Several types of lamps produce light by establishing a permanent electric arc in a gas. This process is known as electric discharge, or gaseous discharge. It is the process used to produce light in fluorescent and high intensity discharge lamps.
In many of these lamps a second process is used to change the color of the light produced. The process is called fluorescence. This color change is necessary if the light produced by the arc is not visible to human eyes or if the light makes objects appear to be the wrong color. This adustment may be done to move the light from ultraviolet into the visible range or to give the lamp a different color distribution to improve the appearance of objects seen under its light.
The color adjustment is done by a powder called a phosphor which has the property of giving off a different color of light than it is exposed to. This powder coats the inside of electric-discharge lamps. It receives light from the arc inside the lamp and changes the light to the desired color as it glows through the glass.
Electric discharge lamps cannot establish or maintain the arc. That is the job of another device called a ballast. All electric discharge lamps require a ballast. Usually the ballast is installed as a separate device but in a few cases the lamp and ballast are made as one unit.
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