mercury vapor lamps (original) (raw)

Definition: metal vapor lamps based on light emission from mercury (Hg) atoms

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Contents

What are Mercury Vapor Lamps?

Mercury vapor lamps are metal vapor lamps based on light emission from mercury (Hg) atoms.

Mercury (Hg) is the only elementary metal which has a substantial vapor pressure at room temperature. Therefore, a lamp may in principle contain mercury only, but often there is also some starter gas like argon or xenon. The lamp can start with a relatively low vapor pressure, which subsequently (within a couple of minutes) increases due to the heating during operation because more of the liquid mercury is evaporated.

Instead of liquid mercury, a mercury vapor lamp may contain an amalgam. When such a lamp is switched off, the mercury is largely bound in the amalgam, which implies that the toxic risk associated with breaking the lamp is reduced: one may collect the amalgam pieces before much of the mercury has evaporated.

Like most other types of metal vapor lamps, mercury vapor lamps need to be operated with a suitable electrical ballast for stabilizing the electric current.

See the article on metal vapor lamps for general properties of such lamps.

Low-pressure Mercury Vapor Lamps

In low-pressure lamps, the temperature rise is moderate, leading to a substantial, but not dramatic increase of mercury pressure during operation. The power density and the radiance of the emitted light are relatively low.

The emission of low-pressure mercury lamps exhibits a characteristic optical spectrum with pronounced spectral lines mostly in the visible and the ultraviolet spectral region, but partly also in the infrared. There is a substantial number of standard spectral lines of mercury, which can be produced with mercury-based spectral lamps; typical wavelengths are 184.5 nm, 253.7 nm, 365.4 nm, 404.7 nm, 435.8 nm, 546.1 nm, 578.2 nm and 1014 nm.

Low-pressure mercury lamps are also used as relatively efficient sources of ultraviolet light for applications like UV curing, lithography, UV spectroscopy, solarium, ozone generation, water sterilization and other germicidal irradiation. A particularly high radiance is not required for such applications. For efficient UV generation, one of course requires a lamp glass with a high transmittance in that spectral region.

For applications where only the visible lamp output is utilized (e.g. lighting), one often requires an absorbing UV filter for removing the ultraviolet light, which could otherwise e.g. be damaging to eyes and the skin.

Further, in combination with a phosphor on the inner surface of the glass tube one can generate white light for illumination purposes (→ fluorescent lamps).

High-pressure Mercury Vapor Lamps

High-pressure mercury vapor arc lamps (a type of high intensity discharge lamps) are operated with a pressure of the order of one atmosphere and with much increased power densities, so that more mercury is evaporated. The high density of the vapor allows for light emission with a much higher radiance.

In such lamps, there is also much stronger spectral broadening. The light emission does not only contain broadened lines, but also a substantial continuum.

High-pressure mercury lamps can be used as white light sources e.g. for lighting, but their color tone has a tendency towards the blue–green (a rather high color temperature), with a deficiency of the spectrum in the red region. The luminous efficacy is typically around 35 to 65 lm/W, which is substantially better than that for incandescent lamps, but on the other hand substantially worse than what can be achieved with metal halide lamps, for example. The device lifetime can be thousands of hours (much longer than for incandescent lamps), but it depends substantially on the type of lamp.

High-pressure mercury lamps have mostly been used for high-power lighting applications and projection displays. Particularly for lighting, they are increasingly replaced by other types of lamps which reach a higher luminous efficacy and/or work with less mercury or no mercury at all. For example, metal halide lamps can now be produced which are much superior to traditional mercury lamps in terms of energy efficiency and color rendering, while the lamp lifetimes can still be very high. Some of those lamps are mercury-free.

Medium-pressure Mercury Vapor Lamps

There are also medium-pressure mercury lamps, which have somewhat intermediate characteristics. They can be optimized e.g. to primarily emit UV-A or UV-C light, e.g. for applications like disinfection, UV curing and other photochemical processes.

Frequently Asked Questions

What is a mercury vapor lamp?

A mercury vapor lamp is a type of metal vapor lamp that generates light from an electric discharge through vaporized mercury. Its characteristics largely depend on the operating pressure of the mercury vapor.

What are the main types of mercury vapor lamps?

They are categorized by their operating pressure into low-, medium-, and high-pressure lamps. These types differ significantly in their output spectrum, radiance, and common applications.

What kind of light do low-pressure mercury lamps produce?

Low-pressure mercury lamps efficiently emit light at discrete spectral lines, especially in the ultraviolet (e.g., at 253.7 nm). This makes them useful for germicidal applications or for exciting phosphors in fluorescent lamps.

What are high-pressure mercury lamps used for?

High-pressure mercury lamps produce intense, whitish light and have been widely used for high-power lighting and in projection displays. However, their light has a blue-green tint and poor color rendering in the red region.

Why are mercury vapor lamps being phased out for lighting?

What is the purpose of using an amalgam in some mercury lamps?

Some lamps contain an amalgam, which is an alloy of mercury. When the lamp is cold, the mercury is bound in the amalgam, which reduces the toxic risk from mercury evaporation if the lamp breaks.

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