Mercury Lamps

The mercury lamp was the first mass produced metal vapour light source which was invented for general lighting purposes and are still nowadays the most popular discharge lamps used around the world.

Due to its lack of efficiency, the popularity of the mercury bulb has diminished around Europe when compared with other lighting sources. However the mercury lamp has not completely died out in other areas such as the USA or Asia due to its low system cost, long life span and its height colour temperature, making it ideal for outdoor lighting. 

Lamp Efficacy

Mercury vapour has the lowest luminous efficacy when compared to other common discharge lamps. Numerous changes have been made over the past 100 years of development to ensure an increase in performance. 

In the centre of the lamp there is an arc tube which is made from quartz with a tungsten electrode disposed on the other end. The tube will contain a few milligrams of mercury and approximately 25 – 50 torr of pure argon which acts as a buffer gas to carry the discharge while the lamp warms up, eventually heating up and vaporising the mercury and bringing to discharge.

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.

A secondary starting electrode (auxiliary) is located next to one of the main ones to provide support for the ignition of the lamp. When the lamp is first energised, the full open circuit voltage is applied across the arc tube. The space in-between the electrodes are so big that the end voltage gradient is not high enough to create ionisation of the gas filled tube. The same voltage as before is used between one electrode and the auxiliary through a resistor, this way the gap is small between the two electrodes therefore making the voltage gradient more sufficient and high enough for ionisation to take place.

After lamp has heated up with the mercury fully vaporised, the discharge then operated in unsaturated mercury vapour at a pressure of around 18 bar for the smaller types – 2 bar for the largest. The arc creates the distinctive yellow, green and violet mercury lines where there are extensive amounts of indistinguishable long wave ultra violet at 365nm along with a wider range of less significant wave lengths. In basic lamps there are usually clear external bulbs, this undetectable UV radiation was totally wasted. Nowadays the external bulb is internally layered with a thin coating of fluorescent phosphor which will convert UV light into visible light.