High-Pressure Mercury Lamp
Lamp that produces light as a result of an electrical discharge, generated between two electrodes, in a high-pressure mercury vapor that is contained in a transparent bulb. In some versions, fluorescent powder is applied that converts the ultraviolet part, which is emitted together with visible light, into visible light to improve the color quality of the light.
High-Pressure Mercury Gas-Discharge Lamps
High-pressure mercury lamps belong to the group of high-intensity discharge (HID) lamps, because they are available in high-lumen output (and thus high-luminous-intensity) versions. High-pressure mercury lamps are available in versions where the discharge takes place in vaporized mercury only and in versions in which metal halides are added so that the discharge takes place in mercury vapor and in vaporized metals from the metal-halide components [2, 3]. These latter types are called metal-halide lamps. Given the special operating principle and construction of metal-halide lamps, they are dealt with in a separate entry “Metal-halide lamp.”
High-pressure mercury lamps, like all high-pressure discharge lamps, are compact compared to low-pressure discharge lamps. They have a moderate efficacy and moderate color rendering. With their cool-white light, they were, until the 1970s of the last century, extensively used in road lighting, especially in built-up areas. Since the introduction of the more efficient high-pressure sodium lamps in the late 1960s, these lamps have in many cases replaced high-pressure mercury lamps.
The gas-discharge principle of high-pressure mercury lamps is similar to that of all other gas-discharge lamps. In high-pressure mercury lamps, the discharge takes place in vaporized mercury at a pressure of around 106 Pa (10 atm). The spectrum of the radiation is a line spectrum with emissions in the long-wave UV region and in the visible region at the yellow, green, blue, and violet wavelengths. The lamp without fluorescent powder lacks red in its spectrum and has a bluish-white color appearance and very poor color rendering. In most high-pressure mercury lamps, fluorescent powders are used to improve the color quality by converting a large part of the (small) UV component into visible radiation, predominantly in the red end of the spectrum. The result is cool-white light of moderate color rendering and improved efficacy. Like all gas-discharge lamps (with very few exceptions), a high-pressure mercury lamp cannot be operated without a ballast to limit the current flowing through it.
Materials and Construction
Outer bulb (often with fluorescent coating)
In view of the high operating temperature, quartz is used for the discharge tube because it has a higher melting temperature than glass.
The discharge tube contains a small quantity of mercury and an inert gas filling.
An outer bulb (usually ovoid in shape) with an inert gas filling isolates the gas-discharge tube so that changes in ambient temperature have no influence on its proper functioning. It also protects the lamp components from corrosion at the high operating temperatures involved. For the smaller wattage lamps, with their lower operating temperatures, normal glass is used, while for the other types, hard glass is used.
As has already been mentioned, high-pressure mercury lamps usually employ fluorescent powder to improve the color quality of the light emitted. The powder is provided as a coating on the inner surface of the outer bulb. Different fluorescent coatings are used to obtain different lamp types with different color qualities and lamp efficacies.
Lamp caps are of the Edison-screw type, with the wattage of the lamp determining their size (E27 and E40).
Approximately 17 % of the input power is emitted in the form of visible radiation. Compare this with the 28 % of a tubular fluorescent lamp and the 30 % of a high-pressure sodium lamp.
Luminous efficacy varies with lamp wattage and with the color quality of the lamp from some 35 to 60 lm/W.
High-pressure mercury lamps are produced in lumen packages between some 2,000 and 60,000 lm (corresponding wattages between 50 and 1,000 W).
As with most gas-discharge lamps, lamp life is determined by emitter exhaustion of the electrodes. Economic life varies according to type between 10,000 and 15,000 h (20 % mortality).
Lamp-lumen depreciation is caused by evaporation and scattering of electrode material (lamp blackening) and by the gradual decrease in the activity of the fluorescent powder. The point at which 20 % lumen depreciation occurs lies at around 10,000–15,000 h.
Run-Up and Re-ignition
The run-up time of a high-pressure mercury lamp to its full temperature and corresponding nominal mercury pressure is some 4 min. The hot lamp will not restart until it has cooled sufficiently to lower the vapor pressure to the point at which restrike with the voltage available is possible. The re-ignition time is in the order of 5 min.
High-pressure mercury lamps cannot be dimmed.
A 5 % variation in the mains voltage changes both lamp current and light output by 10 %. Overvoltage decreases lamp life and increases lamp depreciation because of the correspondingly higher current.
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