"UV lamp" is a family, not one thing
There is no single "UV lamp." UV sources split first into conventional gas-discharge lamps and solid-state LEDs — and the discharge lamps split again into low- and medium-pressure mercury, amalgam and excimer. They differ along two independent axes: their physical form (a tube radiates in all directions and needs a reflector; an LED array points where you aim it) and their spectral character (a sharp line, a narrow band, or a broadband continuum). A lamp's shape does not decide its spectrum.
Radial emitters; usually need a reflector. Tube/cylinder form.
- Low-pressure mercury254 nm · line-dominantThe germicidal workhorse
- Amalgam254 nm · high-outputLP variant for big flows
- Medium-pressure mercury~200–400 nm · broadbandHigh power, penetrating
- Excimer (KrCl)222 nm · narrowbandFar-UVC, low skin penetration
Directional (no reflector); wavelength is selectable.
- UV-C LED (AlGaN)265–280 nm · narrowbandCompact, mercury-free
- UV-A LED365–405 nmThe curing workhorse
Two independent axes: form (tube vs. LED array vs. panel) and spectral character (line-dominant · narrowband · broadband). A lamp's shape does not determine its spectrum.
The most common germicidal type — and the one we dissect below — is the low-pressure mercury lamp: a sealed quartz tube with an electrode at each end, a noble gas and a trace of mercury, run by an external ballast. Apply voltage and the mercury vapour emits UV, primarily at 253.7 nm (germicidal) plus a 185 nm ozone line. An LED UV source reaches the same goal completely differently — a semiconductor (AlGaN) chip, no gas, no mercury.
Anatomy of a low-pressure mercury lamp
Hover a part or legend entry to highlight it
- 1Quartz-glass envelope — transparent to 254 nm; ordinary glass would block it
- 2Electrodes — coated tungsten; release electrons that start and sustain the arc
- 3Fill gas + mercury — argon ignites the discharge; mercury vapour emits the UV
- 4Base — mechanical mount and the electrical connection to the ballast
- 5Ballast — external current limiter; mandatory, sits outside the lamp
1 · Quartz-glass envelope
Ordinary borosilicate glass blocks UV below roughly 350 nm almost completely. The envelope is therefore fused quartz — transparent to the 254 nm and 185 nm the discharge produces. (Some low-pressure lamps use soft soda-lime glass when only the visible glow is needed.)
Solarisation — an ageing mechanism. UV photons slowly degrade the quartz over the lamp's life; the envelope clouds and UV transmission drops. So part of a lamp's output decline comes from the envelope itself, not just the discharge. As a reference point, the 253.7 nm line of a typical low-pressure mercury lamp falls to roughly 70 % of its initial value after about 7,000 hours — the combined result of envelope ageing and electrode wear (Component 2).
2 · Electrodes
Two electrodes sit at the ends, usually tungsten coated with thorium, barium or calcium oxides. These oxides have a low work function: heated, they release electrons easily, which starts and stabilises the arc.
- Hot cathode (classic): the filament is pre-heated, so the lamp starts gently. Longer 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. Instant-on and shock-resistant, but shorter life (~6,000 h). Preferred for mobile devices and frequent on/off cycling.
3 · Fill gas + mercury
- Noble gas: argon or an argon/neon mix (0.5–3 mbar) — the initial charge carriers that ignite the arc.
- Mercury: 10–100 mg per lamp, liquid or as a solid amalgam pellet. In operation it evaporates to about 0.001 mbar — the low-pressure point optimal for 254 nm and 185 nm emission.
How that mercury output is distributed across wavelengths is exactly what separates the lamp types — and it is a common myth that a low-pressure lamp is "monochromatic":
- Low-pressure Hg — line-dominant: ~85 % of UV output at 254 nm, plus a 185 nm line and weaker Hg lines. The germicidal effect is near-monochromatic at 254 nm; the lamp itself is not.
- Medium-pressure Hg — broadband: a continuum from ~200 nm up to visible Hg lines. More penetrating, far less efficient.
Engineering depth: amalgam vs. liquid mercury Engineering depth
Run a traditional Hg lamp too hot (above ~40 °C) and its UV efficiency drops sharply — the Hg vapour pressure overshoots the optimum. An amalgam (an Hg-indium or Hg-gallium compound) holds the vapour pressure at the optimum across a wider temperature window. That is what enables high-output and compact, warm-running designs such as in-reactor water disinfection — at the cost of a slower warm-up to full output. (Amalgam tunes vapour pressure; it is not the same as the metal-halide doping used to tune medium-pressure curing lamps.)
4 · Base
The mechanical mount plus the electrical connection to the ballast. Common standards: G13 (bi-pin, T8 like classic fluorescent tubes), 2G11 (4-pin compact) and G23 / G24q (compact UV). High-output and medium-pressure lamps often use custom bases — they need higher currents and dedicated cooling. The base usually also 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 runs away and the lamp fails within seconds. The ballast is mandatory, not an optional accessory.
| 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 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, is inefficient, and is only legacy stock in the UV field. Electronic (HF, 20–60 kHz) is 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 versus a magnetic instant-start arrangement — especially with frequent on/off cycling. That matters because the largest lifetime cost in a UV installation is often not electricity but the lamp change and the system downtime around it.
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 | 15–20 % | 4,000–8,000 h | High flow rates (municipal drinking water, AOP processes), photochemical breakdown |
| 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 |
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" 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. Validated installations (drinking water, pharmaceutical use) must verify this with measurement sensors — a visual inspection 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). Even a lamp that has not aged can lose a substantial share of its delivered output when the sleeve is fouled or scaled.
- 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 tolerate frequent on/off cycling poorly. Where possible, choose a programmed-start ballast and continuous operation.
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.