A mercury vapour lamp is a high-pressure, electric arc discharge lamp that provides intense illumination over a selected range of wavelengths. Lamps that emit light by the passage of electric current through a gas are called gas-discharge lamps. These lamps were first introduced during the 1930s.
Some of the advantages that mercury vapour lamps provide are intense illumination, high energy efficiency, and longer lifetimes. Mercury vapour lamps are commonly referred to as HBO lamps, where H stands for Hg (mercury), B stands for luminescence, and O stands for unforced cooling.
This article will provide details on the working, construction, and applications of the mercury vapour lamp.
Working and Spectral Output
Mercury vapour, at high pressure, is filled inside a glass tube with two electrodes. When current is passed through the tube, mercury vapour is ionized and emits a light in the ultraviolet region. In order to convert the radiation into visible radiation, the tube is coated with fluorescent material.
The light emitted by the mercury vapour lamp has a bluish tinge. The output wavelength of mercury vapour lamps is distributed over the ultra violet, IR, and visible region, with the majority being scattered over the UV and IR regions.
Mercury vapour lamps consist of two electrodes, that are closely spaced, placed in a high-pressure gas medium. The gas and the electrodes are housed in an optically transparent, elliptically shaped enclosure made of silica. The electrodes are made of tungsten alloys. Tungsten alloys are chosen mainly due to their high melting point. Along with mercury, there is a small amount of argon or xenon that serves as a starter gas.
Some of the principal application areas of mercury vapour lamps are below:
- Molecular spectroscopy
- Area and street lighting
- Photolithography and photoresistance
Sources and Further Reading