescent and compact fluorescent lamps) on the other hand, mercury atoms are made to emit UV radiation as a result of electron collisions. This UV radiation is converted into visible light by phosphors applied to the inside of the tube.

Principle of a fluorescent lamp.