Quick Answer
A UV lamp is, at its core, a gas-discharge lamp: two electrodes inside a quartz-glass envelope filled with a noble gas plus mercury, paired with an external ballast that acts as a current limiter. The electrical discharge in the low-pressure plasma generates UV photons — primarily at 253.7 nm (germicidal) and 185 nm (ozone-forming). UV-LEDs replace the gas discharge with semiconductor AlGaN chips and operate on a completely different principle.
This article breaks the classic low-pressure mercury-vapour lamp down into its 5 main components, places the most important lamp types in context (LP-Hg, MP-Hg, excimer, UV-LED) and spells out the practical implications for operators — ageing, maintenance, safety.
The 5 Main Components of a Low-Pressure Mercury-Vapour Lamp
1. Quartz-Glass Envelope
Standard borosilicate glass blocks UV below roughly 350 nm almost completely. The envelope is therefore made of fused quartz — transparent to the desired 254 nm and 185 nm wavelengths. Some low-pressure lamps use soft glass (soda-lime borate) when only the visible glow discharge is required.
Solarisation as an ageing mechanism: UV photons gradually degrade the quartz over the lamp's life. The envelope slowly clouds, reducing UV transmission, so part of the output decline over a lamp's service life is attributable to the envelope itself rather than the discharge. As a reference point, the 253.7 nm line of a typical low-pressure mercury lamp declines to roughly 70 % of its initial value after about 7,000 hours of operation — the combined result of envelope ageing and electrode wear (see Component 2).
2. Electrodes
Two electrodes sit at the ends, usually made of tungsten coated with thorium, barium or calcium oxides (the emitter material). These oxides have a low work function — when heated, they release electrons easily, which starts and stabilises the arc discharge.
Hot cathode vs. cold cathode:
- Hot cathode (classic): the filament is pre-heated, so the lamp starts gently. Longer service life (8,000–16,000 h) but a 0.5–2 s warm-up.
- Cold cathode: a thick-walled metal thimble instead of a filament. High starting voltage, instant-on. Shorter service life (~6,000 h) but instant-on and shock-resistant. Preferred for mobile devices or applications with frequent on/off cycling.
3. Fill Gas + Mercury
- Noble gas: argon or an argon/neon mix (0.5–3 mbar). Provides the initial charge carriers for the ignition arc.
- Mercury: 10–100 mg per lamp (liquid or as a solid amalgam pellet). It evaporates during operation to roughly 0.001 mbar — this is the low-pressure point that is optimal for 254 nm and 185 nm emission.
Amalgam lamps (higher UV output): when run too hot (>40 °C), the UV efficiency of traditional Hg lamps drops sharply because the Hg vapour pressure overshoots the optimum. Amalgam (an Hg-indium or Hg-gallium compound) holds the Hg vapour pressure at the optimum across a wider temperature range — important for compact or warm-running applications such as in-reactor water disinfection.
4. Base
Mechanical mounting plus the electrical connection to the ballast. Common standards:
- G13 (bi-pin, T8 format like classic fluorescent tubes)
- 2G11 (4-pin, compact)
- G23 / G24q (compact UV, 4-pin)
- Custom bases per manufacturer — especially for high-output and medium-pressure lamps, because those require higher currents and dedicated cooling geometries
The base is more than mechanics: it typically carries the end-of-life (EOL) coding so the ballast can identify the lamp.
5. Ballast — External
UV lamps are negative-resistance devices: without a current limiter the current rises uncontrollably and the lamp fails within seconds. The ballast is a mandatory component, not optional accessory.
Ballast types by starting behaviour:
| Type | Starting behaviour | Effect on service life | Assessment |
|---|---|---|---|
| Instant start | High voltage applied directly → instant on | Aggressive on electrodes, shorter lamp life | Cheap, rare in the UV segment |
| Rapid start | Electrode heating + ignition voltage simultaneously | Good balance of service life vs. complexity | Recommended for most UV applications |
| Programmed start | Pre-heat first (0.15–1 s), then ignition voltage | Maximum service life, gentlest | Premium, for frequent on/off cycling |
Ballast topology:
- Magnetic (choke + starter): flickers at 50/60 Hz, inefficient, only legacy stock in the UV field.
- Electronic (HF, 20–60 kHz): the standard today. No flicker, longer service life, higher efficiency, often with a dimming function.
A practical rule of thumb: an electronic programmed-start ballast can extend lamp service life considerably compared with a magnetic instant-start arrangement, particularly in applications with frequent on/off cycling — relevant because the largest lifetime cost item in a UV installation is often not electricity but the lamp change and the associated system downtime.
