Carbon arcs excepted, mercury vapour discharge lamps were the first discharge lamps commercially available.  Despite being predated by the carbon arc, the mercury vapour lamp was the first commercially available discharge lamps which were actually practical for use in general lighting applications.  Offering huge energy savings over the high wattage incandescent lamps of the time, and previously unheard of lifetimes.  The first medium pressure lamp was developed by Osram-GEC, and launched in 1932.  As technology at this point in time was progressing at an extremely rapid place, so did the evolution of the mercury vapour lamp.  The aluminosilicate material which medium pressure (MA) lamps had their arc tubes constructed had a relatively low softening temperature.  Quartz however had a far, far higher melting point, and could allow the lamps to run at much higher pressures, hence the introduction of the high pressure (MB) lamp in 1937, which made use of a quartz arc tube, bringing with it a considerable increase in efficacy and the ability to create lower wattage lamps.  Since the 1950s the technology has remained fundamentally unchanged, just being refined over the years.  Efficacy increasing here and there as phosphor formulations are refined, and manufacturing methods being perfected bringing decreases in the cost of the lamps.  The most recent progress been in refining the phosphors used, increasing colour rendering properties of the lamps, as that has always been somewhere which mercury vapour lamps have suffered somewhat due to the characteristically greenish tinted colour of the light it produces.

Unknown to a lot of people (us lighting enthusiasts excepted!), a fluorescent lamp is actually a low pressure mercury vapour lamp, and that the discharge itself produces next to no visible light.  The discharge does however produce copious amounts of shortwave UV radiation at 253.7nm.  This radiation while totally invisible, allows highly efficient excitation of the phosphor layer on the inner wall of the lamp.  Germicidal lamps are simply conventional tubes made from UV transparent glass or quartz, and lacking in a phosphor layer.  Electrically they are compatible (Read: Identical) to normal fluorescent tubes of the same ratings.  These lamps are detailed in their own sections however.

Low Pressure Mercury Vapour Discharge

High Pressure Mercury Vapour Discharge

Tungsten Blended Mercury Vapour Discharge


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