- Mercury Vapour Lamp Definition: A mercury vapour lamp is defined as a gas discharge lamp that produces light using mercury.
- Components: It includes an inner quartz arc tube, an outer borosilicate glass envelope, and electrodes made of tungsten with thorium, calcium, and barium oxides.
- Working Principle: The lamp operates by creating an initial argon arc that heats and vaporizes mercury, leading to the main arc that produces light.
- Light Emission: It emits visible light in green, yellow, and violet spectra, with phosphor coating used to improve efficiency and color performance.
- Lifespan and Efficiency: Mercury vapour lamps last about 24,000 hours and have various lumen ratings based on wattage.
In fluorescent lamps, the mercury vapour pressure is kept low so that 60% of the input energy converts to UV light at 253.7 nm. Electrons need minimal energy for transition. As pressure rises, collision chances increase. A mercury lamp includes an inner quartz arc tube that withstands 1300K and an outer borosilicate glass envelope that withstands 700K.
Nitrogen gas between the tubes provides thermal insulation, protecting metal parts from oxidation due to high arc temperature. The arc tube contains mercury and argon gas, functioning like a fluorescent lamp. Inside are two main electrodes and a starting electrode. Each main electrode has a tungsten rod wound with coiled tungsten wire, dipped in thorium, calcium, and barium carbonates.
After dipping, the electrodes are heated to turn these compounds into oxides, making them stable for electron production. Molybdenum foil leads connect the electrodes through a quartz tube.
When the main supply voltage is applied, it reaches the starting electrode and the nearby main electrode, as well as both main electrodes. The small gap between the starting electrode and the bottom main electrode creates a high voltage gradient.
The high voltage gradient between the starting electrode and the bottom main electrode creates a local argon arc, with current limited by a starting resistor.
This initial arc heats up the mercury and vaporizes it and this mercury vapor helps to strike the main arc soon. But the resistance for the main arc current control resistor is somewhat less than the resistance of the resistor used in the initial arc current control purpose. For this reason initial arc stops and main arc continues to operate. It takes 5 to 7 minutes to make all of the mercury to be vaporized completely. The lamp gets its state of its operational stability. The mercury vapor arc gives visible spectra of green, yellow and violet. But there may be still some invisible ultraviolet radiation during discharging process of mercury vapour so phosphor coating may be provided on outer glass cover to improve efficiency of the mercury lamp.
There are five lamps with phosphor coating to provide improved color performance. As the wattage increases the initial lumen ratings for the phosphor coated lamps get available with 4200, 8600, 12100, 22500 and 63000 ratings. The average life of mercury lamp is 24000 hours i.e. 2 years 8 months.
Mercury Lamp data is given below.