Lamp Types Compared
| Technology | Peak λ | Wall-plug efficiency | Service life | Use cases |
|---|---|---|---|---|
| LP-Hg (low-pressure mercury) | 254 nm | 30–40 % | 8,000–16,000 h | Water, air and surface disinfection — the germicidal standard |
| MP-Hg (medium-pressure mercury) | Polychromatic 200–600 nm | 10–15 % | 4,000–8,000 h | High flow rates (municipal drinking water, AOP processes), photochemical breakdown reactions |
| Excimer (KrCl) | 222 nm | 5–10 % | 1,000–3,000 h | Far-UVC for (potentially) occupied spaces — lower skin penetration |
| UV-LED (AlGaN) | 265 / 275 / 280 nm selectable | 5–10 % (as of 2025) | 10,000–50,000 h | Compact mobile devices, point applications, on-demand |
An important asymmetry when comparing Hg vs. LED: germicidal effectiveness is wavelength-dependent, peaking near 265 nm — the peak of the DNA absorption curve. Because a 265 nm UV-LED sits closer to that peak than a 254 nm mercury lamp, it delivers more germicidal effect per unit of radiant power. Comparing devices by electrical watts alone therefore understates LEDs systematically; the correct basis of comparison is the wavelength-weighted germicidal effect, not raw power (see the action-spectra article cross-referenced below).
Practical Implications for Operators
Recognising Ageing
A UV lamp ages primarily through three mechanisms:
- Electrode sputtering — emitter material deposits on the inner wall → visible black-grey "lamp blackening" streaks at the ends, UV output drops.
- Quartz solarisation — the envelope clouds and yellows → UV transmission drops.
- Hg loss through wall adsorption — poorer arc stability, the lamp flickers or struggles to ignite.
As noted above, the 253.7 nm output of a typical low-pressure lamp falls to roughly 70 % of its initial value after several thousand hours of operation. Validated installations (e.g. for drinking water or pharmaceutical use) must verify this with measurement sensors — a visual inspection of the lamp is not sufficient for compliance. The governing references here are AG LUV Guideline 100 and DIN 67506 for secondary-air devices.
Maintenance Best Practice
- Clean the quartz sleeve every 3–6 months (biofilm in water applications, dust in air applications). Even a lamp that has not aged can lose a substantial share of its delivered output when the sleeve is fouled or scaled — measured studies show steady-state output is already markedly lower with a quartz sleeve in place, and fouling adds further loss on top.
- Replace the lamp per the manufacturer specification (typically annually for 24/7 operation) — do not wait until output visibly declines. The final portion of service life often comes with noticeably reduced germicidal effect.
- Avoid cold starts: lamps do not tolerate frequent on/off cycling well — where possible choose a programmed-start ballast and continuous operation. Frequent starts shorten lamp service life.
Safety
UV-C is acutely harmful to eyes and skin (photokeratitis within minutes, DNA damage, long-term skin-cancer risk). Installations with exposed UV-C sources require:
- Viewing windows made of UV-blocking material (boroflint / Borofloat / doped acrylics)
- Protective eyewear to EN 170
- Interlock switches against accidental opening during operation
- DGUV-compliant safety labelling and regular staff training
With Far-UVC (222 nm excimer) the skin and eye penetration is markedly lower than at 254 nm — irradiation of occupied spaces is therefore under discussion, but not yet finally settled in regulation (the EU and Germany are more conservative here than the USA).
Cross-References
These deeper-dive articles build on this structural understanding:
- Ballasts & Drivers — magnetic vs. electronic, start types, dimming
- Reflector Geometries — how the UV gets from the lamp into the reactor
- UV Lamp Technology — lamp-type families compared in depth
- Wavelengths & Action Spectra — how each wavelength acts on each organism
- LED Area Emitters — LED construction in detail (AlGaN chip + SMD/COB/DOB package + AlN substrate + secondary optics + driver). The LED counterpart of this structural topic.
- UV-LED Lifetime & Degradation — how LED components age thermally and electrically; what the anatomy implies for lifetime modelling.
Sources
- IUVA UV Disinfection Handbook (Bolton & Cotton, 3rd edition) — the standard reference text
- ScienceDirect — Mercury-Vapor Lamp overview
- WCP Online — Germicidal Lamp Instruction 101 — output decline and rated-life basis
- ISL Products — UV-C Ballast Start Types
- Crystal IS — LED vs. Lamp Output Comparison
- EDN — Considerations in the selection of UV LEDs for germicidal applications
- ams-osram OSLON UV — Product Specifications
- DIN 67506 (UV-C secondary-air devices) — AG LUV / DIN working group, 2022
- AG LUV Guideline 100 — minimum requirements for UV-C devices
- US EPA IMERC Fact Sheet — Mercury use in lighting
Status: May 2026. This article will be expanded as the planned deep dives (ballasts, reflector geometries, excimer) are published.