Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

High-Pressure Mercury Lamp

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_138

Synonyms

Definition

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.

Working Principle

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

The main parts of a high-pressure mercury lamp are (Fig. 1):
  • Discharge tube

  • Fill gas

  • Electrodes

  • Outer bulb (often with fluorescent coating)

  • Lamp cap

High-Pressure Mercury Lamp, Fig. 1

Principle parts of a high-pressure mercury gas-discharge lamp [1]

Discharge Tube

In view of the high operating temperature, quartz is used for the discharge tube because it has a higher melting temperature than glass.

Fill Gas

The discharge tube contains a small quantity of mercury and an inert gas filling.

Electrodes

The main electrodes consist of a core of tungsten rod with a tungsten coil (impregnated with emissive material) wound around it. To aid starting, a normal high-pressure mercury lamp has not only an inert gas but also an auxiliary electrode. Because of this, a normal mercury lamp does not need an external igniter. The auxiliary electrode simply consists of a tungsten wire positioned very close to one of the main electrodes (Fig. 2).
High-Pressure Mercury Lamp, Fig. 2

Discharge tube showing the auxiliary electrode and two main electrodes

Outer Bulb

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.

Fluorescent Powder

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 Cap

Lamp caps are of the Edison-screw type, with the wattage of the lamp determining their size (E27 and E40).

Properties

Energy Balance

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

Luminous efficacy varies with lamp wattage and with the color quality of the lamp from some 35 to 60 lm/W.

Lumen-Package Range

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).

Color Characteristics

As has already been mentioned, high-pressure mercury lamps have a line spectrum (Fig. 3). The two lines in the red part of the spectrum are obtained by conversion of ultraviolet radiation by the fluorescent powder. The color characteristics are dependent on the composition and quality of the fluorescent powders used. Different compositions and qualities are used to produce lamps with color temperatures between some 3,500 and 4,500 K, with color-rendering index (Ra) values of around 60 for high-quality versions and around 40 for ordinary versions.
High-Pressure Mercury Lamp, Fig. 3

Spectral energy distribution of a high-pressure mercury lamp with fluorescent powder [1]

Lamp Life

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

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.

Dimming

High-pressure mercury lamps cannot be dimmed.

Mains-Voltage Variations

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.

Product Range

High-pressure mercury lamps are available in an ordinary version with poor color rendering (Ra of around 40) and in so-called comfort versions with an improved color rendering of around 60. The bulb is ovoid in shape and increases in size with increase in wattage (Fig. 4). Versions without fluorescent powder are hardly, anymore, produced. Reflector lamp versions are produced with a cone-shaped outer bulb and an internal reflective coating on the front (Fig. 4, right).
High-Pressure Mercury Lamp, Fig. 4

Different wattage ovoid and reflector type of high-pressure mercury lamps [1]

There is one version of the high-pressure mercury lamp, the “blended light lamp,” that does not need an external ballast. The ballast has simply been built into the lamp itself in the form of a tungsten filament (Fig. 5). The lamp can be connected direct to the mains. The light from the mercury discharge and that from the heated filament blend together (hence the name blended light lamp). The color characteristics of this lamp are therefore better than those of a normal high-pressure mercury lamp, but this comes at the cost of a considerably lower efficacy.
High-Pressure Mercury Lamp, Fig. 5

Blended light lamp with high-pressure mercury gas-discharge tube and tungsten incandescent filament combined in one outer bulb. To show the construction, a clear lamp is shown, although most blended light lamps use a fluorescent powder coating on the interior of the outer bulb [1]

Cross-References

References

  1. 1.
    Van Bommel, W.J.M., Rouhana, A.: Lighting Hardware: Lamps, Gear, Luminaires, Controls. Course book, Philips Lighting, Eindhoven (2012)Google Scholar
  2. 2.
    Coaton, J.R., Marsden, A.M.: Lamps and Lighting, 4th edn. Arnold, London (1997)Google Scholar
  3. 3.
    DiLaura, D.L., Houser, K., Mistrick, R., Steffy, G.: IES Handbook, 10th edn Illuminating Engineering Society of North America, IESNA, New York (2011)Google Scholar

Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.NuenenThe Netherlands