A mercury-vapor lamp is a gas-discharge lamp that uses an electric arc through vaporized mercury to produce light. These are used for the production of ultraviolet rays for therapeutic purposes.
The first mercury vapor lamp was invented in 1901 by American engineer Peter Cooper Hewitt.
The mercury vapor lamp can be of two types-
- Air-cooled medium pressure mercury vapor lamp (medium pressure mercury arc lamp)
- Water-cooled medium pressure mercury vapor lamp (kromayer lamp)
1)Air-Cooled Medium Pressure Mercury Vapor Lamp (Medium Pressure Mercury Arc Lamp)⇒
Construction-
- This is often U-shaped
- The burner is made of quartz: this material allows the passage of ultra-violet, can withstand very high temperatures and has a fairly low coefficient of expansion.
- Enclosed in the tube is argon gas at low pressure,
- A small quantity of mercury is also enclosed in the tube.
- An electrode is sealed into either end.
- Surrounding the ends are two metal caps across which a high potential difference is applied to ionize the argon.
- Argon is normally extremely stable and inert, so in order to pass a current through the tube the argon must be ionized.
Mechanism-
When a very high voltage (400V) is applied, the argon gets ionized. It is causing the formation of positive argon ions and free electrons which constitutes current flow across the tube.
The flow of current across the tube, produces heat (joule’s law) making vaporization of the liquid mercury atoms. The mercury vapor formed gets ionized by the flow of current causing a further acceleration of flow current across the valve.
The collision between the free electrons and the mercury atoms may produce:
- Elastic collision (without affecting the atom’s electrons)
- Ionization (knocking an electron out of its orbital)
- Excitation (kicking an electron into a higher orbital of different energy.
Ultraviolet radiation is produced partly as the energy released by the recombination of electrons and the positive mercury ion and partly by the photons released when excited electrons return from a higher energy quantum shell to their normal shell within the mercury atoms.
This process takes about 5 minutes, so this much time is required for the emission of ultraviolet after the burner is started.
The lamp is set at the center of a parabolic reflector made of a special aluminum alloy, supported on a strong stand. the lamp is positioned at a distance of 45 or 50cm.
Tridymite formation-
One of the complications with the mercury vapor lamp is the formation of tridymite (change of quartz to another form of silica). Which is opaque to ultraviolet rays, therefore reducing the total output of the lamp. So it is recommended to replace the whole burner after 1000 hours of burning, due to the formation of a large amount of tridymite.
2) Water-Cooled Medium Pressure Mercury Vapor Lamp (Kromayer Lamp)⇒
- The kromayer lamp is a water-cooled mercury vapor lamp, which eliminates the danger of an infrared burn;
- It has the advantage that it can be used in contact with the tissues, or with a suitable applicator, to irradiate inside a sinus or body cavity;
- A pump and a cooling fan are organized into the body of the kromayer lamp to cool the water;
- After using, the water circulation should be continued for 5 minutes;
- Then the burner is switched off to cool the lamp;
- At the front of the kromayer head, water circulated between two quartz windows, which allows the ultraviolet to emerge;
- The use of this lamp has become less common.