High pressure mercury lamps were developed in 1936, 35 years after the low pressure lamps become available.
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At the heart of the lamp is an arc tube which is fabricated from quartz, with a tungsten electrode disposed at either end. The tube contains a few milligrams of mercury and around 25-50 torr of pure argon as a buffer gas to carry the discharge while the lamp warms up, producing heat to vaporise the mercury and bring it into the discharge.
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The lamp starts with a small arc between the starting electrode and the main electrode. This arc goes through argon gas which easily strikes, even in cold weather. This little arc heats the tube, and over several minutes the tube gets hot enough to vaporize the solid mercury stuck to the sides. The mercury vaporized creates a strong light between the two main electrodes.
To prevent the arc from infinitely getting stronger a ballast limits the current. Some lamps are "Self Ballasted", they use an incandescent filament to act as a resistor, limiting current.
Internal layout of a typical High Pressure Mercury lamp
HRL Lamp
HRL lamps are high pressure mercury lamps with a flourescent coating to compensate for the lack of red in high pressure mercury lamps by converting some of the ultraviolet light emitted into red light, thus improving the colour spectrum of the lamp.
Example of an old Philips 400w HRL Lamp
MBF Lamps
MBF lamps are another type of high pressure mercury lamp with a flourescent coating to compensate for the lack of red in high pressure mercury lamps by converting some of the ultraviolet light emitted into red light, thus improving the colour spectrum of the lamp.
400w Thorn Kolorlux MBF lamp
Just added to the colection is this 'self-balasted' High-Pressure Mercury Lamp. There is no make shown on this lamp, only the wattage.
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This does not matter to me, but it is great to have a High Pressure Mercury Lamp where you can clearly see the internals.