- 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.
A high-pressure mercury-vapour lamp is an HID source, not a high-output version of a fluorescent lamp. A low-pressure fluorescent discharge concentrates much of its radiation near 253.7 nm, while higher mercury pressure broadens and shifts the spectrum and adds strong visible mercury lines. The mercury vapour discharge operates in a fused-quartz arc tube, with a larger glass outer envelope for thermal isolation, mechanical protection and ultraviolet filtering. Exact temperatures depend on lamp wattage and construction, so fixed 1300 K and 700 K limits are not universal specifications.
The space between arc tube and outer bulb can contain nitrogen or another controlled atmosphere, or be evacuated, depending on the design. The quartz arc tube contains a measured mercury dose and a low-pressure argon starting gas. Unlike a fluorescent lamp, a common high-pressure lamp has two robust main tungsten electrodes and an auxiliary starting electrode. Electron-emissive material in the main electrodes can contain compounds of barium, calcium, thorium and other elements, but composition varies by product and generation.
Molybdenum foil seals provide electrical connections through the quartz because their expansion behaviour suits the material over the operating cycle.
When supply voltage is applied to a probe-start lamp, a high electric-field gradient appears across the small gap between the auxiliary electrode and its nearby main electrode. That short gap permits the argon to ionise at a lower applied voltage than the full distance between the main electrodes.
A high-value starting resistor limits the small auxiliary-discharge current.
The ionised argon lets the discharge spread to the two main electrodes. Arc heating then vaporises the mercury, raising arc-tube pressure, lamp voltage and light output during a warm-up period of several minutes. Because a discharge has negative differential resistance, a matched external ballast must limit main-arc current unless the lamp is explicitly self-ballasted. The start probe no longer controls normal operation after the main arc is established. A hot lamp usually cannot restart immediately because arc-tube pressure remains high; it must cool until the available starting voltage can restrike it. Clear lamps emit strong blue, green and yellow mercury lines plus ultraviolet radiation, giving poor red rendering. In a coated mercury vapour lamp, outer-bulb phosphor converts some ultraviolet output to visible light and improves colour balance. The intact outer envelope filters hazardous short-wavelength ultraviolet radiation.
Rated life, lumen output, efficacy, warm-up time and colour performance vary by lamp type, wattage, ballast and manufacturer. Values near 24,000 hours appeared for some products, but they are not a definition of the lamp class. Mercury-vapour lamps have also been displaced in many applications by sources with better efficacy and colour control.
Mercury Lamp data in the historical table below applies only to the products for which it was published.
Spent lamps contain mercury and should be recycled or disposed of under local rules. If a lamp breaks, follow mercury-lamp cleanup guidance. Never operate a high-intensity mercury lamp with a damaged outer bulb: unfiltered arc-tube ultraviolet radiation can burn skin and eyes.
