It is a type of vapor or gaseous discharge lamp which uses mercury in an excited state to produce light.
The arc discharge takes place in a transparent tube inside a larger bulb of glass. The outer bulb reduces the ultraviolet radiation of the inner arc tube.
Radiation from the mercury arc is confined to four specific wavelengths in the visible portion of the spectrum and several strong lines in the ultraviolet.
This lamp is a very important part of many industrial and research application as a source of ultraviolet radiation and this type is specially designed for instrument in lab.